Basement Wall Insulation and Buildout

[rlrobinhood]

Hi all,

I'm in the planning stage for finishing off our basement. Right now, its just poured concrete walls and floors. Its an older house with this portion of the basement built in the 50's. The floor is in excellent shape and the walls are in good shape. There is some effloresence in a couple spots on the walls and the concrete is spalling a little here.

The effloresence is weird to me though. I live in northern MT (59522) and we are very dry with minimal rainfall (10" average annual rainfall). The majority of the wall are perfect, but there are 2 spots on opposite sides of the basement that have this problem and the concrete has spalled to about .3" deep (over probably 50 years). I also taped a 1'x1' clear plastic sheet to the wall in several location and have had them on there for about a year now. The worst I have ever seen moisture on the plastic was just a slight haze. Most the time it has been dry.

For the floors, I think we are simply going to acid stain them. Maybe skincoat, stamp, and then acid stain.

For the walls, I was going to build 2x4" stud walls around the basement. Use pressure treated sill plate with a sill plate gasket. I'll install the walls .5" out from the concrete and have a contractor come in and spray closed cell foam between the studs and the concrete walls, 2" deep between the studs, 2" thick over the concrete sill and wood foundation sill plate, and 2" thick in the sill plate joist area up the the main floor sub-floor boards. Then I figured I'd put drywall up (knowing there will be an air gap between the interior side of the sprayed foam and the dry wall or would it be better to put some paperless batting in? What thickness would you use?). (There will be a suspended ceiling) What do you think about this approach?

Would it be a good idea to use green board instead of standard drywall?

Thanks a million in advance.

[Dana]

If you insulate with 2" of closed cell foam between the studs, with only 0.5" between the stud edge and the you'll have R1.5 framing short-circuiting the R12-R13 foam reducing it to about an ~R8 wall as a "whole wall-R", after the thermal bridging of the framing is factored in. With the 2.5" you could put in R8 econobatts and end up at about R14-R15 whole-wall, but there is a slight risk of wintertime moisture accumulation at exterior stud edge on the above grade portion since it'll be running cool.

Since the studs aren't structural and only need to hold up the gypsum, not the house- either install them turned 90 degrees or use 2x2s (finger-jointed stock if the wall flatness matters much to you), and get the full 2" of foam between wood & concrete. Then use unfaced R8 econobatts or split R13 unfaced sound-deading batts to fill in the 1.5-1.75" deep cavity between the gypsum and foam. At that point you will be well north of R15 for whole-wall R, about 2x the performance of what you've proposed for only a modest (if any) increase in cost. Any fiber insulation needs to be a compression fit- you may have to split R19s if it's deeper than 2" from gypsum to foam anywhere, but you can compress R19s by quite a bit. (At 3.5" a full R19 batt is the same density as an R13 batt, and performs like one.)

Instead of sill gasket under the studwall plate, use 1/2-1" XPS (pink/blue/green sold as insulating sheathing). Sill gasket is pretty marginal as a capillary break, and worthless as a thermal break. Even R3 is enough to keep the wood well above the temp of the slab year round, minimizing it's moisture accumulation, and you can use standard grades rather than pressure-treated. Use TapCons through-screwed to the slab to keep the stud plate from migrating out on you over time.

The efflorescence is almost always a bulk-water issue from exterior drainage (assuming the footing is above the water table), but it could also be wicking up from the sub-soil from trickle-springs or high water table. With 2" of cc foam on the walls there will still be some water vapor making it through to the basement (which is fine), and as long as you don't use batts with facers, poly sheeting, or something highly vapor retardent like vinyl or foil wallpapers to trap moisture in the studwall. If there is at least a foot of above grade exposure on the exterior the concrete will then dry primarily toward the exterior, and you may see minor efflorescence or spalling on the exterior after some number of years, but if that happens a lime-mortar parge on those sections would be sufficient to protect the concrete from moisture wicking damage for the next century or so.

Green board won't be necessary in your basement & climate. In places with higher summertime humidity cool basements can accumulate moisture just from outdoor air infiltration- 70F dew-point air leaking into a 68F basement is a mold-factory! But your summertime outdoor air dew-points are in the mid-40s F according to Weatherspark.com datasets, so as long as you don't have ground moisture issues or you're putting up a winters-worth of firewood in there the basement humidity will stay below 60%RH, maybe even below 50%.

[rlrobinhood]

Dana, Thank you very much for responding. I was hoping you'd see this one. I've learned so much from reading your posts.

I've attached a wall buildout drawing of what I propose to do with all the information you provided. Mainly just for verification. The rim joist areas I assume will just receive more batting to fill the void or should more spray foam be applied here? I also forgot to add the wall board on the inside, but you get the gist.

Another question I have for you is how to handle basement windows. Should I frame out a rough opening with pressure treated wood and then leave a gap for 2" of closed cell foam? See the attached pic for what I'm thinking.

Thanks again so much.

[Dana]

Giving the rim joists at least a 1-2" shot is better than batts alone, since it both air-seals the rim joist to the subfloor & foundation sill, and provides a non-wicking condensing surface to protect the wood from adsorbing moisture in the winter. At 1" the rim joist can still dry at reasonable rates toward the interior from any penetrating-rain bulk-wetting events, but it mostly dries toward the exterior most of the year anyway. Anything over 2" means it must always dry toward the exterior, which is probably fine given your relatively dry climate. Siding type and roof overhang depths affect the drying/wetting rates on the exterior, but aren't much of a decision driver here. With even an inch of foam you can stuff batts in there to bring up the total R. High density R15s (or rock wool R15s) would probably meet code as an ignition barrier for the foam, but the rules have been relaxed for ignition barriers for foamed band joists in most areas anyway. Low-density R19s would probably NOT work as an ignition barrier, due to the high radiant heat transfer at high delta-Ts.

Going out of your way for framing out the window to be able to accept some depth of foam isn't really buying you much from a thermal point of view. Yes, pressure treated is the right approach, as is sealing it with foam. But unless you're buying some really expensive basement windows the window itself isn't going to be any better than R3, which is less than the R-value of a 2x4 (the long axis). The total square footage of that framing isn't very big, so even if it's low-R relative to the rest of the wall, it's still about the same as (or better than) the window it's holding. In very high-R PassiveHouse or "Net Zero Energy" type construction you'd care, but less than 1 square foot of thermally bridging framing on a basement window isn't a deal-breaker for the rest of us. Air tightness is more important than R-value here, since air leaks at the foundation contribute far more to "stack effect" infiltration than anything on the above-grade walls. (Sealing both the bottom and the top of the stack, the static pressure driving infiltration is much diminished.)