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Conditioning an attic with EPS

Nate_Orange | Posted in General Questions on

I’m looking to condition my attic but don’t want to use sprayfoam. I’ve found an EPS mfg that will cut panels to size to I may use them between rafters, and planned on putting EPS 2″ panels under the rafters after.

A few questions:

– Has anyone used EPS to insulate an attic/roof before? Seems like a good idea (similar to a SIP) but I haven’t come across anyone who has done it.
– A key question was to whether seal and insulate the attic floor or roof. We will have remodeling going on that will open up the ceiling, and will likely need the space for storage, so roof seemed to make the most sense although much less common. Are there any fatal flaw I’m missing?
– I am looking to seal the vents, yet have soffit vents that are impossible to get to. Can I seal them from the exterior? With what?
– What would the best way be to seal my ridge vent?
– What other advice would you have?

Thanks everyone,


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  1. GBA Editor
    Martin Holladay | | #1

    Lots of people use EPS to insulate a sloped roof assembly. The key point is that the EPS must be installed on the exterior side of the roof sheathing. (Of course, that means that you'll need a second layer of roof sheathing above the EPS, as well as new roofing.)

    For more information on this method, see "How to Install Rigid Foam On Top of Roof Sheathing."

    If you are thinking of installing rigid foam on the interior side of your roof sheathing -- a method called the cut-and-cobble technique -- you should know that this method is not recommended, because it is associated with moisture accumulation and sheathing rot. For more information, see "Cut-and-Cobble Insulation."

    You should probably also read this article: "Creating a Conditioned Attic."

    The best way to seal soffit vents and ridge vents is with closed-cell spray foam.

  2. Expert Member
    Dana Dorsett | | #2

    The EPS can be on the interior sides as long as there is soffit to ridge venting with at least 1" of unobstructed air between the roof deck and EPS. Sealing the vents or gaps narrower than 1" would not be an option.

    With 1" gap vented EPS insulated rafter bay you could change the interior side foam to 2" polyiso instead of EPS and achieve roughly the same performance as a full cavity fill (no gap) and 2" of interior side EPS.

    For climate zones 4 & higher it takes 11.5-12" of EPS to hit a code-min R49, or 9" of EPS + 2" of polyiso.

    In zones 2 & 3 it would take 9" of EPS to hit a code-min R38. That's do-able with a 2x10 rafter, 2" vent gap, 2" continuous interior EPS or with 2x8 rafters 6" of EPS + 2" of interior side EPS. (The latter could come up a bit shy on R-value, but would make it on a U-factor basis due to the thermal break over the rafters.)

    In zone 1 it takes 7.5" of EPS to hit a code-min R30. That can work cavity-only EPS with 2x10s or 2x12s. With 2x8s 6" in the rafters and 1.5" interior EPS (or 1" polyiso) works.

  3. Nate_Orange | | #3

    Thanks very much Martin and Dana. This was very helpful. Seems like air sealing the attic floor and putting EPS/polyiso there instead of the roof deck is a much easier installation and allows me to get the required R-value more effectively.

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