GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted 汉堡/搜索图标

Community and Q&A

Is an unvented roof assembly with dense pack cellulose and rigid foam above the sheathing safe in a hot climate?

Will4| Posted inEnergy Efficiency and Durabilityon

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 Prime

加入的主要社区建设科学实验erts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay||#1

    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.

  2. Will4||#2

    I forgot to include that detail. My name is Will. Thank you for taking the time to shed some light on my question.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |