insulating unvented inaccessible attic
pipefitterlu42
| Posted in General Questions on
I’m putting an addition on my house. I’m looking for the best way to insulate the ceiling. Can I use closed cell spray foam to the bottom of the roof deck? Will there be an issue with the unvented area between the ceiling and the spray foam? Do I need to vent the soffit and insulate the ceiling joists instead? See the pic for what I’m dealing with.
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Replies
What climate zone are you in, and do you know the code mandated amount of insulation for your area if the addition is conditioned space?
It's difficult to tell the rafter and joist sizes from the picture, but it seems you have not much cavity depth for insulation. Are those 2x6 ceiling joists and 2x8 rafters?
The roof slope is very low, which would generally need a fairly deep ventilation channel if you wanted to make a vented roof, which further reduces the available insulation depth.
There is no issue with an un-vented area between the ceiling and spray foam. It's no different than a conditioned attic, you conceptually just have a very low slope roof with a super small attic. All of the rules for air sealing, etc., still apply like any conditioned attic.
I'm zone 5.....R38 for ceilings/roofs. The rafters are 2x8 and ceiling joists are 2x6. I'm on Lake Erie and the winters are brutal. Was looking at spray foam to keep things tight but didn't want to run into moisture issues with the dead space above the ceiling........I agree with you, there wont be much space once any sort of insulation is added.
Shouldn't be an issue if spray foam is against the roof sheathing and the ceiling is just painted drywall, but technically you are supposed to have some supply or return air movement in conditioned spaces like attics and crawlspaces. The required amount is 20 CFM per 1000 square feet but I haven't read how that formula works for spaces that are very short in height.