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.


189 thoughts on “How NOT to insulate your house

  1. Please help me go in the right direction,
    We have a single floor Ranch west of Boston MA. 25×40 with a bedroom out the back.
    We just had a Heat Pump for the whole house installed to aid in summer cooling (and some additional heat that is not from oil).
    The house is 1951 Plaster Walls with forced hot water to convectors.
    Ok, so now the question.
    Mass Save will give us additional attic insulation. Currently there is 2″ to 7″ of fiberglass insulation in the attic. My problem is 2 fold. We use the attic for storage and the celling joists are 6″. In addition the house has no soffits, but 3 gable end vents and 1 attic fan.
    My plan (in process) is to add 6″ joists above the existing celling joists. Remove the fiberglass and seal ALL air gaps and allow Mass Save to fill the 11 1/2 ” pocket with celluose insulation.
    My question is WHAT do I do out at the band board where the interior room, attic and outside come near each other? I have had Ice Dams (20 yrs ago) but I’m vigilant with a snow rake! I have been cautioned to not use foam there. I want to increase the R value in that area. What is the best way and or best practice?

    • That’s a challenging area because access is so tight. It is hard to check for air leaks and there’s not much room for insulation.
      Even if you don’t have soffit vents, you can still use the ventilation chutes under the roof deck to keep the insulation from pressing up against the roof. Then you can insulate from the attic floor up.
      For more R-value to guard against ice dams, you could use poly-iso sheet foam under the chutes in the rafter bays. If you do that, you don’t need the chutes. Instead you can creat a gap by using 1×2” nailers at the edge of the rafters, then fit the board foam under that. A 2” sheet cut to fit and wedged in right up against the nailers will serve the Same purpose in keeping an air space and added insulation to reduce the risk of ice dams. You can run the insulation up 4’ and leave the rest open from there to the peak.
      The other thing you’ll want to do is really air seal and insulate the attic access too reduce energy loss into the attic. If you have the standard pull down ladder, there’s a great, simple foam cover system that I used in a couple projects. Or just make yourself one from foam board. The key thing is it should be heavy enough to seal tightly.
      Do those things and your project with the deeper joists and blow in cellulose and you should be good to go.

  2. I am working on improving the air sealing and ventalation in a cap cod style home. I found many things wrong but two main issues.

    First, there are large vacant spaces between the top of a closet and the rafters. Only vapor barrier at the rafter level i.e. in the attic, you can see through the plastic in to the large opening below. Should I fill this large area or use rigid insulation to cover the opening in the attic or both? or ?

    Secondly, they filled the slanted portion of the roof with fiberglass batons and did not leave an air space to the soffits. So my thinking is to remove the fiberglass batons (with some difficulty as the space is about 9 ft deep but with 10″ rafters. Then build a “box” with two 2x4sgoing the length of the opening with 2″inch rigid insulation R10 closing the opening i.e. attached to the 2x4s and a 2×8 to cover the end down at the soffit. This would create a 3 1/2 inch air flow. I won’t be able to seal the 2×8 or the 2x4s as it just isn’t that accessable. Then I thought the rest of the space could be filled with densely packed cellulose blown in insulation. does it matter that the home made baffles can’t be sealed? Should I consider putting weather stripping on the wood prior to sliding it into the space?

    The areas from the top of the dormers to the attic rafters does not have an air space so I was wondering if using densely packed cellulose would work. this is about 2′ deep.

    This is a bit complicated to explain so I hope you understand.

    • I’ve seen those areas above showers like this before. It’s strange construction that often leads to very cold showers in the winter! It can also lead to frozen pipes. I’d go the rigid insulation route to create a “ceiling” for the area at the rafter area, if that’s possible. You want it as air-tight as possible so the cold winter air can’t get past the insulation and into the large opening below, so details are critical to get right.
      For the second, that sounds like a good approach, though I’d go with 1×2’s or 1×3’s. Easier to manage and you don’t need a huge gap below the roof, 2″ is fine. I hadn’t thought of weather stripping before. That sounds like a good idea to minimize air flow between the attic space and the new gap. It’s probably overkill, but I wouldn’t discourage you from doing it if you want a little tighter construction.

      You’re probably good with dense pack above the dormer. The only moisture getting in there would be from the attic itself or through the sheetrock, so pretty minimal. Though I’m hesitant about suggesting something in contact with the roof deck that could hold in moisture. If there’s anything you could slip in there so the insulation isn’t right against the roof? That would be best to do before putting in the cellulose.

