How NOT to insulate your house

Sometimes insulation hides bigger problems.

The other day, I was talking with a friend of mine – an energy auditor who was a builder for decades. The topic came around to bad advice that “experts” give about insulating. It’s something that we both feel passionate about because homes get ruined and people get sick when innocent people follow this bad advice. We both adhere to a similar do no harm philosophy of  “if it’s worked for decades as-is, don’t mess it up!”

There’s a science to building and the tighter and more energy efficient you make a home, the more important it is that you do things “right.” It’s like the difference between making a log raft and a submarine. A log raft is leaky, but it’s forgiving because it floats by virtue of the logs. It doesn’t have to be water-tight. A submarine had better be water tight and structurally sound or you’re going to drown and get crushed by the intense pressures of the ocean.

Unfortunately, unlike boats and submarines, homes today are often built in the cheapest way possible, with little regard to physics. Renovations are even worse because people often hire unqualified “low-bid” contractors to do the work without realizing that the few thousand dollars that they save on construction may cost them tens of thousands to fix or even send them to the hospital due to mold or poor indoor air quality.

The problem is, people familiar with building science are extremely rare, as are the chances of finding a builder who knows how to make a healthy, energy efficient home. That’s why you’re here reading this now – you want to learn what not to do when insulating and how to do it right.

#1 – Insulate the attic after air sealing and reducing moisture

Adding insulation is one of those jobs that people think can be done blindly. Just throw some pink stuff up in the attic and you’re good to go, right? Wrong.

If you’re lucky, you’ll get away with this. If you’re not, you could end up with an unhealthy, moldy attic. Why?

Many homes are leaky and have insufficient insulation in the attic. This allows a lot of heat up to the attic. Warm spaces have less condensation risk than colder space so when you add insulation to the attic floor, you’re intentionally blocking some of that heat from keeping the attic warm. This makes the attic colder and a colder attic with moisture leaking from the house can result in condensation, mold and wood rot.

Bath fan ducted into attic

This is one of the main reasons that energy efficiency experts tell you to air-seal the attic where the insulation will be installed and ensure bath fans are properly vented before insulating. Air sealing reduces the amount of moisture that that can get into the attic, reducing the chance that you’ll end up with a mold problem. But, you want extra protection in the system because some moisture is inevitable. Maybe you got away without proper attic ventilation in the old house, but as you tighten things up and insulate, it becomes increasingly important to have good attic ventilation.

I personally don’t care whether you use gable vents or properly matched soffit and ridge vents, just do it right. If you use gable vents, it’s best to have two of them – one on each side of the attic to provide a good cross flow of air. If you use a ridge vent, you MUST have an equal or greater volume of air coming in to the soffits.

And that picture at the top of this post? That’s a wet mess of insulation caused by the builder neglecting to put an air and moisture barrier behind the bath tub. Instead, they were lazy and there’s a big hole with fiberglass draped over it – that’s worse than useless!

#1a – Turn off the humidifier

Humidifiers can destroy a home and make you sick. The more you tighten and insulate your home, the less you need to use humidifiers. And yet, I see very tight, new homes, being built with whole house humidifiers that dump gallons of water into the air. Moisture finds the tiniest cracks and holes and moves into spaces like the walls and ceilings. Much of it ends up in the attic where it can rot out your roof.

My advice to people as they add insulation is always “turn off the humidifier.” If the house is still too dry, find out where the air leaks are and plug those because that’s what causes dry air.”

I know this personally. Before I weatherized my own home, we’d have a humidifier going all winter. The air was so dry that my lips chapped and lightning would fly every time I touched the dog. But ever since we dealt with all the big air leaks, the humidifier hasn’t run once. It’s comfortable in the house all year round.

#2 – Choose your insulation carefully

Insulation is not a “one-size fits all” product. Insulation, like cellulose, that works great in the attic and the walls should not be used in many basements and crawlspaces where it can get wet. Likewise, fiberglass, which is ubiquitous, shouldn’t be used for the band joists in the basement or any other place where air sealing is required since fiberglass is does nothing to stop air movement.

You have to think about why you are using insulation in each location. For example, the band joist is often leaky, letting cold air in. It is also probably very cold because it may be exposed to the outside temperatures, making it a likely candidate for condensation. This tells us that you want an insulation that air seals and keeps interior moisture away. The perfect product for this is closed cell spray foam (as in the link above).

What about wall insulation? Walls are *usually* fairly air tight on the inside and leaky to the outside, with a few holes for electrical outlets and such. In general, walls are pretty forgiving about the type of insulation that’s used, but you can still screw them up. For example, a lot of building codes call for vapor barriers on walls, but using a vapor barrier on the inner wall AND using a tight outer wall can lead to horrendous moisture problems inside walls. This is an entire article itself, so I won’t go into more detail here. Just know that a conventionally built wall is usually forgiving, but if you start doing anything different, you need to be really careful.

Another insulation location is the attic. As noted in #1, you need to air seal before you insulate. But what if you could air seal and insulate in one shot? In fact, spray foam does exactly this. I love the stuff. Rather than having to spend days or weeks in a dirty attic, carefully caulking every crack and filling all the holes with cans of foam, and then carefully laying out insulation, you can instead just remove the old insulation and have a foam contractor come in and in a day or less, your attic will be completely air sealed and insulated. It’s really a superior way to insulate. I used it in all my own renovations and the results are awesome.

There are other places that are critical to consider. For example, cathedral ceilings. I’ve seen a lot of problems with insulated cathedral ceilings, especially those with tongue and groove planks instead of sheet rock. Because of potential moisture issues, I would personally never use a loose insulation like fiberglass in these ceilings. Moisture goes right through fiberglass and will condense on the cold underside of your roof. Even if you use vent chutes to provide an air space above the insulation, you can still have problems. So in choosing insulation for a cathedral ceiling, you really want a material that stops air and moisture from reaching that sensitive roof deck. Again, the ideal material is usually closed cell spray foam.

#3 – Insulate continuously: like a jacket for your home

One of the biggest mistakes people make is ignoring the big insulation picture and how it all connects together. You can do everything I’ve written about above perfectly and not have the insulation work at all. How?

Imagine an ice chest. This is just a box of insulation, sealed on all sides. Ideally, this is how a house is made. But what would happen if you took the top of the ice chest and moved it a couple of inches away from the cooler? It’s not air tight any more and whatever is in the chest would end up at room temperature. Same thing with a refrigerator – what happens if you leave the door open? The fridge has to run continuously to try to get cold while all the heat from the room rushes in. Any child would look at these examples and say “duh!” If this is so obvious, why do highly paid builders often construct houses that are no different than an ice cooler without a lid? Or to use the jacket analogy, would you wear a jacket with the zipper wide open?

Unfortunately, the more complex the home’s design, the harder it is to ensure continuous insulation. Modern homes, with a dozen different roof lines and a hundred walls can be difficult to deal with properly. I’ve seen contractors start by insulating under the roof in one place then switch to insulating the walls in another, rendering the roof insulation meaningless.

