Is an unvented roof assembly with dense pack cellulose and rigid foam above the sheathing safe in a hot climate?
I have a novice question.
I am preparing to construct a roof assembly for a cupola in climate zone 4A. It will be a cathedral roof framed with 2×12 rafters with 11.25 inches of dense pack cellulose. I am planning to add three inches of polyiso on top of the Zip system roof deck to achieve R-60+. I understand the concept of the assembly not needing to be vented due to the rigid foam keeping the sheathing above the dew point in cold weather. However, is it possible that the cool temperature inside of the structure, during hot weather, could reach the sheathing and condense? Does the amount of dense pack cellulose offer a low enough conductive potential?
The structure is fully exposed on top of a hill, south facing, and we have summers reaching the mid 90s with high humidity. The interior will be cooled around 5 months out of the year.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
User-6893474,
First of all, can you tell us your name?
Q. "Is it possible that the cool temperature inside of the structure, during hot weather, could reach the sheathing and condense?"
A. The type of summertime condensation that you are worried about can't happen in the roof assembly you describe, because there are probably two different vapor barriers (the roofing and the 3-inch-thick rigid foam) on the exterior of the assembly to prevent inward vapor drive.
Remember, during the summer, the interior of the house is relatively dry, while the exterior air is relatively humid. The direction of vapor drive during this season is inward.
To prevent condensation on the interior surfaces of your roof assembly, all you need is at least one vapor barrier near the exterior of the assembly. If you have that, there won't be any inward vapor drive.
So don't worry.
I forgot to include that detail. My name is Will. Thank you for taking the time to shed some light on my question.