Ultimate attic insulation

Icy roof deck, not a good sign!

In the first two posts of this overly wordy series, we saw a few ways to insulate an attic while avoiding some of the worst problems that can lead to moldy, rotten attics and roofs.

If you recall, the big problem is that moisture from the house rises up through the walls and all the little cracks around light fixtures, hatches, wiring, and the moisture condenses on cool surfaces. Over time, this will lead to mold growth and potentially, rotten roofs.

How do you know if you’ve got a problem? I’ll give you a hint – if you have ice forming under your roof like in this picture, you had better do something before you have to replace your roof!

The goal is to create a comfortable, efficient and safe home. One that is durable and requires minimum maintenance. Is this an achievable goal? There are durable homes, as evidenced by those that haven’t rotted out over hundreds of years, but they’re not comfortable or energy efficient. There are comfortable, efficient homes, but we’re increasingly seeing issues with mold and wood rot in these because some builders are neglecting to deal with the higher moisture levels. So how do we solve all the problems at once?

In this post, I’ll present one option. Keep in mind – this is my opinion, based on current understanding of building science. There are undoubtedly situations where this will not work. Consider this article my interpretation of the current building science literature and do your own research and consult other professionals before building your dream house. In other words, don’t blame me if you blindly follow what I write and subsequently have problems!

So what’s the solution? It’s actually simple – keep humid air away from cold surfaces and, in the eventuality that water does get in somehow, allow it to drain out and evaporate before it can do further damage. Allowing quick drying is key to avoiding rot.

Please note – the proposed solution is not easy to implement in existing homes unless the attic is very accessible. Unfortunately, many attics are under low-pitch roofs which are a real pain to work in. Or, they’re filled with HVAC equipment and ducts, again, a real hassle to work around. So this isn’t a panacea. But if you are one of the fortunate ones who has an accessible attic, or maybe you are about to replace your roof, this construction can produce a really efficient and durable attic space.

How NOT to insulate your attic ceiling

In the second post, you saw the benefits of turning the attic into part of the “conditioned space” of the house – you can put ductwork up there without worrying; you don’t have to sealing every little crack in the attic floor or the attic hatch; you can store things up there without them baking in the summer and freezing in the winter. You’re just creating an attic that is part of the living space of the house. Nice.

In spite of all of the advantages of spraying foam under to your roof deck, there are downsides – leaks can get trapped by the foam; roofs can be difficult to repair; and there’s still serious thermal bridging issues caused by the exposed wood from the rafters or wall framing.

Is there any hope? I think so!

Remember, the first big problem is moisture coming in contact with cool surfaces. How can you avoid this? You could spray foam over all the rafters and framing. And this can work very well. But it doesn’t address all of the concerns.

The approach I like to see is the use of a rigid board foam product, like foil-faced polyisocyanurate. These products have some of the best R-value of any insulation product. They’re also available at your local home center. You can even order Thermax White Finish board foam that has an interior that can be used as the ceiling finish. It won’t be beautiful, but hey, it’s an attic!

Another feature of this solution is the foil-facing is is a complete moisture barrier, so moisture cannot move through the insulation. That’s a plus and a minus – you have to use it wisely. In this case, it’s a plus because we don’t want moisture moving through it.

The critical step here is that the entire attic has to be encapsulated. Remember back in the first article where I talked about a leaky balloon? That is especially true here. If you have any holes, the balloon (your house) is going to leak. This really isn’t a trick. If you were finishing the attic to be a bedroom, for example, you’d put up drywall and seal all the seams. You wouldn’t leave big gaps to the outside, right? Same thing here, except instead of dealing with heavy sheets of drywall, you’re using light foam board, preferably 2″ thick, giving around R-14 of insulation.

The details are important, just as they would be in finishing a room. All seams have to be sealed. That prevents air and moisture from sneaking through between the panels. Larger gaps at seams should be filled with canned foam. And irregular areas should be sealed up using 2-part spray foam, and is better left to the professionals.

R-14 might not sound like much, but it’s much better than you’d think because it’s covering all the surfaces. In a normal insulation job, the insulation is between the rafters and wall studs which allows thermal bridging that greatly reduces the R-value and provides opportunities for moisture build-up.  So R-14 everywhere is actually almost as good as putting 6″ of insulation between the rafters. But it’s not to current building codes, so you should add more insulation.

To keep things in perspective, a perfect R-14 insulation stops 93% of the heat loss during the winter, so it’s a great start. If you just did this much insulation in your attic, you could keep your attic at 70 degrees on a 15 degree night with the heat produced by two hair dryers. That’s not bad, but we’re going to see how you can cut that even more…

Sources of moisture

We have to pause for a moment and discuss the sources of moisture that can damage the house. They are:

  • Interior moisture in the air when it’s cold outside
  • Exterior moisture in the air when it’s hot and humid outside
  • Exterior water (rain), caused by leaks or bad construction (flashing etc.)

Any solution needs to deal with these issues. Not all climates have all the problems. For example, if you live in the dessert, you’re not usually concerned about high outdoor humidity. Likewise, if you live in the deep south, you usually aren’t worried about it being freezing cold out. The point is, you have to take into account your local climate conditions. This solution works in all climates, though the amount of insulation may need to be adjusted up a bit for extreme climates.

Finishing the job

At this point, you’ve encapsulated the entire attic with 2″ of poly-iso board foam under the rafters and inside the wall studs. It’s the same as being inside a foam ice chest and just as effective at keeping the temperature even. Keep in mind that you have to insulate every surface that is next to the outside. The obvious ones are the ceiling and the walls. But don’t forget overhangs, like over a porch. it’s just as important to insulate those.

