I'm trying to address a poorly insulated and air sealed cantilever, and will have the adjacent downstairs ceiling opened up (so I'll be accessing the joist cavities from the side, rather than by removing the soffit from outside). I'm hoping to get some feedback/advice on my current plan, as this is the first time I've done this sort of project!
There's currently aluminum unvented soffit on the bottom of the cantilever. My current plan is to slide rigid foam board (cut-to-size) in and put it on the bottom of each floor joist cavity, using spray foam to keep in place and seal edges. Then, I will fill the remaining space with fiberglass, trying to fill the entire remaining space (the joist cavities are 15" wide, 9 1/2" tall, and ~40" deep). By the way, we live in Alaska, so it's COLD (currently single digits).
Questions:
* First off, does this sound like a good plan, to achieve both air sealing (w/ the foamed in rigid board down below, resting on top of the metal soffit) and thermal insulation?
* Should I use faced fiberglass, with facing up to the subfloor? Would I be able to install that from the side, without great reach (i.e., do I need to affix that/seal the facing to the subfloor to be effective)? Or, could/should I use unfaced, since the foam board will be sealed on the lower/cold side? Would foil-faced foam board (polyiso, like R-max Thermasheath) serve as a vapor barrier as well, so that I can use unfaced above it?
* What thickness foam board would be recommended? With 9 1/2" height to work with, we're trying to get the best insulation balanced with ease of installation and reasonable cost. [I have considered spray-in foam, but we're trying to do it cheaper.]
* I've got to consider what thickness fiberglass roll/batt is available (once I figure out if I will be using faced or unfaced), as I know I shouldn't compress the insulation. Do I need to have an exact match height-wise (meaning, if I have 7 1/2" of open space above the foam board, I should only use a 7 1/2" height fiberglass product) -- or, can I use a height of fiberglass slightly taller than my space if needed, knowing that it may compromise listed R-value some, but still will be good insulation for that space (e.g., if I use 8" tall batt in that 7 1/2" high space)?
Thanks much for your help/feedback -- any advice would be appreciated!
Katie
Anchorage, AK
There's currently aluminum unvented soffit on the bottom of the cantilever. My current plan is to slide rigid foam board (cut-to-size) in and put it on the bottom of each floor joist cavity, using spray foam to keep in place and seal edges. Then, I will fill the remaining space with fiberglass, trying to fill the entire remaining space (the joist cavities are 15" wide, 9 1/2" tall, and ~40" deep). By the way, we live in Alaska, so it's COLD (currently single digits).
Questions:
* First off, does this sound like a good plan, to achieve both air sealing (w/ the foamed in rigid board down below, resting on top of the metal soffit) and thermal insulation?
* Should I use faced fiberglass, with facing up to the subfloor? Would I be able to install that from the side, without great reach (i.e., do I need to affix that/seal the facing to the subfloor to be effective)? Or, could/should I use unfaced, since the foam board will be sealed on the lower/cold side? Would foil-faced foam board (polyiso, like R-max Thermasheath) serve as a vapor barrier as well, so that I can use unfaced above it?
* What thickness foam board would be recommended? With 9 1/2" height to work with, we're trying to get the best insulation balanced with ease of installation and reasonable cost. [I have considered spray-in foam, but we're trying to do it cheaper.]
* I've got to consider what thickness fiberglass roll/batt is available (once I figure out if I will be using faced or unfaced), as I know I shouldn't compress the insulation. Do I need to have an exact match height-wise (meaning, if I have 7 1/2" of open space above the foam board, I should only use a 7 1/2" height fiberglass product) -- or, can I use a height of fiberglass slightly taller than my space if needed, knowing that it may compromise listed R-value some, but still will be good insulation for that space (e.g., if I use 8" tall batt in that 7 1/2" high space)?
Thanks much for your help/feedback -- any advice would be appreciated!
Katie
Anchorage, AK