Ventilating shed roof overhang
bh1
| Posted in General Questions on
Hi, I’m building a home for a client in the PNW with a shed roof detail. The frame consists of GLB rafter beams, 2×6 car decking, 1/2″ cdx, 2×6 on edge with sprayed foam to r 38, 5/8 cdx and standing seam metal roof. The pitch is 2.2/12 and the high end of the roof cantilevers 4′ beyond the face of the building. That cantilever is non insulated and shows two continuous soffit vents at the high and low end of the cantilever cut into the card decking. I’m wondering if the venting is warranted?
Thanks for your time,
Bill
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Replies
Bill,
It sounds like you are creating an unvented attic. Don't you think the assembly would be safer if the overhangs were sealed as well?
What type of spray foam are you planning to install? New research suggests open cell foam can lead to moisture problems with the roof sheathing. (Some discussion of this issue here: https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing)
Are you planning to install the 1/2 inch CDX in an airtight fashion?
Will the metal roofing manufacturer warranty the roof? It seems like you are pretty close to the minimum pitch some suppliers might allow. Wouldn't a membrane roof be simpler and safer?
Hi Steve, yes the 5 1/2" space above the decking is designed to be filled completely with sprayed closed cell poly over the conditioned part of the building. When you say "installing the 1/2" CDC in a airtight fashion", do you mean adding a vapor barrier over the ply prior to spraying the foam? Is that required when using closed cell poly. I don't know if it make sense to foam or ventilate the overhang. I guess that's why I'm here
Bill,
There's little point in venting the soffits. The only source of moist, interior air is whatever makes it's way through the hard to air-seal t&g car decking at the exterior walls. I doubt enough of it will end up in the overhang to cause problems. Extending the 1/2" cdx and spray foam several inches beyond the walls might be a good idea.
Malcolm, the CDX will cover the entire roof diaphragm as per engineering. But I can move the 2x6 header between the (foamed) heated and non heated space beyond the exterior wall a few inches.
Bill,
I'm not sure that's worth doing as long as the plywood goes out. In fact i'm not sure the problem is serious enough to worry about. The soffits are fine without vents.
I agree with Malcolm. There is no need to ventilate the soffits -- although if there are soffit vents, their presence is harmless.
Bill,
Since the CDX is taking the place of drywall under the car siding, I was asking if you intended to install it in an airtight manner to minimize the risk of moist interior air infiltrating into the attic space.
(I hope you using a closed cell foam with one of the low GWP blowing agents. They are much better for the environment.)
I've always designed ventilated soffits with conditioned attics since we know that water can move against gravity for long distances by cohesive and adhesive means to materials; its just a good insurance policy. Someone told me long a go that "physics always trumps chemistry".