Adding More Insulation to Existing Spray Foam
I’m an architect working on the renovation of a ca. 1800 cape in coastal NH (Zone 5), where we were surprised to discover closed cell foam (looks like a poor DIY job that *might* achieve R9) in the exterior wall’s 2×4 stud cavities. We obviously need to beef up the insulation, and the GC is not keen to devote resources to carving out the existing SPF. Since we’re residing this part of the house, I’d like to go with 1.5″+ mineral wool continuous insulation outboard of the sheathing and WRB (and inboard of strapping + vented cladding assembly). The GC would prefer to strap out on the interior (2×2) and spray additional CC foam to achieve R19 min. I don’t love the idea of additional closed cell foam in the walls for several reasons–but what do you think?
Also: we’re building two small additions (over crawlspaces) as part of this project and I’d love advice:
– We’re just 12″ above ledge, so an insulated slab and frost walls aren’t possible. Would it be better to: a) run a 20 mil pool liner over the floor (dirt/ledge), covered with gravel, and extend the plastic all the way up the interior of the CMU walls? We’d then apply rigid foam (taped) to the underside of the floor framing, and use that surface to support mineral wool batts in the joist bays and spray the rim joist. There will be some plumbing and electrical running in the joist bays, so thinking CC foam won’t work here.
or, b) spray foam the entire joistspace and apply zip or other taped sheathing to the underside of the joists, and apply rigid foam to the crawlspace walls?
Thanks so much!
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Replies
Is the existing foam achieving air sealing? If so you could probably get away with batts or blown cellulose on the interior.
The existing foam is a MESS. I don't think we can count on it for any air sealing value, but maybe a new WRB over the existing sheathing could achieve that?
You could fill the remainder of the interior studbays with open cell spray foam as another option. Personally, I like exterior continous insulation and that would be my preference, so the exterior Comfortboard would work. You could also use EPS for similar R per inch and a whole lot less cost.
I would NOT add exterior spray foam. I think that puts your sheathing at risk, and you'll get the same uneven layer out there as you ahve inside.
I can't quite visualize your proposed foundation wall assembly, can you attach a drawing or sketch?
If your crawlspace is only 12" deep, then you'll probably get better performance insulating the underside of the floor compared to insulating only the crawlspace walls. For best performance when insulating below grade walls, you really need to insulate at least down to frost depth, which you can't do with only 12" of accessible wall in CZ5. I would use polyiso under the joists, at least 1" thick, then fill the joist bays with either batts or loose fill insulation (cellulose or fiberglass). Tape the seams of the polyiso and seal the perimeter to give you a good air barrier. I would absolutely not spray foam the joists or the underside of the floor here.
Bill
Hi Bill,
Here is an in-progress WS detail. For the addition floors, we'll at least 24" clear space under the floor joists to maneuver. Thanks so much for your thoughts.
For the walls of the existing building, I wouldn't go with spray foam either, but am wondering if 2" exterior rigid foam raises any red flags given that the wall won't be able to dry outward well. My preference is to use mineral wool but I don't know that we'll have the budget given all the surprises we're finding.
Many thanks,
Alyson
I don't see any problems with your detail, although I would use a smart vapor retarder on the interior of the wall for extra insurance. Membrain seems to be available again, and it's probably the cheapest smart vapor retarder on the market right now, at least that I know of. Note that your "at least R10" for 2" of rigid foam rules out EPS. I would use polyiso here myself.
If you want to trim some cost, you can get high density fiberglass batts for 2x4 walls that will give you the same R15 insulating value. If you will have the interior walls open, you can get kraft faced batts and use that kraft facer as your "smart" vapor retarder too, although it's a little more labor to install compared to something like Membrain that can essentially do the entire wall in one shot. Fiberglass batts require a bit more care compared to mineral wool to get a good installation, but if you're careful you can get very similar final results with either material.
I would consider loose fill fiberglass in that floor instead of mineral wool. This is another place you can save money, and there isn't a lot of difference between the two materials here. Using loose fill will also mean very low labor costs since it will install quick and easy and there is no need for any cutting the way you'd have to do if using batts. Once you put that polyiso on the underside of the joists, you can blow in loose fill to whatever depth you want, then immediately start putting in subfloor. You just have to change your construction order a bit so that you can insulate prior to the subfloor going on. This assumes you'll have the floor fully open though, which might not be the case in a renovation project.
Bill
Thanks, Bill. This is all super helpful. I appreciate the focus on cost-saving measures--we will likely need them!