Polyiso insulation for my flat roof in Tucson
I have a 30-year-old, flat-roof house in Tucson with 9 inches of fiberglass batting installed between the 2X10 roof joists, no attic.
I need a new roof and I would like to add polyiso rigid insulation on the roof over the plywood sheathing with the new fiberglass roll roofing installed over the polyiso panels. I want the polyiso insulation to increase the R-Factor of the roof particularly for the hot summers to reduce AC load.
Currently, my house has vents in the eaves at the lower end of the roof and vents in the roof at the upper end of the roof between each set of roof joists.
When the rigid insulation is added to the roof, should the vents be sealed on each end to keep the outside air from infiltrating beneath the polyiso insulation. It seems that if the outside air gets under the roof sheathing, it will largely negate the benefits of the polyiso insulation, so the vents need to be sealed.
Thank you.
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Replies
If you want the roof insulation to reduce summer AC loads, you'll be better off leaving the vents open. If you also want to reduce heat loss you'll have to seal them.
The most important thing you can do to reduce solar gains is to install a white roof.
Before you do that I'd look at Insulam from Premier SIPs. It is a product for roofs that is essentially a SIP with one side. It has OSB attached to 5 inches of polystryrene foam achieving a rated R24. 2x6 framing goes all 'round the edges.
You'll want to plug those air intakes/exits and create a "hot roof". You'll also want to talk to your building department about it and run it past them.
You really don't want to roof over foam. Think of people walking on it and how it will hold up.
Good luck
And of course, install a cool roof material for reduced heat gain and longer life.
Flat roofs are installed all the time over rigid foam. They make tapered foam boards specifically for this purpose, though they are often polyiso rather than XPS.
What George is describing is a nailbase insulation, not a SIP. Hunter Panel makes them with polyiso.
If you are more concerned with summer cooling loads, then you don't want a "hot roof", you want a cool roof which requires a reflective surface and venting. If the winter heat load is acceptable now with just the fiberglass, then you'll also have a more durable roof if it's vented, as it's less likely to accumulate moisture.
CR Teeple,
Commercial low-slope (or flat) roofs are installed all the time without venting. In fact, it is far more common for such roofs to be unvented than vented. I suggest that you plug your roof vents.
As Robert correctly noted, many types of commercial membrane roofing are routinely installed over rigid foam. Just be sure that your roofer recommends your specified roof details and approves the use of your chosen membrane over foam insulation.
I'm going to continue contradicting the conventional wisdom.
Unless you have an air-tight ceiling plane, fibrous air-open and vapor-open insulation like fiberglass should be vented to prevent moisture accumulation.
If your primary goal is to reduce summer solar radiant heat gain, then the vented roof will do that better than a "hot roof".
Since the venting is already in place, it makes far more sense to leave it in place.