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.


113 thoughts on “How NOT to insulate your house

  1. 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.

  2. “…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.

  3. 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.

  4. 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.

  5. 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.

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

  7. 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.

  8. 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!

  9.’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.

  10. 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.

  11. 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.

  12. 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.

  13. 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?

  14. 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.

  15. 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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