Question

We need an insulated white roof on our home. Which is the best membrane, TPO or PVC? How thick should the polyiso be?

Asked by Kimberly Trombley
Saint Petersburg, FL

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

Polly Osborne, FAIA, LEED AP

Answered by Polly Osborne, FAIA, LEED AP

Los Angeles, CA

Polly Osborne Architects

October 16, 2010

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?"

Tagged In: spray foam, attic insulation, white roof

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