The next part completes the job. You get a foam installer to spray a few inches – R-20 to R-25, directly to the back side of the foam you just installed. Of course, this can only be done if you’ve ripped the roof and outer siding and sheathing off or are in the process of building a new house. But if you are, this works great. R-20 on the back of two inches of foam board gives a total R-value of 32. If did what code calls for (in most regions) and put R-40 between the rafters, you’d end up with R-25. And, if you did what most people ACTUALLY do, and installed R-29 fiberglass between the rafters, you’d only have an R-21 roof AND you’d run the risk of rotting out the roof.

Building an attic this way reduces the winter heat loss on a 15 degree day so much that an entire attic could be heated with a hair dryer – on low! And summertime air conditioning in the attic could be done with a small window unit. That’s efficient!

Some important features

Why am I so adamant about encapsulating the attic with foam board then spraying foam directly onto the back side of the foam board, from the outside? Because it solves virtually every problem that building scientists and builders fight about.

  • It completely isolates cold surfaces from inner humidity.
  • It provides an air gap directly behind the roof and wall sheathing allowing a ventilated structure.
  • It provides excellent R-value / minimizes energy loss.
  • If the roof or walls leak, the water drains out  so the wood can dry quickly.
  • It should be done in combination with a ridge vent and matching soffit vents.
  • It is compatible with different roof types – slate, metal, rubber, asphalt because of the air channel between the foam and the roof materials.
  • It keeps the attic at the same conditions as the rest of the house.
  • It doesn’t reduce the strength of the roof or walls, like installing foam board between sheathing and the studs/rafters does.
  • Closed cell spray foam between the studs and rafters increases the strength of the house.
  • It works in cold snowy climates as well as hot humid climates.
  • It’s more tolerant of minor building mistakes than other techniques that require “perfect” construction because moist moisture that gets through will flush out before it can do damage.

Given all these advantages, why doesn’t everybody build this way? Because there are generations of builders, architects, code officers and engineers who have done it the old fashioned way and change is hard. Even though they’re in the process of building new homes every day that run the risk of rotting out. Even though they have an opportunity to build a new generation of homes that should never require a roof replacement or suffer from another ice dam. Even though they could make homes that require vastly less energy to heat and cool. They’re not going to do it because it’s “not the way things are normally done.”

In a future post, I’ll show how this construction technique can be modified for all the walls of the house and also show how you can create a variation of this construction that can be used in retrofit situations.

Until then, I hope this series helps some of you avoid moldy, rotten roofs or if you already have one, I hope it helps you find the cause and fix it for good! If you’ve got these problems, send in your pictures and post your thoughts to the comments.


66 thoughts on “Ultimate attic insulation

  1. Hi Ted,

    I’m in the process of converting the previously vented attic with 6 inches of loose fiberglass to a conditioned, unvented attic space. The roof is a 6/12 and am in climate zone 4 with r38 requirement. I am thinking of insulating roof deck with rigid foam board with mineral wool insulation below. I was going to use polyiso but am fearful of the r-value drop at 15C versus doing XPS and sacrificing some of the R-value. Your thoughts would be helpful. Also are there any dangers of putting foam board right against bottom of roof sheathing as opposed to leaving air gap?

    • Hi Mark, that is an excellent question! Honestly, nobody has brought that up with me before so I went to my favorite source to learn more about this in practice. You may want to check out this link:

      They have some good info in that article. Particularly, performance varies greatly between manufacturers and it’s best to put additional insulation on the cold side of the polyiso so as to moderate the temperatures that the polyiso sees.

      With this in mind and for other reasons, I would use the polyiso or XPS under the rafters and put mineral wool above the board, still leaving an air gap of at least an inch under the roof deck. It’s best to always have that air gap just in case moisture gets in there, you don’t want to hold it against the roof deck. The other reason for using rigid foam and then batts is that the rigid foam lets much less moisture through. If you did mineral wool first (towards the inside of the house) then the moisture would go right through it and could condense on the rigid foam.
      Hope that helps.

      • I guess I’m a little confused about the 1 inch air gap. Is this a preventative measure in the event that there is a leak from the outside allowing water penetration or is it to allow air circulation so the roof deck can “breathe” and not rot out from the inside out? A lot of guys that spray foam with closed cell and then use mineral wool or open cell over the top obviously don’t have an air gap. In terms of moisture prevention it seems the XPS would be better than polyiso?

      • The answer is “yes” 🙂
        In the event of a leak, it’s good to have a gap to allow the water to run freely down the rigid foam and out the soffits. Without a gap, the water could get trapped and held in place by capillary action. That water would then soak into the wood.
        However, the more common reason is as you noted – water vapor will travel up and through the smallest of gaps. That can then condense on the cold underside of the roof deck. Having an air gap is beneficial as you noted, as the air circulation can flush out the moisture that might accumulate.
        There’s some debate on spray foaming directly to the underside of roof decks. I personally don’t have a problem with it and did it on my own addition with no ill effects. Since dense closed cell spray foam is very effective at blocking water vapor, there’s very little chance of moisture penetrating and getting up to the roof deck.
        You’re also correct that XPS is better than polyiso in this application. Foil faced polyiso reduces that benefit. But XPS is a great product all around.

      • Thanks for the info, Ted. The attic is going to be conditioned and unvented (maybe this explains why the air gap is not making sense). I wouldn’t expect, even in the coldest winter day, to see a dramatic drop in the attic space below 50 degrees F, where you begin to see the drop in R value with polyiso. Did you use insulation over the top of your closed cell spray foam to get up to the R value requirement? I originally was going for aforementioned approach (flash and batt) but the spray foam has gotten really expensive which is why I’m looking at foam board and trying with tape and home center spray cans to seal as airtight as possible. Again, I appreciate the information enormously.

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