Interior insulation for a garage
I am in the process of drywalling my garage, and so I am taking the opportunity to insulate so that I can heat it up when desired to use as an occasional workshop.
It’s 2×6 construction, OSB sheathing, Tyvek, and vinyl siding, in climate zone 7.
I already have some Roxul batt insulation that I am in the process of stuffing in the 2×6 cavities. The garage will likely never get air conditioned. The debate right now is: Do I use a poly vapour/air barrier?
Is this assembly at all risky?
For heating purposes, having interior foam may help to bring up the whole-wall R-value, as the framing isn’t very efficient with respect to thermal breaks. However, the garage probably won’t be heated frequently enough that spending the extra for insulation would ever “pay back” — especially since the garage doors are a significant weak link in air sealing as well as insulation.
I’ll also likely install a ventilation fan that activates upon door opening/closing to help alleviate IAQ as the garage is attached.
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Vapor barriers are being shied away from in all but the coldest places, and even there they are not necessary. I am not a pro, but after reading here, buildingscience.com, and other good sources for a few years, my new house will not have a VB. Air seal use your Ruxol. You are right about not needing more insulation inside. After 30 some years of no heat in my garage, I finally heated it (intermittently) last year and the difference of 50 degrees and 10 or less is amazing. It also heats up pretty quickly w/ a Toyo stove (10' ceiling, 30 yr old fiberglass R21, and poorly insulated lid). I think you'll be fine, though don't expect to wear sandals.
Darryl,
In Winnipeg, you can use interior poly if you want, although there is no need for it if you prefer to omit it. I don't think you'll have moisture problems in your garage walls either way.
The most important thing you can do is to install the drywall in an airtight manner, paying particular attention to electrical boxes. The air sealing work won't cost much money, but it will yield dividends in energy savings and comfort.
More info: Airtight Drywall