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Community and Q&A

How thick a layer of closed cell spray foam under roof deck?

berryjb | Posted in Energy Efficiency and Durability on

Our roof has 4″ of rigid board closed cell foam with OSB (Atlas Nailbase) installed above the roof deck, but the seams between the sheets of foam were not sealed, as one can see in the attached picture. For those areas I can access, I started spraying closed cell foam on the underside of the roof deck – how thick a layer of spray foam under the roof deck do I need to have for reasonable insurance against moisture condensation at the seams of the Atlas OSB??

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  1. berryjb | | #1

    I should have included that we're located in Maryland

  2. Expert Member
    Dana Dorsett | | #2

    A 4" nailbase is only 3.5" of foam- or was it a 4.5" nailbase?

    Was it polyisocynurate, or was it EPS (Atlas makes both types.)

    Is there going to be any fiber insulation between the closed cell foam and the interior conditioned space? If yes, how much?

    With as little as 2" of closed cell foam the vapor permeance drops to a bit over a half-perm, making it a Class II vapor retarder, which is very protective of the roof deck. If there is fiber insulation in the stackup, in US climate zone 4 (all of MD except Garret County, which is zone 5)) you need at least 30% of the total R to be the nailbase + closed cell foam, to meet IRC code-prescriptives, which shouldn't be too hard to hit (with margin!)

    With a 4" nailbase panel and NO interior foam or fiber, there's effectively zero chance of the roof deck adsorbing enough moitsture at the nailbase seams to be problematic, and even with R20 batts (assuming 2x6 rafters?) it wouldn't be a problem even without the closed cell foam, as long as there was reasonably air tight gypsum board painted with standard latex paint on the interior.

  3. berryjb | | #3

    Dana, I believe the nailbase is 4.5" poly. There is existing 6" fiberglass above the flat part of the ceiling, and no insulation on the cathedral portion - which will be much more challenging to insulate, especially as quite of few of those rafter bays (old house - roughly 3 x 5 rafters) have AC flexhose ductwork - I'm looking for a solution for the cathedral ceiling, because of the gaps left at the seams of the nailbase.

  4. GBA Editor
    Martin Holladay | | #4

    The 4.5-inch polyiso nailbase is rated at R-24 -- but it will perform a little worse when the temperature drops.

    Maryland is located in Climate Zone 4. According to the 2012 IRC, the minimum code requirement for roof insulation in your zone is R-49. So if I were you, I would add at least R-25 worth of insulation on the interior side of your roof sheathing in the cathedral areas.

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