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How to insulate existing attic wall adjacent to heated space?

Cadeng01 | Posted in General Questions on

Existing attic exterior framed walls are uninsulated. 2 attic framed walls adjacent to upstairs heated bedrooms are insulated with batt insulation (vapor barrier at warm side). Batts are between the studs and exposed on unheated side.
Question: Can I add foam insulation board to the unheated (attic) side of these walls to increase R value. What type would I use to avoid creating moisture issues? Perhaps add foam insulation board over 1x or 2x furring on studs to create air space? Thanks for the help.

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  1. Expert Member
    Dana Dorsett | | #1

    Assuming the unheated part is vented to the outdoors, you'll be moisture-safe putting rigid foam on the unheated side if you follow the IRC prescriptives for exterior R-value for wood sheathed exterior walls:

    The type of foam doesn't matter, only the total R-value, but it does have to be covered with a fire-rated thermal barrier to be fire-safe. (Half inch wallboard works, painted or not.)

    If the batts have polyethylene or foil vapor barrier there's a small risk if you create a moisture trap with foil-faced foam. If the only vapor retarder is a kraft facer, it's not a problem, since kraft facers become semi-permeable to water vapor if the moisture levels rise to mold-growth levels.

    You DON'T want to create an air space between the rigid foam and the wall. An air space is just a thermal bypass, rendering the foam less effective.

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