Hybrid Vented/Unvented Attic
I’m interested in better insulating a portion of my attic. The majority of my attic is vented via soffit vents and a ridge vent, however in my kitchen the floor is cantilevered and the ceiling is dropped which eliminates the soffit venting. Currently the dropped ceiling is only filled with poorly fit fiberglass batts. My thought is to fill the dropped ceiling with either closed cell spray foam or dense-packed cellulose. I’d like thoughts on those options.
A second part of this scenario, is that the pinch point created by the dropped ceiling is only a few inches deep, thus not a good thermal break and that part of the roof always melts snow first in the winter (zone 6, southern Wisconsin). Ideally that small gap would be filled with spray foam to achieve the highest R-value possible, but doing so would completely seal off the lower unvented portion. If I were to use cellulose there, should it not be sealed off to be able to dry to the higher vented part of the attic?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Can you add angled framing members parallel to your rafters that extend from the lower, inner corner of the interior soffit? That would allow you to get a decent amount of insulation at the pinch point while maintaining roof venting. Foam, either sprayed or cut-and-cobbled, would get you the highest R-value, but long-term it wouldn't be a lot higher than just stuffing the space tightly with mineral wool or fiberglass.
When you have skimpy insulation near the eaves, it's important to maintain a good vent space to keep the roof surface as cool as possible. It's also important to make that area as airtight as you can get it from the interior.
I'm not clear on what you mean by adding framing members. This portion of the attic is not vented as there is no exterior soffit for air intake.