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What insulations comply with the 2009 IRC definition of ‘air impermeable insulation’?

tignarius | Posted in Building Code Questions on

The definition is: An insulation having an air permanence equal to or less than 0.02 L/s-m (squared) at 75 Pa pressure differential tested according to ASTM E 2178 or E 283. I just learned from a conversation with Firestone that their product ISO 95+, a 2″ polyisocyanurate roof insulation does not meet this defintion. This product is in standard use in the roofing industry. It hs been my understanding that closed cell foams were air impermeable thus complying with the above definition. The Firestone representative said that foil faced polyiso foam does meet the above standard. If this closed cell product does not meet the above definition what closed cell products do, only ones with a foil face?

This is important because in section R806.4 Unvented attic assemblies, 5.3 requires that the air impermeable insulation be against the roof deck and air permeable insulation below that.

I had intended to use a new or used version of the roof insulation as the air impermeable insulation required in R806.4.5.3, because of the response from the manufacturer I ask the question above.

Hope you can shed some claification about closed cell foam and the IRC.

Thank you

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  1. Expert Member
    ARMANDO COBO | | #1

    - You can use rigid foam on top of the roof decking and open cell (Air Permeable) under the roof decking. This is my preferred method for unventilated attics, it allows the roof assembly to dry to the inside and it addresses thermal bridging. I use a full Rvalue required by code with the two foam combination.
    - You can use rigid foam on top of the roof decking and closed cell (Air Impermeable) under the roof decking. This is a so-so method for unventilated attics, but more expensive and you can sandwich your sheathing with 2 closed cell foams. It also addresses thermal bridging, and you need to use a full Rvalue required by code with the closed foam.
    - You can use closed cell or rigid foam (Air Impermeable) under the roof decking only. This is a not so good method for unventilated attics since it does not address thermal bridging.
    The thickness of the insulations depends on your climate zone. Most all rigid foams and closed cell foams are air impermeable, but to be sure you should go to the manufacturer’s website. I prefer to use Dow blue boards for rigid foam and Demilec for spray foams.

  2. tignarius | | #2

    Thank you for your reply.
    The code reference above allows for 'air impermeable insulation' to be used under and in contact with the roof deck along with other steps that must be taken to qualify for an unvented attic assembly. Firestone says that their ISO 95+, polyisocyanurate roof insulation, is not air impermeable.
    Polyisocyanurate insulation is supposed to be a closed cell foam, but the manufacturer says that is not air impermeable. So, if this closed cell material is not air impermeable, what closed cell material is?

  3. bxxKT22MVd | | #3

    Good question! I see someone is paying attention. Icynene open cell spray foam passed the ASTM E283 so yes, it is a certified air barrier. All closed cell foams are. Not many open cell except Icynene.

    I educate building inspectors on this all the time. Most think dense pack cellulose is legal until i show them the code and prove otherwise. Don't get caught using dense pack on a non-vented roof! You will have to tear one completely out one of there days!

  4. Expert Member
    ARMANDO COBO | | #4

    Like I said, R806.4.5.2 allows to use air-permeable open cell foam under the roof decking AND air-impermeable over the roof decking. I like this way in case the roof sheathing gets wet in the next 100 years; it allows the roof sheathing to dry to the inside of the roof assembly. Also, it’s very important to install rigid foam on top of the roof sheathing to avoid condensation under the roof sheathing. That’s why I use this method in unvented attics.
    Do an ASHRAE Fundamentals analysis for your climate and you’ll find out that with all roof and wall assemblies in CZ4+ the potential for that condensation is high. Likewise, it can happen in CZ3 but in very few really cold years, like this past winter in North TX.

  5. tignarius | | #5

    R806.4.5.2 requires the existing roofing to be removed ot get the air impermeable insulation under the shingles. R806.4.5.3 allows the air impermeable insulation to be installed under and in contact with the roof deck so the existing shingles to not have to be removed. R806.4.5.3 is the specific part that I want to comply with. Since ISO 95+ does not qualify as air impermeable, according to the manufacturere, what closed cell foam does?

  6. Expert Member
    ARMANDO COBO | | #6

    My bad Paul, sorry... I assumed that you were talking about new construction or at least no roof installed and I didn’t read about having an existing roof.

  7. tignarius | | #7


    No problem. My original question springs from the response from Firestone that the product I refered to was not air impermeable. I was under the impression, regardless of facing, that polyisocyanurate insulation a closed cell foam was air impermeable.

    If this is the case then what closed cell insulation is air impermeable?

  8. Expert Member
    ARMANDO COBO | | #8

    I'm pretty sure all closed cell insulations, rigid or foam, are air impermeable (or at least the ones I know), but to be sure you need to check the manufacturer's specs. Rigid foam must be super sealed and taped at the joits to be air impermeable and some folks do two layers of staggered joints to achieve that.
    If you are going to use closed cell foam, sprayed closed cell foam would do a better job of air sealing and you could spray the rafters or top cords to avoid some thermal bridging.
    One thing though, you better hope and pray all your moisture management detailing on the roof was perfectly done and you don’t have any leaks.

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