Air-Sealing an Attic with Spray Foam
Our 1929, 1 3/4-story home is in CZ 6 (southwestern Ontario). There is no knee-wall. Rather, the slanted walls in the second story interior are under the pitch of the roof. The attic is walk-up, but the height from joist to peak is only 5′, so there’s really no way to use the space for anything except storage, which we’d like to preserve.
There is currently a lot of air leakage between the 2nd story and the attic. We are planning to air seal and insulate the underside of the roof with closed cell spray foam.
There is woefully inadequate loose insulation in the attic floor. The same material has been pushed down under the pitch below the attic floor (and surprisingly, that area looks good on thermal imaging).
The insulation salesperson says they can’t really remove the loose insulation because it probably has nails and other debris in it. (This sounds a little suspect to me.) He says that they will push the loose insulation further down under the pitch and spray the roof deck as far down as they can. They don’t seem to plan to create a seal where the attic floor joists meet the roof pitch. I think that to get the air seal we want, we need to either seal along that juncture, or else replace the soffits and seal off the eaves from the bottom.
We will not be doing any of this work ourselves, so we are limited to what we can get a contractor to do.
1. Which is our best bet from an air sealing perspective?
2. If we seal from the top, is there a problem with leaving the soffit vents as they are? (We assume there are vents. The soffits are some perforated material, at least 16 years old.)
3. To seal from the top, what should we be asking the insulation people to do?
4. Since there isn’t enough vertical space to make the attic into living space, we are inclined to spray foam over the rafters to mitigate thermal bridging. The insulation person thinks this could lead to dry rot due to the ice and water protector layer under the existing shingles. Is this a real worry?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part