  3. It is a pleasure worth reading this article, as it provides us information regarding how to insulate your house.Such illustrations are given that a layman could even understand. It is an easy to comprehend blog which can be instigated by all at least once. I am sure many people will come to read this in future. You must also check out it has some great insights too.

  4. All the best things on how not to insulate your house and its use are listed here with a brief analyzed discussion and data on them. Such articles are not only knowledge enhancers but also very interesting to read and to learn to compare from. Well, I have visited another site having some wonderful and similar information.

  5. I need help!! We have a 60-year-old brick ranch with a new roof installed 4 years ago. For several years, maybe 17 or so, we have developed mold in the outside corners of the bedrooms. Also, have a dripping problem from can lights during the winter. We live in NE Ohio, so it’s cold in the winter here.
    Now we have begun to get insects that seem to come into the living space when the ceiling lights are on. I have a contract with a reputable company to spray the house 4 times a year for insects. I was told by them that the type of insect was called a plaster beetle that feeds on wet plaster. Upon inspection of the attic insulation, we noticed that the underside was indeed wet.
    We have a ridge vent that runs the length of the house and also installed baffles in the attic for more airflow. I know we will need to remove the old insulation which was only R-19 maybe, and replace it. We are thinking about doing the TAP Insulation as it is an insect inhibitor. My question is why is the moisture there to start with? Could it be due to under-insulating? If we do an R-49 amount, will this stop the moisture buildup? I am beside myself dealing with this mold issue and insect problem. Can you please advise me on solving this problem?

  6. Pingback: Spray Foam Attic Insulation Problems: How To Prevent Them

  7. What is the recommended practice for insulating a doorway out of a basement (unfinished, but used space that is not actively heated/cooled)? The ‘doorway’ is not a bulkhead, but a “doghouse” and is on the north-facing side of the house (in NH). While the walls on either side of the stairs leading from the basement to the outside are concrete foundation, once at the top step everything is framed. I’ll assume the 3 walls could be fiberglass batts with facing or other vapor barrier. Is R21 sufficient for this? There is sloped roof above the structure. I haven’t pulled the existing insulation off yet, but from the outside it looks like there is perforated soffit vents, but no ridge vent (and no vent on the gable). Ideally, I’d like to insulate under the roof, rather than at the “ceiling” of the doghouse to open it up a little (but I’m not married to this idea, if horizontal insulation is a better way than insulating between the roof joists). What R-value would be recommended? Would there be a vapor barrier on either ceiling method? This is a very small an accessible space. If I know the practice, I’m sure I can do this myself.

    • I’d probably avoid fiberglass as it’s so moisture permeable, even with a vapor barrier. The most effective thing might be thick foam board, either XPS (the pink of blue board) or foil-faced poly-iso board foam (best R-value per inch). Tape the seams. You could mount that right to the rafters, rather than going in-between, reducing the number of seams. You could also affix it to the concrete foundation along the stairs, as the concrete is a really good conductor of cold.
      The foil-faced foam might also pass fire code, though you always want to check local building codes. Often, any foam in the home has to be covered with a fire barrier, like drywall or special coating. If you don’t follow code, it can raise red-flags when you go to sell your home, so I always recommend people go “by the book.”

      • Thank you. This is fantastic information. Now I have a slew of new questions! What R value would be recommended for the ceiling and concrete wall applications? Would I just use construction adhesive for the foundation walls or is there another method? And what about the tape? Can a layer or two of non-faced polyiso be combined with a layer of faced, if that is more cost-effective? This is invoking some real scope creep. The stairs go wall-to wall, so now I’m wondering if there is anything under them re: insulation (which I seriously doubt). I’m going to have to pull off one of the treads to take a look.

      • A couple inches of poly-iso board foam is good for ~R-12, so that’s reducing heat flow by about 90%. That’s a good baseline. The walls are closer to ground temperature, but the closer to the surface you go, the closer to outdoor temperature they get. But generally, in this application, no need to go overboard. The ceiling could use more but really, for an unheated basement, blocking air is going to be the #1 priority. If you can insulate everything with some insulation, the air leakage in basements will probably negate any additional benefits. Behind the stairs is tricky because of access. You could pull treads and put in some Roxul or fiberglass, or just glue foam-board to the back of the risers and bottom of the treads. Or you could do nothing and see how it goes over time. Construction adhesive could work. I used that and masonry fasteners specifically meant for attaching foam board to masonry. But if it’s solid concrete as opposed to cinder block, that would be a bear to drill into.
        As for multiple layers, you could do faced and unfaced if you wanted, but I doubt there would be any savings that would make it worthwhile layering.
        Finally – if you have a furnace or other combustion device in the basement, you need to have combustion air available. Modern systems are direct air intake and direct vent, but older systems are natural draft and they draw air from the room. Make it tight and you can have problems with backdrafting, improper combustion and a slew of problems, especially if, for example, there’s a clothes dryer down there too. When you run the dryer, it can actually suck so much air from the room that it pulls exhaust from the furnace back into the house, leading to carbon monoxide poisoning! So if you have combustion appliances down there, you really have to know what you’re doing! Hopefully that’s not an issue but it would be negligent if I didn’t mention that in this discussion.

      • T.D,
        Again, fantastic information. It occurred to me that you are rightfully answering what I’m asking and I’m not fully asking what I’m thinking! If I were to finish the basement, would the above advice change (ie would you not only target air leakage, but also r-value?)? Is there anything in the advice that would have to be ripped out if I do it now, but finish the basement in a few years?

        Just to answer your question, the house was built in 2000. It has a propane furnace and hot water heater in the basement. Both were manufactured in 1999 and both are going to be replaced (also with propane units) this spring with higher efficiency units.

        While not finished, the space is actively used – storage (dry goods and food pantry), workout area, utilities and my home automation center (communications rack, media server, etc). It is mostly dry – on the north wall, which we are talking about, I had some water damage due to a faulty door installation in the space above that wall (50″ concrete wall with studding above to the floor joists). That was mitigated last week with a new higher quality door that was properly installed/sealed. Sheathing that needed to be replaced was. Other wood surfaces were treated to kill mold. All of the insulation on that wall was replaced). On the south-facing side of the house (full poured concrete foundation), there is a small amount of water finding its way in around the septic pipe’s core, but only when there is very heavy rain. I’ll deal with that in the spring when I can dig out the area; reseal/regrade. That is a very small leak and it’s new* as the water doesn’t even make it to the floor, but I’m not going to ignore it. (*I removed some shrubs this past summer, including the root balls, and probably didn’t fill the hole in sufficiently and regrade properly).

        While my initial question was about the specific “doghouse” space, I’m looking at the space as though I, or some future owner could finish.

        I know this is a lot. I debated whether to add this much to the narrative, but I came to the conclusion that the “now” and the “future” aren’t completely disconnected, so this may all help someone else who has plans for a space, but needs to deal with the immediate need too.

      • Thanks for the full description!
        Based on that, I’d propose a simpler, different solution to the stairway entrance – build a door-frame at the bottom of the stairs and insulate that rather than trying to insulate the complex walk-up space and dog-house area. You’d install an insulated door and small wall that’s attached to the foundation and sealed. This would be much simpler and prone to fewer problems than the retrofit we were discussing.
        Later, as you finish out the rest of the basement, this would be tied into wall framing around the basement walls. Which, by the way, I’d insulate the same way – with board foam attached to the walls. You’d then frame inside the foam, which is vastly less likely to suffer water issues. For this application, XPS foam is recommended over poly-iso, as it is more water resistant/tolerant.
        Hope that helps 🙂

  8. Hi Ted,
    I have an issue with what I thought was considered a Flat Roof, but I’m told is a Low Slope Roof. The roof has a 3″ pitch and is 12′ long from attic to soffits. I have 6″ rafters. My bldg. inspector told me to put in soffit vents and drill some holes into the attic for ventilation and then leave a 2″ space between roof sheathing and insulation. I believe this constitutes as a “vented” Low Slope Roof assembly. Now, we had a humid summer here(northern NJ) a couple months ago and quite a bit of mildew formed on the outside of my soffit. Mind you the soffit is out in the open and receives air/wind flow. So with that being said, if mildew would form on the outside with plenty of ventilation, what would prevent mildew forming inside my rafter cavity in a 2′ space with tiny soffit vents leading to tiny attic vents? It just seems to make more sense if I go ahead with a “unvented” approach and not let any air in. Thoughts?

