r/homeimprovementideas • u/NYBAM • 4h ago
Work In Progress Help insulating sofit area under 2nd floor dormer
Hey everyone my house has a large air leak after investigating under the lower soffit that’s under my 2nd floor dormer. My dormer sticks out past my house about 2 feet past the house so under the soffit I can see the floor joist and the 2nd floor subfloor.
My concern is every floor joist cavity I can clearly see into my house. I can see my 1st floor ceilings so I’m pretty sure that needs to be blocked off. I plan to block that and add some rockwool on the inside as I close it up.
My question is what do I do about the subfloor above? It’s bare wood too in every joist cavity and ideally I want to insulate that before closing up the siding again. The floors in that area on the 2nd floor get cold and now I know why .
So do I just cut foam insulation board to fit them close up the soffit again? What about flammability and maintaining a flame resistant barrier when using foam. Also at the bottom of the joist there’s no coverage so if I did a whole bottom sheet in 1/4 wouldn’t that create a stale air cavity now where humidity will build up inside that soffit area (because the 1/4 would go above the soffit area then the ceiling of the soffit would be foam board insulation and the floor joist at the house framing would be boarded up with 2x10.
I was thinking rock wool on the ceilings too but I’m afraid of mold and moisture especially from any unexpected drips from the siding above. The foam makes the most sense for outside but the correct way with regards to safety and moisture build up is where I’m stuck
Also what if I just did the cavity after I board it up with a spray foam insulation kit. I see kits that have fire resistant ratings , but how will that hold up with moisture and rot given it’s in a ventilated space due to the soffit below