Spray Foam For Unvented Cathedral Ceiling, Climate Zone 2
narnal
| Posted in General Questions on
Hi everyone. I am converting a detached garage to a living space in Pinellas County FL, which is climate zone 2 (hot/humid). The garage has a cathedral ceiling with 2×6 rafters. I have decided to go with an unvented roof assembly using spray foam, so that I don’t have to extend the rafters. Should I use open cell or closed cell spray foam?
My understanding is that I would want the walls to be vapor open, but I am unsure of the ceiling. I have heard of condensation issues with ceilings that have open cell spray foam but I believe that was in colder climates.
Thanks for your help,
Nick
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Replies
I talked to an insulation contractor today and he said they use open cell for unvented ceilings most of the time in Florida. Does this sound right?
In your climate, open cell will probably work fine, especially if the living space is maintained around 50% RH.
Has anyone presented a case where the open cell has been a problem when the interior side is reasonably conditioned and, in colder climates, an interior side vapor retarder has been used? Ie, is there good evidence that Lstiburek is wrong? Or that a cathedral ceiling isn't equivalent to a conditioned attic?
"Again, I repeat, it is ok to use open cell low density spray foam in conditioned attics….but the attics need to be conditioned. There has to be a moisture removal mechanism. I recommend air change."
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer
As far as I can tell, data and simulations from some wrongly done roofs has been overgeneralized to malign a cost effective and environmentally lower impact roof design.