One trick to use is the “pencil trick.” Take the architectural drawing of the house and, without lifting the pencil, draw a single line representing the insulation around the house. Wherever you lift the pencil would be a discontinuity where outside air could short-circuit the insulation, rendering it useless.

#4 – Be smart – Don’t break the original home’s design

Going back to the “do no harm” principle, you have to be really careful not to “break” the working design of the house. I’ve seen insulation contractors do really stupid things that lead to moldy, rotten homes. If a house was built 50 or 100 years ago, and it’s never had a mold problem, there’s probably a good reason why. For example, there are a lot of older homes build with plaster and lath. These have an air gap between the exterior stone wall and the plaster interior wall. That air gap allows water to drain and dry out. But yet we see people putting cellulose and other non-water resistant insulation into that gap. That’s like sticking a sponge in there. It will soak up the water and likely end up in a moldy, rotten mess and useless insulation.

There are other stupid things you can do. For example, pretty much every roof that goes on gets a ridge vent. But why add a ridge vent to a 50 year old home that already has gable vents? Worse, that 50 year old house won’t have matching soffit vents. The result is a house that is less energy efficient and more prone to mold and rotten roofs because the ridge vent sucks the humid air out of the house and into the attic.

If something doesn’t make sense to you, don’t let the contractor do it. Sometimes they’ll be offended and say something like “I’ve been doing this 30 years, I know what I’m doing.” Well, maybe they’ve been doing it wrong for 30 years! If you have any doubts, get another opinion or two. Or post a question on my Q&A section. There’s a science to this and there’s a good chance that your contractor doesn’t know the science. I’m not saying that they’re not a good contractor, but they may simply not be knowledgeable about the current best practices. Regardless, if they’re not willing to listen or back up what they say with facts, you should find another contractor.

#5 – The home is a complex system

I touched on this in #4 with the discussion of the ridge vents. A building is a lot like a human body – what you do in one place affects things in another. Just like adding a ridge vent can lead to increased energy bills and a rotten roof, you can do other things that have unintended consequences.

For example, many utility rooms with furnaces or boilers have large vents that provide fresh air into the room. Often, people look at these as sources of cold drafts (which they are) and close them up. The problem is, they were put there for a reason – they supply fresh air for the combustion that the heater is doing. Closing the vents and sealing the utility room can lead to improper heater operation or even carbon monoxide problems which can lead to illness or death. Unintended consequences can be deadly.

Another example of unwanted side-effects of insulation – spray foaming a house without ensuring adequate air supply.

I’ve been known to say “a house can never be too tight.” Unfortunately, you can screw things up by making a house very tight without considering fresh air and humidity requirements. Using spray foam, you can make a house that has extremely low air leakage, so you have to consider introducing fresh air using an HRV (heat recovery ventilator). There are standards for this (See ASHRAE 62.2 and related). Basically, you want a certain amount of fresh air per occupant in the house. For typical families, this amounts to about the amount of air introduced by a bath fan. Not much! But often, people adding spray foam insulation either don’t consider or don’t know about the fresh air requirements, and the results can be horrible.

Concrete example: you make a house super tight at the same time as you remodel the kitchen and add a powerful new range hood. When you run the range hood, it creates a suction so powerful that it can sucks the exhaust fumes from the furnace and water heater back into the house. Again, this can kill you. No joke.

Less extreme example: you tighten up your house and then you find that every time you make a fire in the fireplace, you end up with a huge amount of smoke in the house because there’s just not enough air going up the chimney. To compensate for this, you have to open a window whenever you have a fire.

There are numerous situations like this that can occur if you “just insulate” without considering the complex system that is your home. Please, go into these renovations with open eyes. If you have any concerns, call in a trained energy auditor/building scientist. I’m also happy to answer questions posted here. But be aware that in order to do this right requires a series of precise measurements that take into account a variety of factors about your house and various appliances in the house. In some areas, these measurements are required if you are adding a certain amount of insulation or doing significant renovations.


159 thoughts on “How NOT to insulate your house

  1. Hello,
    We have a house that was built over some time, and we just moved in 7 months ago. Now that we are in the really cold winter here (temps have been running in the 20’s and single digits) , I have noticed that we are getting frost on the roof deck and ice on the nails. We have soffit and a ridge vent. Fiberglass batts for insulation, that have paper facing on the side touching the drywall. We have seemless gutters and our gutter runs out away from the house underground, it’s tied to a drain pipe that runs through my front yard and out into the ditch by the road. Which is about 100 ft away. No gutter leaks, and no gutter blockages. We do have some plywood flooring down in the center of the attic about 4 feet wide and running for about half hte length of hte house.

    We have changed the bathroom vents, and have them vented outside the house and not in the soffit as they were. However, I have noticed that I can see a small amount of steam coming out on cold days when someone is showering. There is a little bit that leaks out of the silver boxes for the vents as well as where the tubing is connected to where the pipe vents out the roof and I noticed some drops there where the pipe goes out. We have 7 children and so there are a lot of showers taken. My furnace is vented out of the first level of the house, out the side of the house through the siding, as is my hot water tank.

    I have put a BatticDoor box over my stair access. It consists of a silver bag that you place a batt inside and a box then slides into it. The box then rests on a foam seal that runs around the door frame. This keeps my attic access door from getting condensation. I had a problem when it started getting cold with access door forming drops that dripped down to the carpet. That has stopped now with this box in place. I also placed a foam strip around the edge where the attic access door itself touches the ceiling.
    I have sealed any gaps around my attic stairs with the spray foam. I had about a 1/2/ in. gap on one side, a little smaller on the other. I ran a dehumidifier last week when the temps were unseasonably warm for a couple days (upper 40’s to almost 60 the one day) and things seemed to be dried out. But now that the cold has returned, I can’t run the dehumidifier or it will freeze up. I went up today to get something out of hte attic and have noticed frost and ice on the nails.
    Before running the dehumidifier last week, the roof deck was wet to the touch. After I ran it, the walls felt dry. WHen I was running it, it was raining hard. But that didn’t seem to be a problem. As I didn’t notice it getting wetter with the rain. It was getting dryer and the dehumidifier was not a large one.
    Only now that the temps have dropped back down into the 20’s and teens has this frosting and nail ice started again.
    I do have 4 recessed lights in my kitchen, and I don’t know if they were sealed or not. Or if any of the pipes that run up were sealed with foam or caulking. I did notice before when I saw the wet walls for the very first time that it seemed wetter by the attic access side. Which is why I sealed that.

    Where would you start with a problem like this? What recommendations would you make for me to try?