    • The general approach the inspector suggests is sound, but your logic is as well. I experienced similar weather at my house in eastern PA, but didn’t get mold. However, the humid air tended to condense on everything metal in the garage, which stayed cool during the warm-humid stretches. So instead of mold, I got lots of rust. But similar concept – some part of the house with thermal mass cools off at night, and during cooler weather. Then when we have a humid spell, the condensation accumulates, leading to problems. No amount of ventilation helps because the ventilation air is super-humid and just causes more problems.
      The thing that could save the ventilated roof is that when the sun shines on it, it heats up and drives off the moisture. But, like you said, if you just have little soffit and attic vents, the amount of air flowing through is near zero. You really would want continuous vents with a high NFA (net free area). The Cor-a-vent products work great for this.
      From other messages, it sounded like you want to avoid using either high-density spray foam in an unventilated roof assembly, though that would almost totally eliminate the possibility of mold in that space. I say “almost” because there’s still a chance (rare) of roof leak, where the water gets trapped under the roof and above the foam, but this is very rare.
      The other thing that you’d want to do is ensure there are no ‘holes’ in the ceiling that would allow humid air from inside the house to get into the roof cavity. If you had foam, this wouldn’t be an issue, but if you had baffles and normal insulation, moisture that got up there would likely cause problems. Recessed lights would be a big no-no.
      Instead of spray foam, you could use thick sheet foam,(2″) like foil-faced poly-iso, spaced 1.5″ to 2″ below the roof sheathing. Use this to form the air channel and insulate. Then, you could either use more sheet foam in multiple layers or some dense fiberglass or other insulation, to fully fill the cavity between the foam board and the ceiling. And as we discussed previously, you could extend your rafters to give more insulation room before sheetrocking.

      • Thank you for the quick response! I just can’t win with this room. I’m only having a ceiling fan and a central air vent in the ceiling, so shouldn’t be too difficult to air seal them. I understand your idea of rigid foam in the rafters. Is that what they refer to as “cut and cobble”? What’s your opinion on leaving the rafter space completely empty and putting the rigid foam on the face of the rafters, let’s say 6″ taped and staggered seams and then drywall on top of that in a unvented situation? I’m thinking that would keep any air entering the rafters and therefore eliminate risk of condensation? And my gut tells me to stick with unvented as opposed to vented.

      • You could do that. Though with that much insulation under the beams it would be really hard to attach to drywall. Or just stick about 4 in of batt insulation into those cavities which will leave airspace above just in case. Then you board foam underneath the beams, in multiple layers like you said to keep the moisture out and everything sealed up tight.

      • Oh well that makes sense I didn’t even think about how I would screw the drywall in with that much 6″ of foam. My mind is fried right now trying to figure out what to do here. Thank you for your help

      • Oh well that makes sense I didn’t even think about how I would screw the drywall in with that much 6″ of foam. My mind is fried right now trying to figure out what to do here. Thank you for your help

  9. You can certainly insulate that way, in strips. You wouldn’t be blocking moisture then, but painted drywall lets moisture through very slowly anyway, so not a big deal.
    With regards to the baffles and condensation – having a little moisture form on the foam baffles isn’t a big deal since it and the insulation aren’t adversely affected by the water. It should diffuse out as vapor as cool, dry air flushes through the channel. The baffles aren’t airtight, so the moisture trapped on the insulation side will find its way out. The baffle is really to simply prevent the insulation from being in contact with the roof and allow air to flow between soffit and ridge.
    Hope that helps.

  10. Thank you so much Ted you’ve been very helpful. I’ve seemed to have a turn of events here. I called the bldg. inspector and he said in my area(NJ, Zone 5) the code for R-value here is R-38. He suggested that I put vents in my soffit, even though I’m not able to vent into the attic. He then said put baffles against the roof sheathing and then R-19 in the ceiling because of my 2×6’s. If his idea of just being able to vent thru the soffit and not the attic is plausible then I might just increase the rafters a couple inches and put 2 layers of R-15 ROXUL for a R-30 value. Opinion?

    • That approach is good too. Full vents from soffit up to ridge with insulation under that works well. Rather than increasing the rafters, you might consider a 2″ to 3″ layer of foil-faced polyiso board foam (R6+ per inch) attached to the rafters, and the R-15 Roxul inside the cavities. That gives you a near perfect moisture barrier. And, since you’re insulating the rafters, the overall R-value is enhanced because it greatly reduces thermal bridging of the rafters themselves. Without that, the rafters will be cold compared to the surrounding insulated area and act as a “moisture magnet”.

