Author Topic: Insulate Roof Deck (external)  (Read 658 times)


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Insulate Roof Deck (external)
« on: January 10, 2022, 02:24:02 PM »
Does anyone have experience with insulating a roof deck (i.e. foam board insulation over the sheathing and under the shingles)? How much did it cost, how did it look and was it worth it?

I just bought a house that requires a full roof replacement. I plan to eventually create a conditioned attic with spray foam insulation in the rafter bays. To get a little extra R-value in the attic, I'm considering having a couple layers of insulation added to the top of the roof sheathing. My contractor recommended against this, saying that, at most, it would give me an extra R-10, which doesn't justify the extra cost of installation. I still need to do the math on this but he's probably right.

I'm also concerned about aesthetics. It's a 90 year old house and I'm not sure how the extra insulation will look from the outside. Thanks in advance!


  • Pencil Stache
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Re: Insulate Roof Deck (external)
« Reply #1 on: January 10, 2022, 02:52:42 PM »
I hadn't heard of this method before, but it makes sense - you're covering the whole roof, so you don't have thermal bridging at the rafters like you do when you insulate inside.

Here's an article with a very useful section:

I don't see how you'd do this without adding the second layer of plywood, and you need some really long screws.  I'd prefer using a contractor with experience for something like this, not one who thinks it's a bad idea.  It looks like there are lots of places to make mistakes.  Home Depot has some 1.5" polyiso board with an R-value of 9.6.  If you use three layers like the section shows you'd get at least an extra R-28.8


  • Walrus Stache
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Re: Insulate Roof Deck (external)
« Reply #2 on: January 10, 2022, 03:08:51 PM »
Also consider what the extra weight would do. Sure, the house might be fine, but I also don't know how heavy the insulation and an extra layer of plywood would be.

I have heard of this method but can't remember the source. If I remember, will edit. Knowing my history, This Old House is a possibility (either adding it, discussing it, or undoing it, no clue).


  • 5 O'Clock Shadow
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Re: Insulate Roof Deck (external)
« Reply #3 on: January 11, 2022, 10:54:25 AM »
We did this to insulate our 'hot roof' about 5 years ago. We added 5" of polyiso board in two layers with staggered seams, for R30, with another layer of sheathing on top. We live in an ~70 year old ranch, with a flat roof (1/12), in a cold place. The roof/ceiling was originally 3.5" thick tongue and groove boards laid from wall to the ridgeline, with a built-up (tar/gravel) roof on top. It's now rubber/EPDM on top of the foam. We added a tiered fascia board and metal drip edge to create some shadow lines and break up the mass, so the house didn't look too much like a cupcake. (Though it sometimes does as we haven't gotten around to updating some of the other trim to match the fascia board!)

Weight didn't seem to be an issue (had to go through city permitting and neither they nor the roofer were concerned anyway). The big screws were easy to sink given the solid nature of the old roof they were aiming for. Of course, our roof is small and simple - YMMV!

If you want to insulate on both sides of the wall for conditioned space, the Cold Climate Housing Research Center (CCHRC) has good info on how to make sure you're not creating moisture/rot issues (I think it's like 2/3 of R value outside the envelope, at least up here in AK).

We did not DIY this, but used a roofer (including full roof replacement, sheathing repairs/replacement, etc) and I think the insulation portion was about $10k of the total.


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Re: Insulate Roof Deck (external)
« Reply #4 on: January 11, 2022, 03:26:48 PM »
I've never done this or seen it done but here are some of my thoughts just the same that you can perhaps clear with whomever does it.

1. You can't apply shingles to insulation so you would have to install another layer of sheathing on top of the foam. Sheathing costs right now are quite a bit higher than they used to be and would be a significant cost adder (along with foam and sealing tape) to a reroofing project. Achieving the same insulation values can probably be done other ways for much less money.

