Insulating cheap and green for a garage roof and door
I have an unfinished garage here in Minnesota, wanting to insulate it and add a heater so it can be used in the winter. Managed to find some R15 faced insulation for the walls off craigslist.
I got a bid for spray foam for the roof and they pitched the benefits, but I’ve still got some sticker shock ($3600 for about 750 sf at 3″ thick R21). Do folks here have recommendations on the best approach for insulating the interior side of the roof (leaving rafters open) that is both affordable and green? If spray foam is the way to go I can do it, just trying my best to weigh my options.
They also pitched me on a fire retardant paint they suggest I spray over the foam, but that’s nearly $1000 extra. Is that necessary?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Hi bfine1234—I think it might be helpful to know how you plan to use it is. That is, are you just hoping to warm it up on a Saturday night for a few hours to watch the Vikings (I'm assuming…) and then let the temp go back down during the week? Or do you want it to always be 68 degrees inside so that you can use it more like a home office?
In either case, it'd make sense to spend time air sealing, and theres lots of information on this site and the QA that addresses that. But knowing how—and how often—it'll be used would be helpful in suggesting insulation strategies.
Mostly for teens to hangout, maybe 2-3 hours a few times a week, the rest of the time unheated.
It seems like there are a lot of factors to consider if you want to leave the rafters open.
If you're looking to go with an unvented roof assembly, I believe you don't have enough closed-cell spray foam in the hybrid assembly for your climate zone. You're probably in cz6 (but could be 7; you'd need 60% if you are), and if thats the case you'd need 50% of the total to be the closed cell. If you hit that number—about r-25— you could put another r-25 of air permeable insulation next to the closed cell to meet code. You can also shoot for hitting the code based on a u-factor basis, but that's dependent on the specific materials and in what quantity they're used.
My guess though is that you're probably not interested in a code-built assembly as its only used occasionally. You may be able to get away with less, perhaps something in the r-38 range (which is about what the u-factor is for your climate zone) given the lessened humidity levels from just a few hours a week of use, but thats entirely speculative, and I wouldn't be in a position to say with any certainty what the minimum you could get away with is.
If the moisture load stays low I know folks who 'get away with' just kraft faced fiberglass batts in CZ6 at the roofline but it's not code compliant nor will it be anywhere near airtight or energy efficient. The risk is cold roof sheathing and no ridge vent for moisture to escape, meaning condensation. How much depends. Nothing crazy in the garage like a hot tub or a shower or an entire snowbank melting off a vehicle every other day? Then the moisture load should be low along with the risk, but again, it doesn't meet code nor will it hold heat for long. The better cost-effective approach is sealed poly and drywall at the ceiling (the truss bottom chords) and then blown cellulose above.
Jason S, assuming a shingled roof and that it is to be untouched, how would the cellulose be blown in? Poly, cellulose and drywall in that order?
bfine1234, why do you wish to keep the rafters exposed? Cellulose and drywall should cost far less than spray foam and would be a much greener approach.