  11. I’ve become so frustrated on trying to find the right answer on to how to insulate a flat roof of a 3-season room being converted into a full time heated/ac room. It’s 12′ x 16′, 8′ high ceilings and the rafters are 2 x 6’s. It’s unvented and can’t be vented. There isn’t any insulation on top of the roof sheathing, just black rolled asphalt, a roofing paper type layer, and the sheathing. The roof is only 4 years old and isn’t an option to have it redone……I’ve been told the only way to go is cc spray foam on the underside of the sheathing. I’m hoping I can go a different route. I was thinking could I increase the depth of the rafters to 7 inches and install 2 layers of R-15 ROXUL for a total of R-30 in the ceiling? The drywall would be an air barrier and the primer/paint would be a vapor barrier. In your opinion would this create condensation issues?

    • I think you’re on the right track. There are probably millions of flat roofs, unvented and insulated that work fine. As long as you have a very good air/moisture barrier between the living space and the roof, you should be able to do whatever you want inside the space.
      The one bit of advice is to leave a little air barrier between the top of the insulation and the bottom of the roof sheathing, just in case you get some condensation, it will have a chance to burn off better from sun on the roof if you don’t firmly pack insulation against the sheathing. That can trap water against the sheathing which is death to the roof.
      If I were in your shoes, I would put in one layer of the Roxul, then under that (across the rafters), two layers of poly-iso, foil-faced board foam. Tape the seams carefully with foil tape and stagger the boards so the seams from the top layer are halfway between the seams of the bottom layer, so even if one seam isn’t perfectly sealed, moisture will have a long way to go to get into the space. The foil is a near perfect moisture barrier too.
      You can then either screw your drywall through the poly-iso foam to the rafters (which can be tricky to find with the thick layers of board foam) or, screw nailers firmly to the rafters and then screw the drywall to the nailers. This has two advantages – it’s easy to screw to the nailers since you can see the wood you’re screwing into and it leaves a gap between the board foam and the drywall, allowing you to run wires above the drywall for flush-mount light fixtures in the ceiling. If you run the wiring before you drywall, you’re good to go. Just don’t cut through the board foam or otherwise compromise the integrity of the air/moisture barrier.

      • Thank you for the quick response. When you say a “layer of poly-iso”, how many inches is a layer? I’ve been getting so many opinions on this and they’re all inconsistent with each other. Your idea sounds right to me. I’m not a builder, etc, I’m a painter, so I’m trying to understand all of this. Most “experts” are telling me the only 2 options are to rip the roof off and insulate above the sheathing(I’m absolutely not even considering this) or closed cell spray foam on the underside of the sheathing(I’d rather not go this route). I believe the concept behind your idea is to stop any warm air penetrating thru the drywall by using the rigid foam. This way the sheathing will stay cool in the winter and not condense because of the lack of warm air in the cavity?

      • Starting at the end of your comment – yes, the goal is to eliminate the ability of moisture/warm air from inside the house getting up into that cavity, or at least reduce it to such an extent that it can dry out naturally without doing any harm.
        As for the amount, in this application, every bit is increasing the existing insulation, helping keep the room below cooler in the summer. In the winter, it’s insulating from the cold, but mostly being used to stop moisture, so it’s less about R-value and more about stopping moisture and the foil face is what really stops the moisture, so the thickness doesn’t matter much. Here’s a thought, based on another discussion I had – Use a thin layer, maybe 1/2″. Mark the location of the rafters, and apply nailers over that so you can have something obvious to attach the inner layer to. Apply a second layer, of foil-faced foam board, the same thickness as the nailers but in-between the nailers (or thicker, if you want to leave a gap for running wires). Finally, you can add your inner drywall, attaching it to the nailers.
        With regards to the suggested solutions others have made, they’re fine. If I were doing a rehab from scratch, I’d probably do the spray-foam to the underside of the roof. That would be easy to do if the inner ceiling is gone. The downside is that you couldn’t do that yourself and it might be more expensive. I’m not sure of the going rate for foam, but for a roof application like this, I’d want at least 4″, maybe 6″, and that much high-density foam can get expensive.

