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Applying Polyiso foam on the outside and inside of a wall?

GBA Editor | Posted in General Questions on

I am remodeling a home in southern Maine with 2×4 walls. It will have a heat recovery ventilator. I plan on putting 1 1/2′” polyiso foil faced foam on the outside with joints caulked and taped. I then planned to install R-13 betwen studs covered with 1/2″ polyiso foil faced foam on the inside with drywall over that. My reasoning is in summer time this will keep any warm moist air from condensing on the back of the drywall when the house is air conditioned. Am I right or is there a better way?

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  1. Sean @ SLS Construction | | #1

    That would be adequate in my climate zone & possibly in yours as long as it is properly detailed, flashed & air sealed. My only recommendation would be to change out the batts with a sprayed in cellulose or fiberglass product instead. While foam would give you a better RValue, the cost might be prohibitive

  2. Christopher Briley | | #2

    Hi Gary (fellow Mainer).

    You've described a classic vapor sandwich assembly. By that I mean the foil faced polyiso acts as your vapor barrier (and air barrier) and it does so on both faces of your assembly. This means you have to have everything right, and have zero failures in your installation because if there are any failures your wall assembly can not dry out. To quote a friend when presented with this type of assembly,"what happens in the wall, stays in the wall."

    First, I would swap the fiberglass batt insulation for dense-packed cellulose. Its performance is better and more importantly it will pack the cavity tight and eliminate convection within (and across) the wall. Then I would consider educating yourself on the air-drywall approach developed by the building science corporation. This is using your painted drywall as your interior vapor barrier. It would inhibit moisture migrating from the interior, allow your wall assembly to dry to the interior, and allow you to just use 2" foil-faced polyiso on the exterior (saving you the work of another layer on the interior)

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