Question
We need an insulated white roof on our home. Which is the best membrane, TPO or PVC? How thick should the polyiso be?
Our attic is ridiculously hot 8 months out of the year even though it is insulated, and our AC ductwork is up there. The roof is 1/3 flat and 2/3 medium-low pitch. We figure we could cut our energy consumption in half if we could greatly reduce the heat radiating into the attic.
Answer
Although I'm not really a fan of either TPO or PVC, I have to grudgingly admit PVC has a better reputation for lasting a long time.
What I might recommend, rather than replacing your roof on the pitched side, is to consider a non-vented attic. This is done by putting the insulation in the rafters instead of laying it on the attic floor. This way the attic becomes a conditioned space, so your ductwork is now inside.
- This is a delicate operation, though, because you do not want to develop an area where moisture is trapped between the roof and the insulation. This can reduce the life of your roof.
- There are several foam products (mainly closed cell insulation) on the market that address this concern, but they can be rather expensive.
I recommend finding a local home foam insulation installer who understands the issue and knows how to install the right product properly. I first learned of this concept from Building Science Corporation and have used it successfully in Southern California.
Another option is to add insulation above the roof sheathing prior to installing a new roof.
- A good reflective and high-R-value insulation placed to resist heat before it ever gets into the attic is extremely effective.
- Again, an experienced installer will use details that prevent roof decay caused by the moisture of trapped air and condensation.
For more information:
Read Steve Saunders's Q&A "I'm converting my attic into a non-vented conditioned space. Should I use open cell or closed cell foam to insulate?"