  2. Hello,
    We have a house that was built over some time, and we just moved in 7 months ago. Now that we are in the really cold winter here (temps have been running in the 20’s and single digits) , I have noticed that we are getting frost on the roof deck and ice on the nails. We have soffit and a ridge vent.
    We have changed the bathroom vents, and have them vented outside the house and not in the soffit as they were. However, I have noticed that I can see a small amount of steam coming out on cold days when someone is showering. There is a little bit that leaks out of the silver boxes for the vents as well as where the tubing is connected to where the pipe vents out the roof and I noticed some drops there where hte pipe goes out.

    I have put a BatticDoor box over my stair access. It consists of a silver bag that you place a batt inside and a box then slides into it. The box then rests on a gasket that runs around the door frame. This keeps my attic access door from getting condensation. I had a problem when it started getting cold with access door forming drops that dripped down to the carpet. That has stopped now with this box in place.
    I have sealed any gaps around my attic stairs with the spray foam. I had about a 1/2/ in. gap on one side, smaller on the other. I ran a dehumidifier last week when the temps were unseasonably warm for a couple days and things seemed to be dried out. But now that the cold has returned, I can’t run the dehumidifier. And I went up today to get something out of hte attic and have noticed frost and ice on the nails.
    Before running the dehumidifier last week, the roof deck was wet to the touch.
    What can I try to correct this?

    • Thanks for the thorough description. Did you seal the areas that you saw steam coming out? That’s a dead-giveaway of the source of the moisture. In most of these places, you can clean the surface (I like using baby wipes then a dry paper towel) then use foil-tape (available at HomeDepot/Lowes) to seal metal boxes. You can also use the canned foam to seal up openings from the walls up into the attic. Just doing these two things might eliminate the problems.
      The other thing to look at is the humidity level inside your home. If you were getting condensation on your attic access door, then the humidity inside your home may be higher than recommended.
      During the winter in a cold climate, I’ve found, it’s best to keep the humidity between 30-40% in the home. If you start pushing higher, there’s so much moisture in the air that condensation problems often arise. So if you’re using a humidifier, I’d dial it back. A good, tight home should NOT require additional humidification. If you feel like you do need humidification, that’s a sure sign your house is leaky.
      For reference, I used to use humidifiers in my home because we’d get bloody noses and cracked skin all winter. After replacing windows and really working on air sealing the house, I’ve thrown away the humidifiers and am more comfortable than ever.

      • Sorry for hte double post, there was an error when I tried posting it originally and when I refreshed the screen it didn’t show my post as having gone through. I did add some additional information I thought of before I reposted it.
        We do not use any humidifiers in here at all. Although I am not sure what my house humidity is. I will have to get something to check that.
        I noticed on my boxes for the vents, there is a small hole built into the top of them from the manufacturer. And I could see steam coming out of that small hole when my daughter was taking a shower and I went to check for steam. Can that small hole be sealed? As I have two of those boxes and both have that small hole.

      • No problem. WordPress can be a little odd with posts…
        Ok, no humidification. That’s good!
        Most of these metal boxes leak horribly. I’ve sealed all of them without problem. However, just in case, do you have a make and model number? I could look at it and see if I can determine anything from the photos/docs.
        The other area to seal is where the boxes meet the sheetrock. Usually there are huge gaps around them. Nobody seals them. For this purpose, I use an amazing tape called Polyken (
        This stuff sticks permanently (on clean surfaces) and is awesome for sealing these metal boxes to ceilings. I pull off the interior grill to get access, then tape to the inner surface of the box and down around the sheet rock, maybe 1″. You have to measure carefully so that you’re sure the grill will cover it otherwise you’ll see the tape. And if you stick it to the sheetrock and try to remove it, it will rip the paper right off the rock!

      • Ok, loads of holes. You can seal it up. If you want to be safe (concerned about heating) use an LED bulb. Much less waste heat /energy than an incandescent bulb.

      • Thank you so much for all your tips! I really do appreciate this! I do have LED’s in them now. But I will go ahead and seal them up. And get the pipe better sealed. And hopefully this takes care of the issue.

  3. Ted, great article and insight…thx for sharing and caring.

    Quick question on using CC spray foam insulation…

    I have an old 1900’s home that am remodeling. The former owners either (DIY) or hired someone to add a few additions back in the 70’s…one being a 6′ x 10′ kitchen. Besides the foundation sinking (another topic) they added a shed roof off the original roof/house. its low slope, thinking 3/12. When I tore out the Flat drywall ceiling (made the kitchen very claustrophobic) noticed the plastic moisture barrier, then batts, signs of moisture. It was poorly constructed (used 2×4 rafters) and it looked like it had leaked in the past…just a poor job overall. Also NOT having any soffit vents, nor rafter venting + low slope and possibly poor insulation, this roof leaked due to heavy winter snows, heat escape, roof/ice dams etc.

    With the advent of these CC foams thought why not foam the exterior walls (from rim plate/floor, up the walls, into the shallow 6″ soffits (no vents) continuing underneath the shed roof (installed new 2×6″ rafters) and decking and create an impenetrable R/value + Air + water tight shell to control heat loss, air leaks and lack of soffit vents – basically remove all the elements creating the icicles/heat loss/roof leaks that are plaguing this addition area.

    Yes, this small shed roof ties into the upper attic (where addition was added) but will be insulted w/ batts, (2) Ridge vents etc…Just thinking I seal off this addition roof area w/ foam, treat the attic as it was originally built (sans addition).

    Would appreciate your input?


    • That sounds like a good plan. Seal everything up tight. As long as there’s enough R-value to keep the inner surface of the foam near room temperature, there should be minimal condensation risk. It’ll be just like a normal cathedral ceiling.

      • Hi Ted, I heard 1″ = R-7, 2″ = R-15…so filling up the 2×6 cavity @ 5.5″ (or going Hybrid) + applying the 1″ Ship-lap..should be a beefy R-25+….that’ll be tight and right.

        While I have you, in regards to my Attic space…since their are NO Soffit vents in that roof either (approx 20×20/vaulted ceiling) + (2) gable vents (for cross ventilation) + the tongue-groove flooring w/ cellulose underneath…also has (2) drafty windows on each side (act like vents)…would spraying CC foam where the floor meets the rafters (at bottom plate) help reduce heat escape along the 2nd story walls? again NO soffit vents so thinking the air leaks from the walls to underside of roof (@ soffits) is where enough heat is escaping to form the icicles…

        At some point I may want to finish off the attic so will either go 100% CC Foam between rafters (from floor to peak) or do you think it’d be a waste of time/expense to use a combo of installing continuous soffit vents, plus 1″ rafter gap, w/ foam board/batts etc. I do not have a ridge vent so thinking option B is was more work/time/money that 100% CC foam and closing off the gable vents?