      • Ted,
        I’m a little confused when using baffles to create ventilation between the roof sheathing and batt insulation. If condensation or moisture occurs, what’s keeping the condensation from forming on the outside of the baffles(the side the batt insulation touches) instead of the roof sheathing? I would think both the baffle and the roof sheathing would be of equal cold temperature….And in a totally unrelated question: In reference to my ceiling, would it be an effective remedy to thermal bridging to cut rigid foam in 2″ wide strips and sandwich them between the face of the rafter and a 2″ wide furring strip? I’d like to increase the depth of the rafters, but trying to avoid laying whole sheets of rigid foam across the entire ceiling.

  12. Bought a 1918 house that had an attic full of mouse droppings in the insulation. Had to get rid of all the insulation. Attic also has no vents. What should we do with it now?

    • while the insulation is out is a perfect time to check to see how well are sealed the floor of the attic is. so I would first seal up the attic floor wherever it doesn’t look to be airtight to the space below. as for attic venting, if you haven’t had any problem in a century I wouldn’t mess with it. I’ve seen people introduce moisture problems by adding ventilation to houses that have worked fine for decades.

  13. I just bought an attic fan to blow heat out of the space, but I saw a home with blown in foam, so now I need to see about doing that as I think it will save my air units in the attic and bring down my air conditioning bill as it would run less. I need to know if I can leave the existing blown in insulation laying on the rafters above the rooms. I was told that could lead to moisture and mold but don’t see how that can happen as I barely run my heater unit and it has a heat pump which I thought would remove some moisture in the air. Also what would be the cost on doing @ 2000 Sq Ft of foam?

    • Hi Dave, leaving the insulation in the attic would create a cool, enclosed space with minimal air exchange. The moisture can settle in there leading to mold problems. Here’s why that can occur.
      – Condensation occurs when air (all air contains moisture) cools to below its dew point. Air inside your house rarely causes a problem because most people keep their houses at a temperature between 70F-75F and humidity at maybe 40%-60%. The dew point for air with this amount of moisture is at a temperature of around 50F-55F so there’s no chance of condensation.
      – When the air from inside your home goes into the attic that is insulated that is insulated on the bottom and top, the attic space can very easily be at a temperature below 55F. In the dead of winter, it would likely be colder. So any air that gets up there is going to condense.
      – If that insulation were removed from the floor, now the heat of the house can more easily help keep the attic space above the dew point. It’s still not guaranteed to, but it’s much less likely to get really cold in there. Therefore the risk of condensation is vastly reduced.
      – Even better would be if the heater would be allowed to blow a little bit of the warm air into the attic during the winter so as to keep the temperature up a bit and also circulate some air so it’s not stagnant in the attic. Think of the attic, when insulated just blow the roof, as becoming part of the living space of the house.
      This is admittedly less of an issue in mild climates where winters don’t get cold. But why risk it? Best to remove the blown in insulation.

      I should note, regarding your comment on heat pumps, they do not remove moisture from the air. They only heat it. The reason why heat pumps have a reputation for leading to dry air is because most ducts are leaky. When you pull in cold air from outside during the winter, that air has very little moisture, leading to drier air inside the house. This also makes air conditioners work much harder during the summer because the leaky ducts pull in hot, humid air.
      So the idea of foaming under the roof when you have units in the attic is a big win. Not only are you providing a much more temperate climate for your system, but you seal out the outdoor moisture/dry air, allowing your entire home to be more comfortable and energy efficient.

  14. My house was built in 1802 with ballon framing. I used close cell spray foam and have a metal roof.
    the humidity in the house on the top floor is never below 75 so we put in a dehumidifier without any changes. Not sure what to do? The walls sweet on very hot days. Any suggestions.

    • The thing you have to determine is where the humidity is coming from. Often, old houses have dirt floor crawlspaces or basements under the house that let tons of moisture come up from the ground. That rises up through the house (humid air rises) so upper floors get the most humidity.
      If you have a high water table around your house, water vapor can also seep in through the foundation walls.
      That’s the starting point. Figure out where the moisture is coming from.

  15. We recently bought a 30 yo pier and beam house with no insulation. Siding is attached directly to studs and there is some roofing felt as a weather barrier between the two. There is no sheathing. It’s a very leaky house and we are wanting to insulate it so it can be used year round more comfortably. Gable vents in the attic. No heating or a/c, but plan to install a multi zone mini split heat pump. It’s in northeast Texas where temperatures may range from 10-100. What are your thoughts on using closed cell foam for this house? Thanks!

