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Does it make sense to install two staggered layers of rigid foam above a roof with spray foam insulation – or just one layer?

Jodi Gunderson | Posted in Green Building Techniques on

We’re finally getting ready to replace the roofing on an old building that has recently been insulated with spray foam. The 2×6 walls were filled with open cell foam and shaved flush. We chose to use closed cell (Lapolla’s 4G) foam between the rafters due mainly to the fact that we were not going to put the minimum r-20 (zone 5) rigid foam on top of the old board sheathing. It was just too thick and would look way out of proportion with the building. The design of the roof didn’t lend itself to reducing the fascia height either. In fact, the thickness of the roof will be doubled with just 3″ of rigid foam, a 1 1/2″ vent space, and new plywood deck sheathing.

On a separate note, we decided to insulate before the tear off and reroof in order to keep the filth and contaminants (cement-asbestos tiles) from falling into the building through the spaces between the sheathing.

I’ve read here that it’s best to use two layers of rigid foam with taped and staggered seams when adding exterior insulation. I’m wondering if in our situation, with the spray foam beneath, if it makes more sense to go with just one layer to save time. We plan to use 3″ (total) of EPS foam which I had planned to seal the seams with canned foam during install.

I am aware of the risk we will have of the roof sheathing being sandwiched between two vapor impermeable layers. Therefore I’m also wondering if it would make sense to use a traditional asphalt impregnated felt for the underlayment between the old board sheathing and the EPS sheets. The thought being that there might be some drying potential through the EPS to the vent space above the foam.

Any thoughts or suggestions are appreciated. Thanks!

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    With closed cell underneath the sheathing, I don't think you can install rigid foam above the sheathing. You need some way to address sorption and promote drying. Flash and batt might be an option with a combination of R-20 of closed cell foam and R-29 of air permeable insulation (all underneath the sheathing).

    See this article for additional detail: https://www.greenbuildingadvisor.com/articles/dept/musings/flash-and-batt-insulation

  2. Expert Member
    Dana Dorsett | | #2

    There is no problem with putting rigid foam above the roof deck with closed cell foam on the under side of the deck. With most roofing materials the roof decks can't dry toward the exterior anyway- a standard #30felt + asphalt shingle layup runs about 0.1 perms, a minimal Class-I vapor retarder (aka vapor BARRIER) but the roof deck can dry (albeit slowly) to the interior through several inches of 2lb foam.

    Lapolla's 2000 4G product is 1.4 perms @1" (R6.8) and even at 7" (~ R49-ish) it would run about 0.2 perms, twice the drying rate through asphalt shingles, but only 1/5-1/4 the drying rate through 3" of Type-II EPS into a vent space. Even if the underlayment of the EPS is an impermeable membrane, as long as the closed cell foam is at least 2" thick (~0.7 perms, a Class-II vapor retarder) the roof deck is adequately protected from interior moisture drives, and there could even be R30+ of fiber insulation on the interior side of the closed cell foam without high risk of moisture accumulation in the fiber. But using vapor permeable underlayment is recommended.

    With R12- R12.6 of EPS above the roof deck I'm assuming you went with 5-5.5" (~0.25 perms) closed cell foam (a nearly-full 2x6 cavity fill) on the underside to hit a center cavity total of ~R49 code minimum?

    Most rigid foam will develop seam gaps over time from creepage and shrinkage. It's not a bad idea to install two layers so that no gaps are the full depth, but the "whole assembly" performance hit over time isn't huge if you do it in one layer. Some manufacturers have ship-lap foam to be able to deal with the shrinkage & creepage gaps, but I can't immediately point you to a specific vendor of 3" ship-lap EPS.

  3. Expert Member
    Dana Dorsett | | #3

    According to Jodi:

    "We plan to use 3" (total) of EPS foam which I had planned to seal the seams with canned foam during install."

    ...and...

    "In fact, the thickness of the roof will be doubled with just 3" of rigid foam, a 1 1/2" vent space, and new plywood deck sheathing"

    That's why I assumed an exterior R12+ and 5-5.5" of Lapolla 2000 4G on the underside of the roof deck, and using a permeable underlayment for the EPS to give the roof deck some drying capacity toward the exterior (~1 perm) as well as the ~0.25 perms on the interior.

    It's not a bad stackup, but it's a lot o' polymer compared to R20 rigid on the exterior and R30 open cell below.

  4. Jodi Gunderson | | #4

    Thanks for the replies. Yes, we have +/- 5" of the Lapolla installed under the roof deck now (2x6 framing). It is a lot of closed cell foam but I had to compromise somewhere. Our code is R38 for roofs. I'm aiming to go past that though it probably isn't quite R49. Should be close.

    I will be ordering the EPS sheets soon and will look into the shiplap edges. I had originally planned to use their nailbase panels but didn't like their system for the vented panels nor the edge splines involved. I'm not opposed to laying two layers - it probably wouldn't add much more time anyway. The EPS is made to order so I may look at possible differences in cost. Would I want to use 15# or 30# felt under the EPS? Should I consider taping the seams as well?

    Thanks again Dana for taking the time to explain It all. I now feel confident in this stack up. After all the time taken to research the science behind it, and weighing all the options and constraints, it helps to get confirmation that what you plan to do should work out ok.

    Thanks again for all the help!! Here are a couple drawings that would have helped a while back. It's an old railroad station that is being renovated into a home.

    Cheers

  5. Steve Knapp CZ 3A Georgia | | #5

    [Deleted this comment.]

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