      • The ice dams and subsequent icicles are typically a combination of inadequate insulation and heat loss from the interior, so likely, yes, good chance of that being true in your case and sealing everything up there would most likely eliminate the problem.
        Full foam solution is likely the best long-term answer. Like you said, no vents, so doing something like you describe would likely be a waste of time. However, if you have any concerns about the roof decking and its long-term viability, you might be best off with plan B, and install spacers and board foam. Then you could just spray inside of that. The advantage is if you have to replace the roof in the future, it won’t be glued in place by the foam. Removing the roof deck would ruin the foam job, which would be a real shame. So spending extra effort today makes future maintenance much easier/less costly.

  4. I am in the process of insulating a pole barn constructed garage. We are located in Holland, MI and winters are cold, summers hot. However, I only intend to insulate for heating purposes. It basically is built the same as stick frame at the roof as the building is sheeted with OSB 7/16 walls and 1/2 ceilings. I have Tyvek wrapped the whole place and sealed all the windows and doors. The foot print is of an L design in that there are two bays connected to each other with dimensions of 24’x25′(12’high ceiling) and 17’x40′(16’high ceiling). The floor has 2″ foam with a vapor barrier and a 6mil poly that I also put down. There will be hydronic radiate heat and a efficient design layout was provided by the engineer that sold me the tubing and boiler system. I plan to use an R38 batt insulation for the ceilings (2’oc) and the walls(2×4) would be R11 batt (2’oc). The wall girts are 2×4 space 2’oc horizontally. This creates a 1.5″ gap that I could either fill with 1.5″foam board, leave it or by 5″ batt and remove 1.5″ where the girts go. The last option would be a PITA for labor. I looked into spray foam, which would fill the gaps, but it is too cold out now and was $$$. The place with the Tyvek and good installed doors/windows has made the place quite air tight already. So, what would you recommend? Also faced batts or not. The walls will be 1/2″ plywood. The ceilings will be either 5/8 drywall or 1/2 plywood. Thanks for any input.

    • Given that construction in your climate, I wouldn’t use the foam board as that could create a moisture trap since the inner surface of the relatively thin foam board would still tend to be chilly enough to induce condensation formation.
      Having said that, if you wanted to up the insulation to thicker batts and just compress them at the girts, that gets around having to carefully cut them around the girts and gives you better insulation than thinner insulation all around.
      I know it’s considered bad practice to compress batts, but that’s in more normal situations. For example, in an attic where you have pipes and wires, it’s better to carefully cut the insulation because squeezing them around those things can leave air gaps. In your case, the full wall cavities will be filled and the only compression that would take place is on the surface where the girts press. Not a big deal. In fact, the slightly compressed 5″ batt will have better R-value than a thinner, uncompressed bat of the same thickness because a dense batt actually has a better R-value per inch.
      As for facing or not, I believe in your climate, faced batts are recommended, but you should double check that with your local code officer or builder just to make sure.

      • Thanks for the advice. In looking for insulation options 6.5″ (R19) is readily available. Thus, I would have to compress it to make it fit the void of 5″ space or stud further out from the outside wall. Since you said more compress is a higher R. How much compression is too much? Another option is to just remove 1.5-2″ of batt and place them in between and then put R11/13 to fill the rest of the void. Thanks again.

      • That amount of compression should be fine. You’ll end up with a higher R-inch, i.e. you might get R3.5/inch instead of R3.1/inch. So, 5″ times 3.1 = R15.5 uncompressed but you’ll get 5″ times 3.5 = R17.5 in the 5″. So it’s less than the full R19 but much better than normal.
        Like you said, doing the extra work would be a PITA for minimal gains, so keep life simple. If the structure is tight, the marginal difference in R-value will hardly be noticed, if at all.

  5. is almost impossible to get to the perimeter of our attic due to the low slope, and there is at least a 3 foot eave overhang. Where there is a span in the eave that does not have a soffit vent, could the insulation be blown in and enter the eave/touch the bottom of the roof? Installing baffles will be quite the trick, is going around the entire house putting baffles between every single rafter really necessary? Thanks so much!

    • The part of the eave that overhangs doesn’t need to be insulated, so I’d just insulate the part you can easily access in order to avoid blocking the soffit vents. Since you don’t need to insulate the area anyway, you don’t have to worry about those areas where you don’t have vents. This should considerably reduce the issue of having insulation compressed against the roof deck which could trap in moisture and rot your roof.
      The other thing you can do is to blow the insulation in then use a rake with a long handle to remove the insulation in those areas where it is in contact with the roof. You don’t need much space, just an inch or two, so it’s pretty easy to do after the insulation is blown in.

  6.’ve been working toward zero net energy for years now and have come a long way. for the walls I’m using poly iso closed cell (R-max), custom cutting bats and spray foaming around the edges, trimming and then taping the seams all the way up into the dropped ceiling, which is one of the least favorite things I’ve ever done, there is less than 2′ of headroom for me to wriggle around in, no fun. I’m achieving somewhere around R 25-30 total, including the wall material, which isn’t bad considering I’m not adding to the thickness. I am also using a 1/2″ sheet of poli iso over the walls on the inside to block thermal bridging. my understanding is that I am clear from creating condensation issues because the dew point will occur at about the 2″ mark, within the closed cell insulation. I have virtually eliminated any convection current driven channeling in the walls which should keep a water pump from forming. my specific question right now is how to best insulate the basement concrete slab. the house was built in 53, so no insulation under it, it is an infinite black hole of energy and it has a large footprint, so lot of surface area. currently i’m planning to use 3.5″ of rigid foam, which is the most water phobic. I plan to lay it down and foam in the seams, trim and tape over, essentially gluing the edges to the insulation in the walls for a continuous seal. on top of the foam I plan to alternate two layers of 1/4″ plywood and then cover with Vinyl Plank. my question is whether I need to do anything for moisture on the slab. if moisture ever comes in, there is nothing for it to soak into so I think I am OK, but it would be nice to be able to bounce all this off of someone in the know and a stumbled across this site so Ted, how about your thoughts? any tips would be appreciated. the trouble is that zero net energy is hard to achieve with a remodel because there are so many things about bad common construction techniques that make it hard to insulate adequately. the whole thing is ridiculously time consuming and simply would not be affordable at this point if I was not doing the work myself as part of my role as part time stay at home dad. we have two electric cars, an air sourced heat pump, a heat pump water heater, heat pump clothes dryer, all LED lights and energy star appliances, all powered by a solar array.

    • Great job! That’s a lot of good work you’ve done. Very challenging for a retrofit as you say.
      My house has the same basement slab issue that I was going to remedy like you specify with layers of foam over the slab. But the ceilings are too low, so I just went with a simple thick foam pad and carpet, which has made a huge difference in spite of low R-value.
      The thing to consider is that many slabs are in the ground which is already 100% humidity in many places. The moisture slowly seeps through and comes out. Same with foundation walls. As long as they’re not subject to freeze-thaw cycles, nothing much happens. It might get nasty in that layer between the slab and the foam, but you’re locking it all under the foam. And it’s really little different than the dirt under the slab.
      Ideally, the insulation goes under the slab, but having it over the slab, blocking the moisture, doesn’t really do much harm, you just don’t get all the benefits of that thermal mass.