    • Foam insulation could certainly improve the situation a lot. Are you planning on gutting it? Ripping off the siding then spraying foam onto the back of the interior walls then adding sheathing? or something else?
      Just keep in mind that the entire dynamics of the house will change if you seal it up tightly. That’s an entire article if you go down that road.

      • Thanks. It’s pretty well gutted at this point. I assume I would have to seal off the gable vents. would I have to spray under the floors?

        I was thinking about spraying directly onto the weather barrier, so hopefully if and when the siding is replaced in the future, the insulation is maintained. Is that a problem? Do I need sheathing to use spray foam or is that plan ok?

        I guess I need to read up on venting if I were to seal it up tight.

        Thanks again!


    • Most likely, spray foam. You want something highly moisture blocking since any water vapor will get trapped by the metal walls and condensation will be a big risk.
      I would suggest reading up on existing homes made this way and see what others say. It’s such a different building method that there may be other concerns that I have not experienced before.

  17. live in a basic construction, “cookie cutter”, home in Nampa, Id. It was built in 2001. If you aren’t familiar with the climate, it’s cold and dry in the winter, and hot and dry in the summers. Humidity isn’t something I ever thought we would have a problem with in a desert climate. But, we have one, or so it seems. Four years ago we replaced all our windows. But, we now have mold build up on several of them. It’s not a minor mold problem either. I’ve cleaned it up several times and still it grows. I have also noticed condensation on the inside of the windows. Other issues we are experiencing are, mold in our bathroom. (No window, but we do have a vent fan that is original to the home) The exterior walls in our bedroom are freezing in the winter and warm in the summer, with occasional dampness. Our master bedroom is always a completly different temperature than the rest of the home. There is also a noticably cold section in the floor of our bay window. Our master closet also exhibits temperature fluctuations significantly different than the rest of the home. It has one small exterior wall and a large interior wall it shares with the garage. On of the other rooms, that also has mold issues in the window, is always hotter than the rest of the home, and I think there is always a “scent” inside. My husband differs in opinion with that. We have no idea where to start. Is it possibly an insulation problem? It’s incredibly frustrating, and I feel like it doesn’t matter what I do the mold is always there.

    • Given that you live in a “dry” climate, the moisture causing the mold has to be coming from inside your home. Do you use humidifiers? If you do, then you need to keep the humidity lower, maybe much lower, in your home. During the winter, if you have a humidity of more than around 40%, you’re going to have moisture problems on colder things like windows and exterior doors.
      If you’re not using humidifiers, then you need to be diligent about using the bath fan during and AFTER taking showers. Bath fans need to be run for about 1/2 hour after taking a shower in order to flush out the moisture, assuming the fan is working properly. You can test the bath fan by holding a sheet of paper up to the fan. It should suck strongly onto the fan. If it doesn’t, you need to get it working and venting properly.
      As for the cold walls, they clearly aren’t properly insulated. If you can find someone who does blown in insulation, they can inject insulation right into those walls. That can help a lot if done properly.

      So, for mold, the solution is to get rid of the extra humidity in your home. Make sure the dryer is venting properly. Make sure the bath fan is venting properly. If you have a gas heater, make sure it’s venting properly. Get yourself a cheap (<$30) humidity monitor and keep the humidity lower until most of the condensation issues go away. This might be 40%. It might be 30%. It might even be lower.
      here’s a humidity meter that would work:

      If you keep the bath fan running all the time, it will pull in more fresh, dry air from outside which will help keep humidity levels down. With that, and the humidity meter, you can figure out how much you need to run it to avoid condensation. Plus, the added blown in insulation for the cold walls and you should be much better.

  18. I renovated a 1957 Cape-style hose in 2009, doubling its size into a Center Hall Colonial. The entire second floor is new construction as is the hip roof. The stand-up attic space is fully vented on all four sides by soffits and a ridge vent runs along the full length of the peak. I was present during the construction to check that insulation was being done according to specifications. Hollow box corners were created and filled with foam, insulated lighting cans used, caulk and foam were used to seal gaps as well as along sill, floor and top plates. Gaps between sheetrock and bath ventilation fans were also caulked. Finally an energy audit company was used to do a blower door test and examine the entire house from basement to attic to address any deficiencies.

    The attic has R-30 fiberglass insulation covered with a full plywood floor. A second layer of R-30 batts were laid perpendicularly across the plywood flooring and all soffit vents were kept clear of the additional batts. The pull-down stairs hatch was covered on its top side with foil-backed foam insulation and the attic stair opening has a hatch covering of dense foam board with high R factor.

