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!
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Replies
Justin,
Let's see if we can encourage some responses. On the floor, This article suggests at least R-30 in CZ6 (https://www.finehomebuilding.com/2012/03/08/how-to-insulate-a-cold-floor).
And here is an article from Dr. Joe that includes additional advice on insulating floors (https://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance). Note that Joe suggest including an air gap between the air permeable insulation and the subfloor.
If Dana offered a plan for insulating the ceiling, I would go with it.
Hi Justin,
Floor
Q1: Code-minimum for new construction in climate zone 5 is R-30 in the floor system, or “as much as will fit between joists,” one of the more comical exclusions in the code book. 2” XPS (best avoided for climate reasons, but if it’s already there, you might as well leave it) will perform at about R-8.4. With R-30 batts, as long as the foam is taped or there is another air control layer, it should perform well and be safe from moisture concerns. Floors are more forgiving than walls about the permeable: impermeable ratio.
Q2: Some drying to the exterior can occur through 2” of XPS, so you could probably have an impermeable layer at the interior, but it would be safer to keep it as vapor-open.
Q3: Yes, adding rigid foam on the inside of the rim joist is safe (in a heating-dominated climate).
Walls
Q1: That sounds like a decent approach. The Rockwool will be R-15 in the cavity. Polyiso would be a better choice for the exterior as 1 ½” will get you about R-9 (aged), a bit less in very cold weather but still better than the alternatives. I’m not sure what current MA code requirements are but the 2018 IRC in CZ5 allows R-13 cavity plus R-5 exterior, so you’re doing better than code. Put some effort into air sealing and you will have a decent wall. Be careful about the type of tape you use; during the heating season, warm, moist air will be doing its best to get outside, and you don’t want to block it with a large impermeable exterior surface at the posts. There are vapor-open tapes available.
Q2: It's best to vent behind an impermeable exterior cladding; without venting there is a risk of moisture accumulation. ¼” Slicker Classic might be enough but I’d rather see at least ½”, vented top and bottom, to ensure air flow. The polyiso will block moisture movement from the interior but it will still occur at transitions, posts, etc.. I’ll make a pitch to use a more sustainable material than Azek. It’s a nice product to have when nothing else will work, but there are many other options that would work for you, most with lower embodied carbon emissions.
Roof
Q1: Dana’s advice sounds good. Please seek out HFO-blown spray foam, which is available to all installers but not fully adopted yet. It has much lower carbon emissions than the previous blowing agents, about 25% the climate impact. Closed cell foam ages to about R-5.6 to R-6 per inch, so 7” will really be about R-40. 2x8 rafters are about R-8.7. Because thermal bridging has a big impact, assuming rafters 16” on center, the average value of that roof assembly is R-10. Adding the Bonfiglioli strips and the flash-and-batt insulation that Dana recommends, the roof average is R-40. One quarter the heat loss for just a little extra effort.
Q2: Both are fairly safe from moisture problems, but because closed-cell foam can shrink away from the framing, leaving a path for moist air to reach the sheathing, I’d add a variable permeance interior membrane.
Michael,
Many thanks for your reply. I have taken your feedback and other noodling into account, here are my follow-ups:
Floor
- All set with the floor.
Walls
- I have changed tracks and will use Zip-R, with 1.5" polyiso, outside the posts, and then r-13 rockwool.
- Zip-R is very hard to come by right now. Would I run into any trouble if I simply glued unfaced 1.5" polyiso to regular zip and sheathed the wall that way? The glue only has to hold until I get it up on the wall. Cheaper and I can do that without a 12-week wait.
Roof
- Which vapor permeance interior membrane do you recommend?
THANK YOU!
Following up on this.
I have 4-5" of CCSF between the rafters, 2" of XPS under the ratfters, and strapping perpendicular affixed with 4" deck screws. I plan to add rockwool now before drywall.
With the first round of spray foam, it was November in MA, and some delamination occurred between the rafters. I asked them to come back, they did, cut it away, touched up and it was good. I have since noticed though a few more minor gaps where foam has pulled away from the rafters. Space is unheated right now.
I realize in a perfect world, this shouldn't happen.
Realistically, does it "always" happen to a degree and should I proceed under that assumption?
Or should I address now and any separation is unacceptable?
I can:
1) Ask them to come back and repair, again
2) seal the minor gaps with my own touch n foam CCSF kit
3) Leave it alone and add a smart vapor retarder under the strapping and before the drywall
Feedback much appreciated.
Thx
Justin,
Are you sure they installed closed cell? It is usually rock hard and difficult to remove.
It's closed cell I was told. It's absolutely hard and you can't just peel it off -- tap on it and it's hard. When they returned they cut the delaminated areas back with saws and then re-sprayed.