Insulating a cantilever room (NorthEast)
dudleyr1032
| Posted in Energy, Heating & Insulation on
An 2nd story addition was built onto my home in the mid 80’s. Under the 12×24 cantilever room is the same size outdoor deck. The room is cold during the winter and it’s time to do something about it! The floor joists are 2×10’s with 3/4” plywood subfloor. The current insulation between the joists is R-30 batts. There is no air sealing. Just the insulation stapled up there. My question is, should I have it spray foamed, spray foamed with batts under the spray foam, spray foam and foam board??? Any insight as to how to properly insulate is greatly appreciated.
PS. It gets cold in Boston!
Replies
Is the deck attached to the cantilever? Do you have access to the space from below or just from the attic?
The deck below the room is completely open and I have 100% access to the room above.
I'd go in from below, air seal, and insulate with Roxul batts.
There's some good stuff in this piece: https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor
I had a smaller version of what you had, a 6x20 overhang which extended our great room. It was COLD. It also was a true cantilever, and the great room floor bounced. When I did my great deck project, I asked my engineer to include a footing and grouted/rebar CMU column on the plans for each corner of the cantilever. Jacked the cantilever a touch, put in treated top plate, and boom, no more bounce. Then I dropped the plywood they had used in '77 to finish off the underside. I cut 2" foam and carefully positioned and used expanding foam around it, and then filled the middle with Roxul and did the foam at the bottom in the same manner--exactly what's depicted in that article Andy Engel posted. I also had a vent in two joist bays, so built foam up around those and airsealed well. Dramatic, instant results.