2. Conventional wisdom says that shingles need adequate ventilation to dissipate heat in many parts of the country or it can lead to premature shingle wear and failure. This is generally done by venting the attic underneath the sheathing. If you are adding insulation on top of the sheathing, there would be no room for ventilation. I would assume you would have to put down some spacers that run from peak to eave on top of the insulation and underneath the bottom of the top sheathing and provide a way for air to ventilate underneath that surface. I'm not sure what that would look like at the eave or peak of the house but I'm sure there are ways to do that. Again, this would add more cost to the project. But I have read more recent research that says that ventilation is not so necessary to shingle insulated roofs as spray foam has become more popular.  The added shingle temperature, which depends on climate and color, could be as little as a few extra degrees which might reduce the life by a year at most. Still, many shingle manufacturers will still void warranties unless there is adequate ventilation provided. Something to research.

3. I would assume this would fall out of the realm of a lot of roofers who just want to come in, strip and replace shingles. They would need additional tools to deal with the sheathing and fastening it through the layer of foam, spacers, original layer of sheathing and into the rafters that they might not normally use. I would expect high labor costs and maybe an extra margin padded into quotes just to deal with all the unknowns.

4. Just cost wise, I think it would be much cheaper in the long run to just spray the bottom side of the sheathing with spray foam insulation and then if you want more R-values, apply a layer of foam to the bottom side of all your rafter joists. This achieves the same R-Value as what you propose but saves on an extra layer of sheathing, the spacers mentioned above, labor, etc. while still maintaining the aesthetics of the house on the outside and your roof replacement would then be a straightforward job. This doesn't answer the question of whether or not you void shingle warranties or shorten their life expectancy since there would be no ventilation in this case either.


  • Pencil Stache
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Re: Insulate Roof Deck (external)
« Reply #5 on: January 11, 2022, 03:41:59 PM »
The general term I'm familiar with is "nailbase."  There are code restrictions base on your climate zone so that you don't end up with a condensation layer between your original sheathing and the new sheathing.  This will specify the thickness and R-value you need to add.

The product is usually sheathing already bonded to a layer of insulation, depending on the R-value requirements, there may be a layer or two of insulation prior to the final layer.. The contractor generally builds a lip around the edge of the roof using 2by material, and then adds the insulation and nailbase to build up the new roofline.

There are definitely benefits to it. Generally it's easier to do than remodeling something like a vaulted/raftered ceiling with additional insulation.

edit: To reiterate what some others have said. You need to find a contractor who is familiar with it, most will just want to slap a new roof on. Unless you live somewhere really hot, I wouldn't worry about the shingle degradation. If it's a metal roof or something, you'll want to buck it out anyway.
« Last Edit: January 11, 2022, 03:45:56 PM by BDWW »


  • Walrus Stache
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Re: Insulate Roof Deck (external)
« Reply #6 on: January 24, 2022, 09:35:21 PM »
I did this with my build.

You do have to get the correct exterior-interior R numbers correct, as BDDW mentioned, to avoid a condensation sandwich.

I used 2 layers of 3" polyiso over sheathing. The underlayment was placed on top and then purlins were installed using 10" screws. The metal roof was then screwed into the purlins.

Try and source the rigid foam used.

Jon Bon

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Re: Insulate Roof Deck (external)
« Reply #7 on: January 25, 2022, 05:43:35 AM »
How much head room do you have in the attic ? It would be MUCH easier to extend the roof rafters downward (face nail a 2x10 to the 2x6) that way you would get about 9 inches of room for insulation. That's r-30 in cheap stuff and what r-60 in spray foam?

Yeah if you make your roof 3-4 inches higher with external foam you are going to have to redo your all of the trim and fascia board around your house. Likely your gutters as well. I saw some dude do it on a blog, it looked like it cost  about 1 million dollars.

If a new roof costs 2k I would say what your proposing would be 10x as much. I would think there has to be easier and cheaper ways to accomplish your goals.