  7. I just had blown-in insulation installed in entire house (built in 1920, with several additions & improvements since), 950-square foot bungalow.

    The entire house now smells very musty.

    Could there have been must/mold already in walls that the insulation disturbed and pressed toward the inner walls creating this overwhelming smell??……
    ….. or Could the insulation that was blown in been musty, causing this horrific odor?

    • On the first question, odors wouldn’t pass through solid walls, so that’s not likely. Now, the musty smell could have several sources that come to mind:
      – The insulation can make the house much tighter, meaning less fresh air circulates into the house, flushing out odors. That’s largely the point of the insulation. This is why, in many areas, contractors are required to test the tightness of a house before and after insulating to get an idea of “natural ventilation” – how often the air in the house turns over. This is also done for safety purposes because a very tight house can lead to safety issues with combustion devices (oil or gas furnaces, gas stoves, dryers etc.)
      – Do you have hot air heating or radiators? If you have a forced hot air system, it’s possible that stale air is being sucked into the ductwork due to leaks, then being blown into your house. Combined with a tighter home, the air could get very stale quickly.

      Until you’ve resolved the issue, you might want to leave a bathroom fan running full time. That will force air flow through the house and will help to reduce the odors.

      The best thing to do would be to have an energy auditor come in and do what’s called a “blower door test” and ask them what they think based on the numbers. This would also tell them if there’s significant leaks in the ducts and could also help to determine where the odors are coming from.

      • Thank you for your quick response, Ted!

        The blown-in insulation was just done 4 days ago.
        They did begin their work with that ‘blower door test’…. and I was told later in the day that this house was SO OPEN, that they couldn’t even get a reading. And I have NO doubts about that being correct, as we’d feel constant breezes throughout this house.

        I don’t really smell that musty smell all through out the house, just when i”m near the walls…. which is where our dinner table rests…. especially that location, reeks of musty odor.
        I’ve been sitting in that location for almost 30 years,… that smell there is new, since the insulation. I swear its from that.

        The insulation they blew in was like a white/grey fleece material.

      • Aha! Ok, that’s good information. Did they tell you how leaky it was after the insulation? Given the age of the house, it’s common for them to be very leaky. Also, construction can be prone to moisture problem which would lead to the musty smell. Do you have a dirt floor basement or crawlspace under the area that smells? Musty odors are usually mold related, and molds grow where there is high humidity. So I would check all around that area of the house for where smells could be coming from, but most likely, under that floor.

      • ok…. so… after my last response to you, I went around and did a ‘sniff test’… around the inside walls of the outer walls that were insulated….

        …..that smell is radiating from that one wall, beside the kitchen table….. that wall has paneling on the lower wall, with a chair rail, and a newly installed 3-pane vinyl casement window above.

        None of the other full walls are radiating that musty odor I smell.

      • Good detective work. I fear that you should have someone open up the wall there to inspect the inside for rot. It is possible that when they installed the new window, they were not able to do the flashing properly which has led to water getting into your wall. The sooner you can deal with it the better.

        However, you left one question unanswered – do you have a basement or crawlspace under there? If you do, you should inspect that are directly below to see if there is any sign of water or mold that could be causing the smell.

      • Yes, there is basement directly below…… with a cement floor. (there are other crawl spaces down there, but not in that area.)

        Now that you mention it, I wouldn’t doubt at all, that the flashing for the window wasn’t done correctly 😦
        That is gonna be one hell of a job in the Spring, trying to correct that, with all that insulation in there now. Ughhhh

      • (edited) I highly recommend that someone explore what is happening at floor level behind the paneling prior to Spring. If there is a water leak, the rot could become severe in that amount of time leading to a weakening of the structural integrity of your wall.

        I speak from experience. We moved into a home that had a leaky sunroom wall that the sellers had covered up. On Christmas, several years ago, I noticed some mold on the paint. By the end of the night, I discovered that all the 2×4’s that support the wall had rotted out, leaving us with an extremely dangerous situation.

        A contractor experienced in diagnosing water damage will have a “moisture meter” with long probes. They need only drill a couple of small holes in the wall to insert the probes. They can then determine the level of moisture hidden in the wall. They should also have a “non-destructive” moisture meter that allows them to detect moisture hidden behind the paneling.

  8. I am about to build my family’s forever home and am having trouble understanding the best way to insulate my house. Your post was very helpful. I’m wanting to do open cell foam. From what I understand, if I foam the entire house, I need to have some sort of vent system, that way my house can breathe a little. Having vents in my attic are pointless if I foam the floor, seeing that it creates an air barrier, right? What is the best set up for me? 2 story, crawl space, electric heat pump with propane backup.

    • A tight house definitely needs a fresh-air system. Depending on where you live, you want either an “HRV” (heat recovery ventilator) or an “ERV” (energy recovery ventilator). Here’s an article on that.
      Venting the attic if you have foam on the floor is still important because moisture can still build up in the attic. If the attic is all closed up, then you have what is called a “dead air space”. Without ventilation to flush out the moisture, you can end up with problems.
      Your builder should be able to give you specific info for your climate since some building rules vary depending on your local temperature and humidity throughout the year.

  9. “…but using a vapor barrier on the inner wall AND using a tight outer wall can lead to horrendous moisture problems inside walls. This is an entire article itself, so I won’t go into more detail here.”

    This describes my current reno project with R32 Roxul (rock wool) in the walls including Roxul R6 sheathing insulation (dense, more rigid rock wool); 6mil vapour barrier inside, Tyvek house wrap outside. R 60 Roxul in ceiling/attic space above. I live in Ontario Canada where it can reach -25 F in winter.
    Can you direct me to an article regarding proper use of vapour barrier, house wrap, insulation to help me prevent future mold problems. All help and insight is greatly appreciated. I am currently applying the Tyvek house wrap. Thank you!

    • Tyvek is actually a specially made material. It has a high perm rating, so it allows water vapor to pass through it. At the same time, it sheds water, so liquid water doesn’t get into the wall. It can do this since water vapor is a tiny molecule compared to liquid water. So you should be ok with that type of construction.
      The main time that people get into trouble with this construction is when the flashing isn’t done properly. This lets liquid water into the wall cavities which the Tyvek doesn’t let out. However, if water is getting into the walls, it would most likely get stuck in the wall and rot it out even without Tyvek.

