The closed cell spray foam doesn't require an air barrier between it and the roof plywood because it is both a moisture barrier and non-air permeable. Meaning the warm air from the garage would never reach the cold plywood of the roof to condensate against it. Any condensation would merely collect on the surface of the foam. You need a 1" air gap between fiberglass insulation and the roof plywood because warm air from the heated garage can permeate the fiberglass, then hitting the roof and creating condensation against the plywood. Without an air gap the moisture has nowhere to go and the wood can't dry back out.
How big of a concern is it with my only intermittently heated garage, whose to say? The 200$ for the plastic panels that create/maintain said air gap is cheaper than a new roof, so I'd go ahead and spend the extra money.
Yes, if using faced insulation I would then need some sort of 15 sec fire barrier between it and the rest of the garage. Although my worries of a grinder spark reaching 12ft up into the rafters and still able to catch fire to the paper are fairly minor.
I just got a quote for 1100$ to get a 3.5" thick (equaling R20) layer of spray foam put on my roof. By comparison if I use R19 fiberglass bats and do it myself I'm looking at approx. 600$ based on Home Depot website pricing for all the materials. I could theoretically spend the extra money on R30+ fiberglass and be at 800$, but in both cases I'm having a hard time via Home Depot's website of getting unfaced insulation. So then I would also have an added cost of some sort of fire barrier. I'm not tiling my roof with drywall (would have to cut it down pretty small to get it between the joists) and I'm not sure what else I could use. I guess I could always just put the paper facing towards the roof and call it a day. $1100 is a lot to swallow for the spray in stuff, but it does save me a lot of work/time. It also saves me a lot of itchiness. If I do it myself though I can always just do a small section at a time and spread that cost out over a few paychecks.
Just for fun, based on the square footage of my roof the mathematical perfect world difference between R20 and R30 insulation is about 260btu's of heating requirement to maintain my garage 40deg warmer than outside and 340btu's at 50deg warmer than outside. My garage heater should be putting out about 20,000btu's. Currently with my garage completely uninsulated it should take about 40,000btu's to maintain that 40deg temp differential. With figuring my garage door at R0 and not counting the wall that faces the house (as its both insulated and the other side is at 70deg) it should take 9,600btu's to maintain that 40deg temp differential once I get done insulating the walls and roof. That drops down to 3,500btu's in a perfect world with the R5 garage door foam insulation kit. In the end my little heater should have no problem keeping things toasty once its all said and done.
Of course, not counting the electric bill I could just save all my money and simply buy a second 6kw garage heater for about $200 installed price. That gets me up to my 40,000btu's of heating requirement and saves me a ton of work and money. I don't pay the electric bill anyways and its not like I'm heating my garage 24/7.
