Converting Porch to Enclosed Space
I have a covered porch on 6×6 posts with sliders, 26×14 that I want to enclose as a 4-season room.
I live in coastal MA, zone 5.
I need help with my final insulation plan, and am seeking input.
Floor: 2″ XPS (r10) skinned and airsealed across bottoms of 2×10 joists (the XPS is already there), r-30 unfaced fiberglass batts between joists, subfloor (3/4″ OSB, ply or advantech), pine finish floor. Because of the limited roof overhang, there is not room to put significant rigid foam over the exterior of the band joist (more on the walls in a minute).
Question 1: is 2″ XPS on underside of joists enough? I believe 25% of total R is acceptable?
Question 2: no vapor barrier of any kind on the top of floor assembly, correct?
Question 3: If no rigid foam on exterior side of band joist, can I add 2″ XPS or 3″ foil faced polyiso on inside of band joist to reduce thermal bridging there?
Walls: 6×6 posts leave little room for me to run rigid outside of the posts, because that will leave me too little roof overhang (only 5.25″ at present). So, I am thinking (inside to outside): 2x4s, framed flush with interior side of 6×6 posts with rockwool in the cavities, then 1/2 zip system, window bucks, 1.5″ EPS or XPS. That would bring me flush to exterior side of existing posts. I would tape all foam seams, and tape over the posts themselves tying that into the foam. I realize the posts will be a thermal bridge, but I don’t have alot of room to add exterior insulation around them unless someone has an idea?
Question 1: Any concerns or comments on this wall assembly?
Question 2: I plan to side this entire thing in azek (colonial sunroom style with a wainscoting look), but still want to prevent moisture hanging behind the siding — is something like 1/4 slicker classic over the foam overkill?
Ceiling/Roof: 2×8 framing, 1×10 pine board sheathing. Roof shingles in good shape. Unvented cathedral ceiling. I realize that just spray foaming will leave me with a lot of heat loss through the framing, but get me to r-49. Dana Dorsett had suggested Bonfiglioli strips, with 1.5″ foam and 1″ furring strips underneath, allowing me to shoot 4″ of closed cell foam, then add r23 rockwool. I would drywall over either arrangement.
Question 1: Just closed cell spray foam will cost me more, but be much less labor — how much of a performance difference will I get going with the strips?
Question 2: Any concern with rotting out my roof sheathing in either scenario?
Thank you everyone for your feedback!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part