Better to spray foam walls or crawl space/floor?
Hi, I’m building a 20×20′ apartment. I know I’m doing flash-and-batt in the cathedral ceiling (2×8″ rafters with 3.5″ of closed cell + 3.5″ batts). For the walls/floor, my plan is/was to use traditional batts (less expensive, lower GWP, easier repairs). Air sealing is important to me, so I was going to use Certainteed MemBrain on the walls before drywall goes up, and caulk the subfloor and bottom plates.
I have a quote for the flash/batt on the roof, but I’m not sure I trust the installer (very few reviews, some are fake/duplicates, and the 2 pictures from reviews look kind of sloppy/lumpy). They also seemed really keen on open cell in the unvented roof/walls, which I understand to be risky.
I found another installer that seems more reputable, but they have a $4k minimum on spray foam, and the roof only gets me to $2300 because it’s such a small space. Either the walls or the floor would get me over the minimum:
— walls: 2.25″ in the 2x4s would get me to code for $2400
— floors: 3″ to meet code for $1700
(There’s also a chance I’d get the crawlspace encapsulated/conditioned someday, but it’s had a history of water intrusion so I don’t want to seal it up until I know that’s solved.)
Are there benefits/risks to one vs. the other?
— walls:
—— pros:
——— helps air-seal the most difficult surface faster
——— more robust than MemBrain that could easily be punctured when doing repairs
—— risks:
——— could it hide door/window/flashing leaks?
——— more difficult to patch/repair?
— floors:
—— pros:
——— avoids saggy/gross fiberglass
—— risks:
——— what happens if there’s a water leak onto closed-cell foam? (perhaps not worse than w/ caulk
——— difficult to remove for future encapsulation
I’m leaning toward spending it on the walls, but let me know if you disagree!
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Replies
I would do the crawlspace walls and rim joist. If you are worried about flooding, stop the SPF 3" to 4" from the bottom. Most of your foundation losses are the section close to and above grade, this bit lost won't change your energy use much and would still be enough space to add in drainage if needed.
For conditioning, the simplest is an exhaust fan until you sort out water and radon issues if any.
As for sealing your walls, the best is taping the seams of the sheathing, not sure if that ship has already sailed on your project. A proper install of a warm side air barrier as you propose is also a good idea.
Hi Akos, thanks for your reply! Good idea, I'll ask the insurance installer about that approach for the crawl space to make sure it'd still pass inspection.
The siding is 80-90% removed but there's still [patched] house wrap over the sheathing, and I think we'd have to remove the last bit of siding, trim, electrical panel, and maybe windows in order to tape the sheathing properly. So warm-side air barrier it is! (This whole thing has been an exercise in accepting constraints/limitations... if only I had unlimited time/money/energy, I'd DIY it the best possible way and enjoy every second!)
Akos has the answer.
- MemBrane and/or gyp as the air seal on the walls is, as you noted, easily breached. Instead of filling the walls with spray foam, consider flash with foam and then batts. The spray foam will give you a solid air seal.
- The most difficult area to properly air seal is the rim joist to foundation. Spray foam it and you solve that issue.
You DEFINITELY want a conditioned crawl. Spray foam it now so you don't need to remobilize for it later.
I see cathedral ceiling as a design flaw. Simply to much important stuff needs to fit in too little space.
I except the fact the you are unlikely to agree so please be sure to read this article or read the new threads that seems to get posted each week titled something like “how to fix my cathedral ceiling”.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
I see spray foam as a red flag on a set of plans. Almost aways there is a better lower cost way to build without the spray foam.
Walta
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Where do you live that R-34 is acceptable in a roof?
I agree with Akos. Spray foam is airtight but it doesn't address most of the areas where air leaks actually occur, at the edges and transitions between materials. It's like using a sledgehammer to install trim. I find that making the sheathing layer airtight is by far the simplest and most effective approach.
R-38 is the requirement for the roof in central NC, but I think I could actually get away with less since it's a cathedral ceiling. The flash-and-batt solution I mentioned should get me to R-39 though.
I do agree, taped sheathing would be nice, but I don't think I can get behind the house wrap in most places since it's already attached and underneath windows, trim, siding, etc. I will be caulking around all top and bottom plates, between subfloor boards, blocks of studs, etc since that's the best I can do. An interior smart vapor barrier will be part of the equation too.