Spray foam attic with cathedral ceiling
I’m considering spray foaming my attic for energy efficiency and elimination of dust. The attic currently has blown in fiberglass insulation originally installed when the one-story house was built in 1993, which I plan to get vacuumed out prior to spray foaming. Last year I had a metal installed with synthetic underlayment on my A-frame roof. I have seven soffit vents on each side of my home. I also have a cathedral vault ceiling, which limits access in the attic. No ridge vents only a few 18″x 18″square style attic vents placed across the roof for attic ventilation.
I’ve gotten a few bids for vacuuming and then spray foaming my attic. All of the bidders propose to seal off the peak of where the cathedral ceiling apexes in the attic. I question if this puts my cathedral ceiling (20ft x 20ft) at risk for wood rot and mold, since soffit vents will continue to allow air to flow in and up the tight (maybe insulated with batts) cavity between the roof decking and drywall of the cathedral ceiling. I don’t have any attic vents above the cathedral side of the roof.
I’m in Austin, TX and envision hot summers where extreme hot air is trapped in the cathedral ceiling sealed off from the rest of the attic without proper outflow. Am I overly concerned about this being an issue? The only other option I can think of is to tear down the dry wall of the cathedral ceiling and spray foam then re-drywall, but this doesn’t sound very cost effective. Any experiences with this, alternatives, or suggestions? Please advise.
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If your existing cathedral ceiling is insulated with fiberglass batts, then you are required by code to include a ventilated air space between the top of the fiberglass batts and the underside of the roof sheathing. This ventilated air space needs to be connected to soffit vents (to allow outdoor air to enter the ventilation space) and to a ridge vent (to allow the air in the ventilation space to escape).
So the answer is no -- you can't block the ridge vent. And if there is no ridge vent, you need to create one.
Here are links to two articles with more information:
How to Build an Insulated Cathedral Ceiling
All About Attic Venting
-- Martin Holladay
Martin,
Sharing more info and hoping for some suggestions. My cathedral vault cavities (drywall) in the attic only goes up about 2/3% the underside of the roof decking. The rest is open attic space where studs and roof decking exposed. See picture attached. Any suggestions on how to tackle this situation when spray foaming the attic?
There are a variety of half-measures you can try, but the best way to solve this is to remove the drywall from the cathedral section and insulate that section properly.
Alternatively, you can abandon your plan to use spray foam in your attic, and instead install one or more thick layers of rigid foam above the roof sheathing. If you do this, you'll need to install an additional layer of roof sheathing above the new rigid foam, followed by new roofing.
-- Martin Holladay
What's your thought on removing each of the batts and probing each cavity with a wand extension to spray foam as alternative to drywall removal?
In an Austin TX climate it's not very risky to just dense-pack the rafter bay cavities with cellulose. (Including in the then-unvented attic section. It's not exactly to the letter of IRC code, but it should work just fine. See table 3 of this document (and read the whole thing until you understand it):
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
According to the WUFI simulations, up to R30 of cellulose works just fine in Miami, Houston, & Phoenix (with some risk in Houston if it's a tile roof), and even in somewhat cooler Atlanta as long as the interior isn't kept at a high-humidity in winter. Austin is on the dry edge of zone 2A, making it's wintertime moisture accumulation levels between those of Phoenix (zone 2B) than Houston (zone 2A, but on the more humid side. of 2A.)
Then it's time to re-roof you can further mitigate the risk you can add an inch of foil-faced polyiso under the new shingles, which would make it in line with the IRC code prescriptives for unvented roofs.
Thank you for sharing this cellulose option. Would dense packing the rafter bay cavities with cellulose work with the existing batts in place or should I try to fish and remove the batts out prior to cellulose application?
Most cost effective way to save energy is to run a BeOpt simulation, it can calculate the payback and the savings for you. It's a free download.
But really, quickest payback is going to be mini split ($500 install, this will also bypass your leaky dusty ducts), you should also consider slight positive pressure to get rid of the dust.