Insulating a Flat roof with 2 feet of air space
JamesA
| Posted in General Questions on
I live on the North Costal of BC (Zone 5) and recently reconstructed an old war time house and removed an old pitched roof and installed a new flat roof. The perimeter is soffit vented and the trusses provide me with 33 inches of air space between the finished ceiling. What would be the best way to insulate this low-slope roof? I have sealed all plumbing and electrical penetrations and told the wife no recessed lighting. 😉 Would fiberglass batts work?
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Replies
Hi James, you in sunny Prince Rupert?
Pretty hard to passively vent a flat roof as there is no convective action. On a peaked roof the air will flow into the soffit and out the ridge, but a flat roof will not get the same effect.
If the roofing is already on you may want to look at closed cell spray foam applied directly to the roof sheathing. If the roofing is not on, you could insulate at the exterior, on top of the roof sheathing.
You may want to bring in some professionals for opinions and help.
Not sure if you have a GBA paid subscription, but this Martin Holladay column from 2013 covers the topic quite well, including recommendations from BSC and Bruce Harley.
Insulating Low-Slope Residential Roofs
Unless you’re careful, your low-slope roof can end up with damp sheathing
https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs