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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
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.
Would this still be considered a conditioned space? The space below is conditioned.
If you don't supply or return air from it, it's not really connected to the rest of the conditioned space. See IRC R806.5 for unvented attics (R408.3 for unvented crawlspaces). And actually it looks like newest code updated it to 50 CFM per 1000 sq. ft.
Edit: The code distinguishes between vented and unvented. And in your case, you are unvented. Not really any ambiguity to it in my opinion. So R806.5 applies to you, just not sure if they have an alternative supply air formula for short height spaces.
Thanks. I think I'm good to use closed cell spray foam, keep it unvented and dont need to circulate air. Looks like all of section 5.2 applies to zones 1-3. I'm zone 5
Before we jump on the spray foam band wagon.
Let’s talk about the facts
1 Spray foam is the most expensive way to but an R of insulation.
2 Sprat foam is the least green way to buy an R of insulation.
3 Spray foam is the riskiest way to but an R of insulation.
The way I see it large areas of spray foam in new construction is a design flaw made by someone looking for an easy answer with zero regard for the budget or the environment. It seems that ship has sailed and the building is up.
I can’t tell from the photo in you have room for R38 of cheap fluffy insulation and a vented attic but designing in and building the space would have cost much less that the spray foam bill will be.
Note if you spray foam the roof and fail to condition the attic air the risk is the dew point of the attic air will get higher than the surface temps in the attic. When this happens, things get wet, if they get wet often enough and stay wet for long enough mold starts growing and things start rotting.
Walta