  10. am so glad you are answering questions!!! I live in a 720sq ft cottage in humid, hot South Florida, built in the early 20’s, and it was built using very interesting methods. The walls were not constructed using studs, rather the exterior (and interior) walls were built with tongue-in-groove bead board. Every wall is a pretty solid sheet of 1 1/2″ bead-board. On the exterior is tar paper and cement board siding, and interior is a few layers of paneling (3 different layers), and now drywall. The ceiling also consists of tongue-in-groove bead-board nailed directly to the 24″ on center joists. Currently, the insulation looks like shredded cellulose, but I am just not sure. The attic space is ventilated with gable vents on either side of the house. The flooring is a nice old pine floor, but there is no sub-floor. The is also I good sized crawlspace under the house. I love my little house, but I know I can to better insulation wise. In the attic I want to clear out the nasty, dirty stuff, and replace it with something new, that I can partially cover with decking for added storage, and I want to figure out how to add additional support to the underside of the flooring and add insulation there too. I loose a lot of cold air through the floor. SO my question is what do you recommend? 15″ of fiberglass batting or could I use Polyisocyanurate panels, or some combination of materials to get to R-49 up in the attic that I can the sheath in plywood decking? I know it will be odd, but I am thinking of adding panels of plywood under the existing flooring between the joists, but I still need some sort of insulation under that. Eventually, I would like to remove the drywall and various paneling, redo the wiring, add rigid panel insulation, and refinish with new drywall.

    • That’s impressive construction! I’m surprised it didn’t get really moldy in there from all the moisture.
      The first thing that comes to mind is avoiding “unforeseen consequences” with anything you do. In the past, with all the airflow, the interior climate would have been much like the exterior. After you installed A/C, I would think that it would be prone to condensation on the inside due to the colder temperatures from the A/C. The A/C however dehumidifies, so it must be doing its job well to be combatting all that humid air.
      If you’re doing a major remodel as you’re describing, here are some things to consider:
      – Your climate is the opposite of many in the north, so the rules have to be reversed. You have hot, humid air inside, and temperate, dry air inside.
      – Condensation occurs when warmer, moisture laden air comes in contact with a cooler surface. Therefore, the vapor retarders need to face the direction of the hot/humid air. In your case, that’s “out”.
      – The way to keep moisture out is to ensure that any loose insulation, like fiberglass batts, is protected from the moisture. So you could have it against the ceiling (the floor of the attic), but you’d want something above it, like the poly-iso sheeting carefully installed with taped seams, to prevent moisture from getting past.

      Here’s a schematic of how to think about it:
      Moisture (outside the house) -> vapor barrier -> Insulation -> house inside ceiling or walls
      The trick is how to get R-49 insulation AND provide a floor for the attic for storage. If you really want to go that route, your best bet would be to change where the insulation layer is to the underside of the roof. This would make the attic much more comfortable and you could do whatever you wanted with the floor. You’d have to close up the gable vents and treat the attic as if it were an upstairs bedroom, if you get what I mean. Totally seal up the attic, insulating the “ceiling” and walls.
      For your crawlspace, you reverse the process. You definitely want a full plywood subfloor with insulation under it. The amount there is much less important than the attic. The key thing is keeping air from blowing up through the floorboards. Adding plywood between the joists wouldn’t help much structurally unless you were really careful about installation. And even then, you’d be a lot better off pulling up the pine flooring and laying plywood over the joists then re-attaching the flooring. But that adds a level of complication too because now you’d have to trim your doors and anything else that depends on the current floor height.
      If you’re not married to the idea of reinforcing the floor, then the easiest thing would be to have a couple inches of polyurethane spray foam blown right to the underside of the floorboards. This will totally stop wind from whipping through and add a great moisture barrier. It would also “glue” the flooring to the insulation and make the entire floor feel much more solid. That would actually be the recommended approach for the attic. They’ve shown that foaming the roof like this actually makes the house more hurricane-proof because it glues the roof on.

      • I didn’t really want to go the route of making the attic a livable space, and definitely not closing up the gable vents, the storage is for things that are not overly heat sensitive. That is way more than is feasible. I was thinking of a layer of spray foam on the attic floor, and then adding fiberglass batting over that. Whatever I do, I have to add additional height to the floor as the joists are 2×4’s which means only 3.5 inches of cavity to fill. I could do R-15 fiberglass between the joists, and then add R-30 perpendicular to the joists and forgoing the storage decking. I have to do something before the next summer comes.

      • The approach isn’t to actually make the attic into living space, it’s to put the insulation layer where it can be most effective and yield a structure that is least likely to develop moisture problems.
        You can probably get away with doing it as you describe since the foam will provide enough insulation that you’re unlikely to create a moisture trap between the fiberglass and the foam. But I wouldn’t do just a thin “skin” layer of foam. You’d want at least 2″ in there, which would give you close to the same R-value as filling the joists with fiberglass. Then go perpendicular as you said with high-R batts.

  11. Thank you for the very informative article. We have a problem in our 1972 townhouse in Miami FL. I hope you can assist. Approximately 2-3 months after our remodel 1 year ago, my husband and I have been smelling a cigarette/smoke smell even though no one in our household or neighbors smoke, we are plagued with headaches, coughing, watery eyes, itchy nose. I am ready to move out of the house. As soon as we step outside the symptoms subside and no more smell.
    Our remodel entailed, new windows and doors, shingle roof with ridge vents, removal of the old cellulose insulation, sanitize and install new Certainteed insulation in attic, new A/C and all new attic duct work. I have had home inspectors, AC installers check ducts and ac for leaks and clean coils, roof installer to check roof and vents, and the insulation installer out to make sure the insulation is installed properly and not covering up the soffit vents. I also had an air quality testing company come out to check the air. No one can determine the issue nor do they smell the anything. Neither my husband or I have ever had allergies, before and have lived here many over 20 years. I considered the spray foam insulation but the roofer said it will reduce the life of the shingle roof do to heat? Please any advise is appreciated.

    • That’s a tough problems, especially since you are obviously smelling something that others do not. You know the problem exists since you’re there all the time.
      Here’s some things to think about:
      – is the smell always there or does it come and go?
      – has anyone else smelled the odor? For instance, when you have guests, has it come up?
      – If the smell comes and goes, you should note the specific conditions

      Keep a notebook handy and log every circumstance when you smell it. Log: date, time, weather (is it sunny, rainy, windy, etc.)
      I ask about weather because humid air releases smells. If it only happens when it’s foggy or rainy, that could indicate the smell producing area is getting wet and releasing its odor.

      Log conditions inside the house. Are any vent fans running? This would include kitchen or bath fans, or any other fans like that. It would also include your dryer.
      I ask this because any air vented out of the house has to be replaced by air coming into the house. This “make up” air gets sucked in from any place air can get in.

      How did they check your ducts? A visual inspection is worthless as an inspection tool. The only valid way to test the ducts and A/C system is for them to seal all the vent then attach a powerful, calibrated blower to the ducts. This pressurizes them and lets them measure the exact leakage. If they just walked around looking at your ducts, that doesn’t tell them anything.

      There’s debate about spray foam and roof life. Us energy people like to point out that a south-facing roof gets vastly hotter than a north facing one because the northern one doesn’t see much sun. Do they complain and tell you not to face the roof south? Regardless, I wouldn’t take any measures (which could get very costly) until you determine the source of the problem. A lot of contractors are more than happy to try to fix things because they make money doing so. However, if they don’t know what they’re trying to fix, then the chance that they’ll fix your specific problem is virtually zero.

      During your remodel, did you put in new carpeting or furniture? These can be notorious for outgassing which can cause the problems you’re experiencing. I just had some staining and polyurethane sealing done in my house and it gave me those same symptoms. I had to put plastic up to seal off that section of the house and run a large fan to vent the area for several days before the odor was tolerable.

      Another thought – it sounds like you didn’t have ridge vents before. I’ve heard of problems occurring when ridge vents are installed. Even if soffit vents were installed, often they allow far too little air in to match the air going out the ridge. So the ridge vent ends up sucking air from the house below. As with the exhaust fans, that air has to be replaced, so the house sucks air in from anywhere it can outside.

      You’ve probably got a much tighter house than you had before the remodel. As such, the outside airflow through the house is much less, as desired. However, the unintended consequence is that it’s much easier for the air inside to get “stale” unless you install a fresh air ventilator. Same thing with my house. Did a similar remodel and the house started getting very stuffy/stale (i.e. it stunk!). Fortunately, I knew about this phenomenon and installed an ERV (energy recovery ventilator). I can’t imagine living in a home without this unless I lived in a climate where we could leave the windows open all the time. In most climates however, you close up the house for winter and never open a window. And during the summer, you close it up and run the A/C. The air can get yucky fast.

      The installation of an ERV is the one thing that I could say will benefit you regardless of the source of your smell. You typically install an air intake vent for the ventilator up high, where you know it is away from any odors. It sucks in fresh air, conditions it with the temperature and humidity of the exhausted stale air, and blows it into your home. They’re required in modern, tight homes for exactly the reasons we’ve been discussing. I consider them essential items.

      As you can see, there’s many possibilities. If you have any more questions, feel free to write.

  12. rim band joists were insulated with fiberglass insulation by a contractor this past winter who said he had no problems using fiberglass in the basement. Everything was fine until September when the humidity level shot up to 70-80%. I get the levels down to 57% with a dehumidifier then they shoot up again. Am I right to believe the Fiberglass insulation has been absorbing moisture and is causing the sudden rise in humidity the last few weeks? Guess I should switch to closed cell spray foam.

    • Actually it’s unlikely that the fiberglass is to blame. It takes considerable water to raise humidity levels, as your dehumidifier bucket (if it’s not self draining) will show.
      Those gallons of water are most likely coming in through either the floor or from moisture-laden air coming in from outside. It could also be moisture coming in through the foundation walls. Most likely, it’s air infiltration from the outside but if the ground around the foundation is wet from summer rains, a lot of moisture can make its way through the walls.
      As you noted, closed cell spray foam would be a better solution as it will prevent moisture from condensing on the band joist (since they’ll be covered by foam) and it will air-seal the band.
      I would definitely check the moisture levels in the walls and floor. It may be that you’ll end up wanting to seal them as well if you’re getting a lot of moisture in there. Having to run the dehumidifier constantly to keep the humidity down will cost a fortune over time, so it’s worth dealing with the issue.

      Another tip – walk around the house when it’s raining and make sure that your gutters are draining far from the house. A rainstorm can dump literally thousands of gallons of water right at your foundation. I’ve seen many homes with moisture problems due to gutters draining close to the foundation. You want clean gutters and all the downspouts at least 6+ feet from the foundation. The further the better.

  13. Pingback: Before You Insulate, Read This | Remodeling Kitchens and the Home

  14. situation is this…I have a low slope roof/ceiling that gets a lots of ice build up,the wood stove is in this room and there is a metal roof,thus far there is no evidence of water damage. My solution I believe is to put 2″ insulation up on the ceiling to stop the heat from escaping and causing the ice dam. Question…should I put the insulation directly on the ceiling or create an airspace with some strapping? Thank you,Darwin.

    • That’s a good question. I would be inclined to leave an air-space to allow air flow that would allow any moisture to flush out. This would also keep the metal roof cooler if you permitted outside air to flow under the roof (above the insulation). That should minimize the ice build up.

  15. think that being able to air seal our attic when we add insulation like you said would be helpful! We definitely don’t want to have to deal with mold or wood rot when we have our attic insulation done, so being able to air seal it is something I’ll have to look into. I feel like being able to have some attic insulation could help us out and see if we can have a more energy efficient home!

  16.’m wondering if you have thoughts about insulating a summer cottage which is not used (or heated) in winter. It’s about 900 square feet, and the front half has a cathedral ceiling, while the back half has an attic. Up until a year or so ago, the ceilings were all unfinished (between bedrooms and attic, rafters at the roofline). However, so far we’ve insulated the cathedral ceiling section (with fiberglass insulation; the contractor used wood paneling as a ceiling finish). The wall dividing the cathedral ceiling from the attic was also finished (with a thin wood paneling). Currently, the back half of the house is not insulated, but we want to insulate it. We are not sure if we would insulate at the roofline, to match the cathedral ceiling in the front, or at ceiling height, under the attic floor–and how we would avoid creating a moisture problem that has never existed up to now. Also, the attic has only a single gable end vent. The attic used to be open to the cathedral ceiling, but now there isn’t airflow in that direction. Still, the attic is still open through the bedroom ceilings. If we finish the ceilings with sheetrock, though, it seems to me we will need more attic ventilation. There is no ridge vent or soffit vents, and running a fan in the end gable would be an option only when the house is occupied (power is turned off during the off-season). Would we be better off not to insulate? (We do not use air conditioning, and don’t heat during the snowy season. We do use a wood stove on cool early spring and late fall days.) If we do insulate, how do we avoid creating a moisture problem?

    • Challenging! First, condensation is almost always an issue in winter when the roof gets cold and there’s moisture sources in the house. With the construction that you describe, the moisture from the house would almost certainly get up past the paneling and into the cathedral ceiling structure. If you used the house during the winter, I would strongly recommend using sheetrock instead of the thin paneling. You could still have problems if moisture from the ground can get into the living space. I would monitor the ceiling for signs of condensation -drip stains and such.
      In the attic space, since the bedroom ceilings are leaky, you’re probably best off insulating under the roof, like the cathedral ceiling. But you want to make sure the “ceiling” under the insulation is air tight, blocking moisture from getting into that insulation and condensation from forming on the roof underside. Since you’re in the attic, you might want to use foil faced foam board as the attic ceiling material. That is light, easy to work with and, when the seams are taped, forms a very good vapor barrier. But, you have to check with your building code official to see if they allow this without covering it with drywall. Some places allow it, others don’t.
      If you do this, you would,of course, close the gable vent.

