Unvented Roof for Cold Climate – Detail Question
Hello!
I’ve been researching the feasibility of converting to an unvented attic with a cathedralized ceiling. I’m aware that the best option is closed-cell foam, and that’s the route I’m planning on taking, barring any complications.
I found a detail in the Detail Library (Unvented Roof for Cold Climate, Raised Plate; Asphalt Roofing) that seems to show a standard batt-style cavity insulation with properly taped and sealed board insulation below. This contradicts what I’ve read elsewhere in articles and posts (even on this site); what makes this detail different, and other than the obvious of ensuring proper sealing, is there anything specific not mentioned in the detail to ensure this gets installed correctly?
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
epalumbo,
Can you link to the detail you are describing?
https://www.greenbuildingadvisor.com/cad/detail/unvented-roof-for-cold-climate-raised-plate-asphalt-roofing
epato,
Thanks. That's a very odd detail to find here for an un-vented roof. The title and the drawing seem to directly contradict the text.
I don't know of any code that allows permeable insulation to be in contact with the sheathing above without either having:
- A vapour diffusion port - and only in the warm climate zones 1, 2 and 3.
- Continuous exterior insulation above the sheathing.
The code compliant cathedral ceilings that we know work are shown in this article:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
If there is sufficient insulation value in a low-permeability foam board, it can safely be used inside the fluffy insulation rather than on top. I've done it myself in Zone 4, So long as the potential dewpoint is inside the foam layer, the fluffy stuff will always be dry. In the projects I've done, we used foil faced polyiso and taped the seams with foil tape to make the interior VB very tight. While it does work, it is still is riskier assembly than putting the foam on top of the sheathing with fluffy stuff inside.