Insulating my roof
by Bryan
(Halifax, Nova Scotia, Canada)
How can I insulate a flat roof with a 12" cavity without risking moisture problems?
I have a flat roof, built with roof boards directly on top of joists, so, there is about 12" of cavity.
I'm wondering about whether I can use batting in the cavities or if I should be concerned about leaving room for air flow in the cavities? I'm considering doing a new roof and want to have it insulated, so was considering taking off the old boards, putting batting in and then re-sheathing with new plywood. One of the roofing contractors suggested putting Polyico Insulation for R20 on top of old boards, but, I'd also like to have the possibility for a roof deck and not sure that would work to have a deck on top of the Polyico.
Can you provide some guidance?
Answer from Green Energy Efficient Homes
I can't give you a definitive answer as I'm not a roofing expert, but here are a couple of points you should consider:
Moisture is the main issue you will need to address if you insulate your attic. You need to stop moisture from inside the living space from getting into the attic, and you need to provide a way for any moisture getting into the attic from outside to get out. The solution you suggest, sounds to me like a risky one because you aren't doing anything to block indoor moisture from getting into the insulated space inside the roof, and it's not clear how thorough the waterproofing from above will be (although even if it is perfect from above, you'll still get moisture within from below).
To prevent moisture from the living space from getting into the attic, you could paint the ceilings of any rooms below the attic with two coats of oil-based paint or latex vapor barrier paint (regular latex paint won't do). In a flat roof with an open air space (where the roof joists are not also the ceiling joists), you could install a vapor barrier on the inside the attic above the ceiling joists, but this doesn't sound like your situation - it seems the joists for the ceiling are the same as the joists on which you are laying the roof boards.
If the living space walls are open along the tops you may still get living space air leaking into the roof space from the walls, in which case you'll need to seal these openings from above when you remove the roof boards. Also, any ceiling openings for wiring or ductwork are potential sources of air leaks that need to be addressed before adding insulation to the joist spaces. Much of this should become clear as you remove roof boards to see what you have underneath.
If the insulation you add is permeable, you'll need a way for moisture to vent out, unless you're able to completely seal the attic space from anything outside. I can't think of a way to do this effectively without putting a vent at each end of the opening between each pair of joists, above the insulation level. That's a lot of vents, assuming your joists are on 16" or 24" centers. And since you need air space above the insulation, you'd only be able to put about R-24 (6" of batt) because with less than 6" of air space above the batt you won't get effective circulation
Placing rigid foam board insulation on top of the open air space may help a little if you then cover the foam to protected it from weathering, but that does make a future roof deck a challenge. (I haven't heard of Polyico so can't comment on how well that would work.) Another option would be to look at roof decking materials for flat roofs that come with insulation below. In this situation you would have your 12" of empty air space, followed by a roof deck containing 4 or 6" of insulation below the deck itself.
My hunch (and it's just a hunch) would be that injected foam such as Icynene insulation would do the trick at a relatively low cost. It will seal the attic airspace from both indoor moisture and outdoor leaks since it's impermeable, and it has a good R value of around R-4 per inch. I doubt you would have moisture problems here as long as the roof membrane doesn't leak - the risk would be joists getting wet due to a leak from above, and having no way to dry out because there's no open airspace around them.
Not that many insulation contractors carry Icynene but a local contractor may have an equivalent product in your area. That's not to say they'll be keen to do this - the contractors I used were happy to fill a 1 1/2 inch space in my walls with Icynene but were definitely not keen on filling an 8" deep cavity above my front porch with it, because Icynene isn't really intended (from what I can tell) for filling really wide cavities. There's also a small risk that, if it's not done properly, too much of the foam can be injected, which can cause wall envelopes to buckle; my basement stairwell wall bulges out a good inch because of this, and there was just 1 1/2 inches of airspace there; with a 12" airspace you could blow the drywall or plaster to bits!
Sorry I can't provide a definitive answer but hopefully the above gives you some food for thought, and some idea of the humidity risks involved in sealing a flat roof attic.
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