Basement Cavity wall insulation options
Hello from Seattle, Marine Zone 4. I have a project that is in the middle of construction. We have raised and put a new foundation/basement on a 1910 house. The new heated basement doubles their existing living space. Originally we were installing 1″ R5 rigid (XPS is what we used) with a 2×4 wall with BIBS- fiberglass. The owner now wants to gain an inch by using 2×3 walls with rigid in the cavity. At first this seemed plausible, but the more I read the more concern I have. Our Energy Code permits R-5 continuous insulation + R-13 cavity wall. We will also have a slight gap between the rigid and new wall due to some not completely plumb concrete walls, which if we used BIBS would just be filled in. Looking at the R values, polyiso would be what we needed to use, but the foil face concerns me. Before I tell my client to stick with a 2×4 wall, I wanted to see if there was another approach. Already installed is the R5 on the new 8″ concrete walls. Thank you!
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Moisture passes easily through concrete, so if there is fiberglass against the concrete, it can get damp and moldy, which is why the building code frowns on it. (cold damp concrete + fiberglass - not a good combination) Foam insulation works, but typically needs to be pinned to the concrete to eliminate the air space where mold can form. We use 2” or 4” of polyiso, (climate zone 7) which works as the aluminum foil keeps the moisture from passing through. I’d guess the XPS would probably work; just unclear to me if 1” is sufficient. You don’t mention the floor, another cold damp surface; hopefully that is also insulated with either EPS or XPS.
Thanks. There is 2" of XPS under the slab and the foundation wall has waterproofing on the exterior of the foundation wall. Our code allows R-5 continuous and then R-13 cavity. So, in theory, there would be no fiberglass touching the concrete. We already have the 1" R-5 in place.
Spray foam would be the easy solution here. At an advertised R-value of about 6.5 per inch, it'd give you about R16 in the cavity. It would also seal any gaps and cracks. Not the most "green" solution because of the amount of resin and blowing agents, but it would work. A lot less labor than trying to cut & fit rigid iso insulation, too. Seems like a lot of effort and expense either way, though just to get another 1" of room. Some times that 1" is critical, but for a new basement finishing project, it probably isn't.
kdelucas,
These are exactly the type of proposed changes that increase both the complexity and time involved for little gain.
In even the smallest room or hallway, an inch more space is un-noticable. Unless there is some compelling reason (like a built-in cabinet, or ?) just stick with your plan and use the 2"x4"s.
I hear you, but the client is seeing the space now and wanting to maximize SF and inches seem to matter to him...we are packing a lot into this new SF.
2021 IRC Table N1102.1.3 shows R5ci + R13 for CZ4 Marine. Polyiso is a great choice, so you are good to go 👍👍👍
So to be clear unfaced polyiso? Concern is for the combined perm rating of 2.5" of polyiso. In this condition, aren't I still trying to dry to the inside? This same client also wants to put a vinyl soundproofing material behind the drywall....which I already told him that that was not a good idea. https://www.acoustimac.com/soundproofing/soundproofing-massloaded-vinyl-1lb50sf
Mass loaded vinyl (MLV) sheet won't really do all that much in terms of soundproofing a wall. You end up spending a lot of money for very little gain, and you end up with a vapor barrier too which can cause problems (and almost certiainly will in this particular wall assembly).
I'm not sure what the rational would be for soundproofing an exterior foundation wall, since the concrete is already going to do a pretty good job of that. If you client really insists though, I would either put a double layer of 5/8" drywall up, or hang that double layer of drywall on resilient channel for even more performance. Resilient channel especially will do much more for sound proofing than that layer of MLV, and it won't cause any problems with moisture in the wall.
If your client is planning on doing something noisy in that basement (like a home theater), I would recommend isolating that basement area from the living space, which would mean more emphasis on interior walls and the ceiling of the basement room than the exterior foundation walls. A double layer of 5/8" drywall with green glue between the panels and hung from hat channel would do wonders for sound proofing the ceiling.
MLV is something only used by people who don't really know about building soundproof rooms. MLV is much better to damp resonanances in things like metal panels in automotive applications than it is for soundproofing walls in occupiable structures.
Bill
No, you don't want the basement walls & floor to “dry to the inside”. That’s what happens in most basements, but basements, unlike floors above grade, are surrounded 24/7/360 by a moist environment, whereas the rest of the house the humidity levels vary by the day or hour. It’s actually pretty easy to design/build a basement that keeps most of the moisture IN the concrete or ground rather than IN the house. Result is a dry basement and a drier house (which is where air sealing comes in, since 97% of moisture migrates through AIR; so seal air leaks & humidity levels stay comfortable in winter.
We typically insulate the floor with 6’” EPS (zone 7) with 2” on top of the footing, and 2’ around the slab, so it’s completely thermally isolated.
(And stays - dry - at room temp) then a 10 mil poly vapor barrier, then concrete. Walls typically 4” polyiso with foil covering. Works great!
Typically, pour concrete basement walls can cure in 28 days, but that doesn’t mean they have no moisture inside, and that can take several months depending on your climate. The same goes for block walls but to a lesser extent.
Depending on which waterproofing system you used, it’ll dictate what you do on the inside. Your detail doesn’t mention your drainage and waterproofing methods, but I will assume you installed drain pipe and a sump & pump system, plus you also sprayed/rolled the wall with a membrane to stop water from coming to the inside.
If well sealed, the polyiso will stop any moisture left inside the concrete wall from migrating to the inside. Usually the wall will finish drying out thru the top of the concrete wall to the exposed opening to the outside.
Building a 2x4 wall with treated plate and sill sealer, adding impermeable insulation on the inside (cellulose or fiberglass), and finishing with drywall will accomplished your prescriptive code. Vinyl wall coverings are usually highly impermeable materials, and if for some reason, any moisture get behind the wall, it would be very difficult to dry-out.
There are many ways to achieve a highly acoustic wall system without vinyl, from double drywall and spacers to different kinds of panels. Stay away from impermeable panels.
I tried to load a couple of files for Sound Acoustical Assemblies, but the files are too big. Search for USG and National Gypsum.
If you really must have that extra space skip cavity insulation and go for 3" of polyiso on the foundation strapped out with 1x4s for the drywall. Otherwise as Peter suggested, spray foam is your better options.