    In early January this region experienced a prolonged, bitterly cold period of temperatures in the single digits before wind chill factor. The cold snap broke with two days above freezing- the second day reaching 61°F with high humidity and fog from the melting snow. This was followed by a very rapid deep freeze. A day later was sunny that began warming the east, south, and west exposed sides of the roof. I happened to go up to the attic to notice the entire roof sheathing of the north side from soffit to ridge was very wet. There was also evidence water had condensed on roof nails and dripped. I used a large fan and a dehumidifier to dry the sheathing over two days.

    The house is single occupant. Short showers and dual ventilation fans are used in a large Master Bath. Heating is hydronic radiant floor heat. Humidifiers are attached to the air handlers on each level and humidity levels kept around 25% when the temp is 20°F or lower and between 30-35% when above. This is to prevent Maple harwood floors from drying out as the radiant heat lines are beneath the flooring. Humidified air by evaporative pad method comes out the AC vents at ceiling level on each floor. The air tends to be slightly cooled going through R-8 insulated flex ducts. I have been monitoring the attic humidity and it has remained low during the current cold weather following the last thaw creating the problem. The sheathing has now remained dry along with the weather.

    If I condition the attic space by attaching sheets of 8″ foil-faced foam board to the common rafters creating vented spaces from soffits to ridge vent, is there a high probability of outside humid air condensing on the underside of the roof sheathing in similar situations of a thaw, followed by a flash freeze followed by a thaw? These situations have been rare but I suspect the water vapor condensed in the past without being observed. I’m concerned about the roof decking eventually rotting from condensation in the colder months. Hence the idea of conditioning the attic space to mitigate further condensation. Is this viable or are there other options to consider?

    • So, first I’ll say, congratulations on working hard to do everything right with the construction. It can be very difficult to have renovations done this way.
      The humidity you experienced during the rapid change from cold to warm/humid then back to cold is very common with a vented space. When you had the warm, humid weather, all the ventilation did its job, allowing that warm, moist air to fill the attic. Unfortunately, when you then had the rapid freeze again, the roof cooled quickly and that moisture all condensed on the inside roof deck. This type of wetting cycle won’t do any long term damage as long as it’s relatively infrequent and has an opportunity to dry out quickly.
      Your suggested solution would probably help a bit as the amount of humid air under the roof deck would be limited to just that small amount in the rafter bays as opposed to the entire attic volume.
      Just as an FYI, I have a ventilated attic and we experienced exactly the same conditions during that weather change. While I didn’t check our attic, I’m sure the same thing happened. But I’m really not concerned since that occurs so infrequently.
      If I were you, I’d monitor the conditions up there over time, especially after these types of weather events and make sure that the roof deck dries out in a few days and you don’t start getting signs of mold growth. If things look good, then you’re not going to have your roof rotting out. However, if the wood stays wet for long periods (more than several days at a time) and frequently, there’s a good chance you’ll have long term problems.

      • Thanks very much for your quick reply. I’m rechecking the integrity of original caulking around ceiling AC vents, bath ventilation fans and return registers to make sure it has not cracked or shrunk from drying or opened from the house settling. I have a remote temperature humidity monitor in the attic so those parameters can be closely monitored. I reduce the humidistat setting at night to suspend the humidification. Humidity usually remains at a comfortable level overnight.

        In considering foam board to condition the space, all the ducts and the air handler will be in an environment closer to the internal temperature of the home. Besides reducing interior humidity that can come in contact with roof sheathing, cooling requirements should be lower in the summer. When interior humidity is required on coldest, dry winter days, the output from the AC vents will be warmer.

        I’m sure with your suggestions and my increased diligence, condensation can be greatly minimized to those rare acts of nature beyond anyones control.

        Kind regards,


  19., I am much less educated on the terminology of these things, but I will try to describe my issue the best I can. On my home, when I go up into the attic against the side walls – they do not attach to the roof. There is about a 6-8” gap that I can stick my hand in and cold/hot air blows into the attic freely. This is the same on both sides of the house. However, at the ends of the roof angles there are the normal baffles for the ventilation. So my question is, are these gaps between the roof and wall intended to be there for ventilation purposes? Or would I be ok sealing them to stop the airflow that is coming into the house? Thank you very much for your help in advance!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s