      • Thanks for the reply. I’m hoping I can get a few clarifications. First, to clarify on my end, the house doesn’t have a basement–it’s built up off the ground on supports. I’m sure the older windows (1970s) aren’t entirely airtight, though. Also, we want to finish the bedroom ceilings with drywall–I’m not sure if that makes a difference in your suggestions. Regarding your suggestion of foil faced foam board as the attic ceiling material: is this a type of insulation used instead of something like fiberglass which goes between the roof rafters? Or is it something which is used in addition? We do use the attic for storage (a necessity with a small house), so we do not want to lose much of the already-limited low clearance in the attic. Also, can you explain why we would close the gable vent? And would we still close the gable vent If we used a fiberglass insulation? Thank you.

      • The more finished the living space, the better, as it will reduce the air and moisture flow from that area up to the attic. The faced insulation board can be in addition to fiberglass between the rafters. This will bring the attic into what’s called “the conditioned space” of the house. Just like the cathedral ceiling in the main section. That’s why you close the gable vent – leaving it open would be like leaving a window open in the house.
        I will warn you about unintended consequences. The house,as it is, sounds pretty leaky. This actually keeps the humidity way down in winter, because cold winter air is very dry. As you tighten the house, you give it it’s own environment, and can trap in moisture. I’m not saying that you will have problems, just that it’s possible.
        It’s good that the house is off the ground. As long as the floor is tight, you shouldn’t have much moisture building up inside.

  17. We have a home that was built in 1929. The original roof was clay tile. That was removed some time in the 70s (according to a neighbor) and a cedar shake roof was put on. We bought the house 1 year ago and knew we would be replacing the roof (my insurance company insisted). We liked the idea of going back to the original Spanish style tile, but the price was very prohibitive. So, we had a metal product put on that looks like tile. It’s been great but now that winter has rolled around I am having some ice damming issues on the north side of the house. Our attic is under insulated. It has some loose insulation on the floor of the attic, just up to the top of the joists and it looks pretty compacted. I have gotten two quotes one for adding loose insulation and one for spray-in closed foam. Our current set up has no ventilation. No eve/ridge vents or attic dormer vents. So, the company that wants to add to the current insulation wants me to make sure I have ride/eve venting added first and wouldn’t recommend the spray-in foam due to risk of condensation building up and possibly causing water damage and mold in the attic and plaster ceilings. The company that wants to do the spray-in foam said I would not have to add ventilation and that condensation would not be an issue if I go that route. Being close to a 90 year old home I am assuming my issues are due to the way the new roof acts with the current insulation and climate. We live in Southwest Michigan (climate zone 5). I would lean towards the spray-in closed foam, but I worry about condensation and mold. What’s your advice?

    • I have a house in Edmonton Alberta, and I had an ice dam. The solution is to keep the attic cold, and keep the house warm. So cut intake holes in your soffit (use soffit baffles so the insulation doens’t hinder the intake), add ridge vents to the top to allow hot air to escape. Then completely seal the attic floor with caulking etc so no hot air can flow into the attic, then insulate with blown in. Closed foam both seals and insulates, but is more costly.

      This will stop the conditions that cause ice dams, which is caused by your hot attic melting roof snow, which then re-freezes on the cold roof overhang causing ice buildup.

  18. Thanks for good article, I want it to do in my house in Lithuania, that house is 2000 year’s build. And have attic, but it’s full of holes and in the winter cold going in. I have a question what about summer, does hot air comming through holes in the roof too?

    • Especially in the Summer with the hot sun attics will get very warm. But that is okay if the attic is insulated on the floor because that will keep most of the Heat out of the house.

  19. I am finishing my attic, a 1940’s home that use to have the attic finished. Anyway, the unfinished portions of the attic were vented by gable vents, I am planning on switching to eave and ridge vents everywhere. My question is, should I leave an airspace between the rafter insulation and the sheathing above the finished area, to allow venting above the finished part of the attic?

    • Yep, you always want to maintain an airspace between the insulation and sheathing. I’ve seen some really ugly situations (mold, roof rot) that arose because the insulation didn’t have that air space and the water was trapped in the insulation and stayed in contact with the sheathing.
      I’ve also seen problems even when there was an airspace if the humid air from inside the house got into the space and condensed on the sheathing, even with good ventilation in there. This happened when there were recessed lights that let lots of humid air in there. I strongly advise against recessed lights for exactly this reason.

  20. Boak and Sons installed cellulose without baffling into our attic. They also covered the soffits with the insulation,suffocating our attic n causing a mold problem. We had to have all insulation removed and replaced the roof n a lot of the sheathing to get rid of the worst of the mold. Hired a mold professional to treat remaining mold. I’m afraid to use the blown in insulation incase we have any future problems. We had to pay $1200 just to remove the 3y.o. blown in insulation. What kind of insulation to do recommend?

  21. Our state is requiring us to insulate our new basement… even with a wood stove there as our primary source of heat. We have a plastic barrier on the exterior areas that are ‘underground’, and would prefer to not insulate the interior; but if we have to, then use something like P2000, to cut down on condensation problems, etc. I’m concerned the air will become much too dry. I would like to know what the ‘science of insulation’ is in that situation. 🙂

    • Great question!
      The accepted “best practice” for insulating basement foundation walls is to install a layer of XPS rigid foam board (the pink or blue foam you can buy at big-box stores). Here is an excellent article on this topic. That article also contains other references which go into this topic in great detail.

      P2000 appears to be a low density styrofoam board covered with an aluminum barrier. Like many products with a shiny aluminum surface, they seem to inflate the R-value claims due to the radiant barrier ability of the aluminum. However, this diminishes over time as it gets dirty and is also ineffective if anything is placed in direct contact with the aluminum surface.

      I haven’t used it but would personally stick to XPS foam board which is readily available at big-box stores. Or Poly-iso board with an aluminum backer, also readily available and which provides excellent R-value per inch.

      Be sure to check with your local building official about codes. Most locales require covering foam board with sheetrock due to the fire/smoke hazard.

      Wintertime dry air is due to a leaky house that lets cold, very dry air in from the outside during the winter. You’re best off reducing the amount of uncontrolled moisture coming in through the basement walls and dealing with the air humidity in a controlled manner.

  22. Thank you for the advice that finally makes sense. I’ve always asked why even new houses have “open” attics, and got the deadeyed stare from realtors, and even inspectors. It just screams “mouse and squirrel nest” to me. I have the answer for the silly “I’ve been doing this for 30 years” comment. First, I ask what facts they can prove in what they say. Then I say thoughtfully, “Well, give you 3-5 years to get familiar with the work. So that leaves your SOP behind by about 25 years, then?” I don’t worry about being rude at that point, because anyone who tries to bully me like that isn’t getting a dime from me. The issue I’m having is with an intractable insect and mouse pest problem that is self perpetuating. The reason is the attic is “open” and the insulation up there is some kind of fluffy stuff (gray-white), that I hope isn’t asbestos. The house was built in 1973. I like the spray-foam idea for which insulation to replace it with, but first, that space needs to be sealed and de-humidified. Thanks for pointing me in the direction of a building analyst. Much appreciated advice.

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