More Q&A Spotlight
Over the last couple of years, the sudden increase of wildfires in regions not accustomed to widespread conflagration has given a darker meaning to the phrase “global warming.” In areas that are historically prone to such events, strict guidelines for fire prevention have been mandated for all new construction—and these areas are growing. For roof assemblies in particular, the assembly and/or the roofing materials often must be rated Class A, the highest rating for fire resistance. In the high fire zone of California’s Bay Area, GBA member “Schmoot” is in the process of building a house. Staying within the guidelines, he’s installed a fire-rated, nonpermeable Pac-Clad HT underlayment and a Pac-Clad standing-seam metal roof. He’s also going with an unvented attic, as soffit and ridge vents create a ventilation flow that’s a perfect channel for introducing burning embers into the attic.
In the roof assembly he’s planning to insulate with a flash-and-batt strategy—spraying 2 in. of closed-cell spray foam against the underside of the roof deck that creates an air seal that theoretically precents any condensation forming on the interior side of the plywood, followed by unfaced batt insulation, likely mineral wool, which he likes for its sound deadening qualities and because it is vapor open so the assembly will have some inward drying potential. But Schmoot still wonders what might happen if any water gets between the roofing materials and the spray foam. Will the sheathing be able to dry?
He also wants input as to whether he should insulate the enclosed soffits, which will be finished with stucco. His architect wants to fill them with spray foam, claiming it will help with fire resistance. Schmoot’s contractor, on the other hand, argues that fiberglass will work just as well.
He asked,…
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7 Comments
Good article. Other than Vulcan vents what else is out there for soffit vents in WUI areas?
https://osfm.fire.ca.gov/what-we-do/fire-engineering-and-investigations/building-materials-listing
freyr_design,
Thanks. Good resource.
A Flash and Blow may be a better solution! A monolithic layer of closed cell spray foam applied directly to the TOP of the roof deck will add an additional air, vapor, thermal, and water control layer that eliminates the risk of structural damage that is caused by spraying the underside of the roof deck. The spray foam is then covered with a thick layer of Blown in Rock wool that provides the fire protection. see https://www.greenbuildingadvisor.com/article/introducing-the-insofast-x-bracket-system
See Flash and Blow attachment below
insofast,
Connecting a wall rain-screen cavity of that depth with the roof vent channel above is a terrible idea in a wild fire area. Here in BC it is expressly prohibited in our building code.
The IRC also prohibits it, Malcolm, but the language isn't particularly clear and I've never seen it enforced: https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction#IRC2021P1_Pt03_Ch08_SecR806.1. "Required [roof] ventliation openings shall open directly to the outside air ..."
Michael,
When Joe Lstiburek looked at the risk of fire in rain-screens he said that if the cavity was 3/4" or narrower it would not sustain flame spread. Code aside, connecting the two is probably safe if the wall cavity is no more than that, But an 1 1/2" vent space made of combustible 2"x4"s is introducing risk where it isn't necessary.
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