Unvented ceiling
mtntosound
| Posted in GBA Pro Help on
Hey all,
I know this has been asked a million times on this site but I’m having trouble finding any answers for my specific situation.
I have a 2.5 /12 sloped roof that I can’t/don’t want to vent. Trying to determine the best course of action, this is a new build. House will be to close to a property line to vent from the soffit and all the products I have seen that allow you to start your vents on the top of the roof require a greater slope to use them. I’d prefer to not use spray foam in the ceiling but haven’t ruled it out. Roof is a shed roof built with 14″ tall i joists.
Any ideas would be appreciated, thanks.
Jesse
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Replies
What climate zone is this house being build in? What is your ultimate R-value goal here?
Different answers call for different approaches for "...the best course of action..." on a new build house with 14" I-joist rafters and very low pitch.
https://www.greenbuildingadvisor.com/article/all-about-climate-zones
Hey Dana,
Thanks for getting back to me.
Climate is a marine 4c. I'll have an r value of 49 most likely. I guess my main concern is if I try and set it up for cold sheathing, can I air seal it well enough to not let any indoor air make it up to the sheathing? I'll also have an hrv unit so should be able to keep moisture at a consistent rate.
Jesse,
Flat-ish roofs are hard to vent (there is less air movement from stack effect and your roof would have no ridge vent. Varying wind would still offer some air movement due to pressure imbalances though...)
Your climate is tough on vented roofs. Vented "cold" roofs seem to be prone to mold growth. Unvented cold roofs also seem to be prone to mold growth. Mushrooms grow on roofs where you live. Yikes!
If it were me, I would try and get some rigid foam above your roof deck (3-4 inches of Polyiso would be awesome). I would then ask my insulation team to add insulweb in such a way that only half of the I joist cavity is insulated. If this isn't an option, then consider simply spraying about 7" of open cell foam into the rafters along with the exterior foam. This would achieve a favorable ratio of exterior to interior insulation.
I would avoid the closed cell foam but it might be the simplest option, just as you said.