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Shed/garage insulation question

notaproVA | Posted in General Questions on

Hi everyone,

I have a shared/garage that I’m trying to make into a workshop. The construction puts it somewhere between a pole barn and a standard framed structure so I wanted to get some advice on how best to proceed. I read the pole barn articles and several others on this site, but since this structure falls somewhere between the two I still have some questions.

To start, my goal is to “take the edge off” the current temperature extremes (Zone 4a with interior temps ranging from as high as 100 degrees in the summer down to freezing in the winter), to make it into a reasonably habitable 80-50 degree range with a small ac unit or heater.

The building is an old (1930’s) 16’ x 16’ garage with ~2×4 wall framing and 2×6 rafters. The roof is a traditional asphalt shingled roof with a recently installed ridge vent. The walls however are simply corrugated sheet metal nailed to the framing.

I’d like to avoid removing all the sheet metal, so my thought is to install 2” rigid foam centered between the wall studs and use spray foam around the edges to ensure an airtight seal. After installing the insulation I’d cover the interior walls with plywood or OSB leaving an air gap between the insulation and plywood (as well as the insulation and metal siding). For the roof, I was going to add 4” of rigid foam between the rafters and again use spray foam seal around the rafters, leaving a 2” gap between the roof and the insulation for air to flow from the soffits to the ridge vent.

I was considering a product like silver board with foil facing on one or both sides to help create an air barrier as well as a radiant barrier. Would a foil faced rigid insulation with spray foamed edges work well enough in this scenario? Is there an advantage to using insulation with foil facing on both sides? Any recommendations on foam type if this plan would work?

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  1. Expert Member


    I think it is as good of an option as any, for the money. Its not a huge space, and not permanently inhabited, so a lot of the rules here can be flexible.

    I would probably opt to just fill the bays with something like comfortbatt, and forgo the air gap. Its going to be exposed on the exterior either way, and it will at least not absorb much water if any gets behind there, and provide some amount of sound dampening. It will probably save you about 2 days of work compared to cutting and foaming all the rigid boards as well.

    The thing that's probably hurting you the most in this scenario is the metal siding attached to the studs, thermally. In the summer those can roast in direct sun, and all of that heat is transferred directly to the studs, same in the winter, but for heat loss.

    I'm not sure how much you want to spend on the project, but an additional layer of eps between the studs and the plywood might go a long way in taking the edge off, especially in the summer.

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