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Community and Q&A

Vapor barrier for garage roof without drywall?

Peter274 | Posted in Building Code Questions on

I recently purchased a house that has an attached unconditioned garage with an uninsulated roof. The garage gets 15 to 20F hotter than the exterior temperature in the summer. Also the roof originally had skip sheathing, and plywood was added over that when the roof was redone at some point. Tar paper was left between the skip sheathing and plywood, so that is currently exposed to the garage interior. The garage stinks of tar in the summer when the roof heats up. I’m located in Portland Oregon Zone 4c

My goals are to reduce summer garage temperature and also create an air barrier between the garage and tar paper. I would really like to avoid adding drywall or osb etc on the interior, although I know about the 15 minute fire resistance requirement. I was thinking of using stone wool insulation placed directly under the sheathing between the joists, which I understand would not require a thermal barrier material for fire code. However to create an air/vapor barrier I would need to add a plastic film material on the interior side of this assembly. I’m not clear on how this works with the thermal barrier requirement. Is there a product, perhaps some kind of foil, that would not require a thermal barrier but would be air tight? Or any other solutions/ suggestions appreciated!

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  1. Expert Member
    Dana Dorsett | | #1

    A water-vapor permeable broadsheet product like housewrap can be detailed as an air barrier without creating a moisture trap.

    Assuming there is no roof venting, perforated aluminized fabric radiant barrier mounted on the underside of the rafters would lower the temperature of the garage, and while it's not an air barrier, would minimize the mixing of air with the volatile asphalt gases with the room air. It works even better if there is soffit-to-ridge venting in the rafter bays, and using either perforated or unperforated fabric type radiant barrier (which is a vapor barrier), which purges the contaminated air to the outdoors via convection. Most of these products are designed to work in open attics, with fire-ratings that don't require thermal or ignition barriers. If roof venting is complicated, start with just perforated RB and see if it meets your goals, add the vents only if it still stinks.

    Whether that's cheaper or easier than rock wool + housewrap depends on the particulars. 500 square feet of aluminized fabric RB in 4' x 125' rolls costs about $65-70 from the two major box stores. (If the local store doesn't stock it you can usually get it shipped to the local store for free buying it online.) If it's cheaper than rock wool and you don't intend to heat the space in winter, that's probably the right thing to do.

  2. GBA Editor
    Martin Holladay | | #2

    As far as I know, housewrap isn't rated for indoor use or exposure to the interior -- there may be code issues that preclude its use, because some types of housewrap may be flammable. When in doubt, ask your local code authority.

    Certainly drywall is safer from the perspective of fire safety.

  3. Peter274 | | #3

    Thank you Dana and Martin. I think I will end up trying just a radiant barrier to start. Looking at the 2015 IRC the relevant sections seem to be R302.9 and/or R302.10 which set limits on interior facing finishes/insulation, for flame spread and smoke development under the ASTM E84 test. Turns out there are a few radiant barriers on the market that easily meet the prescribed E84 test limits, for one Reach Barrier Silvertanium.

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