Walkout Basement Insulation Challenges w/stepped concrete walls
VTLiving
| Posted in General Questions on
Hello All,
We are trying to finalize an insulation strategy for a partial walkout basement in central Vermont. The concrete walls are 8 inches thick, and some of them are only a few feet high, so they will have stick-framed walls atop them.
For the concrete walls themselves, our current thinking is to put 2″ of rigid EPS foam board (likely Scoreboard) up against the inside of the concrete (glued and taped), build a 2×4 stud wall inside of the foam, fill the stud cavities with mineral wool batting, then perhaps eventually install drywall as we get around to it. [This meets the Vermont energy code of R-10 continuous + R-13 in the cavities, but runs afoul of the no-fibrous-insulation suggestion].
The challenge is how to handle the stick-framed walls that rest on the concrete. The builder plans to rip down 2x10s to be exactly 8″ thick, and we’ll need to add 2″ of additional material (likely ripped strips of 2″ ZIP-R) to furr out the studs to match the thickness of the foam board below it. We somehow want to insulate that 10″ space in between the studs, and then put up Intello wrap to serve as a smart vapor barrier (to let these insulated walls dry inward as needed). Then, of course, there will be the 2×4 wall inside of that, with mineral wool batting, as described above. On the outside, we plan to use 1/2″ ZIP sheathing (with a rainscreen and some sort of metal siding).
Our inclination is to use mineral wool batting in the 10″ space, but the batting only comes in thicknesses of 3.5″, 5.5″, and 7.25″. 5.5″+3.5″=9″, which would leave a 1″ gap. Would it be problematic to leave a 1″ air gap in this assembly, or should we be looking to fill that with some sort of 1″ sheet product? If we should fill the gap, do you have recommendations for a product (and placement — against the inside of the sheathing or at the far interior edge of the cavity)? We suspect that any such product would need to be pretty vapor permeable (like unfaced polyiso), but we’re not certain whether this is important.
As a quick note, our preference is to not to use cellulose, so we’re looking for ways to make this work with roxul and board products.
Thanks in advance for your advice!
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Replies
VTLiving,
I would do exactly as you are suggesting for the lower walls against the concrete, but would frame and treat the walls above just as you are planning to do with all the rest of the exterior walls in the house.
So a 2"x6" outer wall with whatever combination of insulation you are using elsewhere, and leave both the upper part of the 2"x4" inner wall, and the void in-between the two walls empty.
Why not InsoFast & skip the stud wall?
https://www.insofast.com/resources/interiors-resources/about-basement-insulation-and-insofast
Would you run the InsoFast all the way up (covering both the lower concrete and the upper stud wall)? These "half and half" walls are a bit of a pain, especially since in some places the concrete is below grade, and other places above grade. :-/
VTLiving,
They are quite simple if you break them down into their two distinct parts. The concrete stem-walls get insulated like any other ones, as do the framed walls that bear on them.. Whether the concrete is above or below grade makes no difference to how it gets treated.
The exterior stud wall above the concrete is exactly the same as any other exterior stud wall in the house. There is no reason to run foam on it unless you are doing the same on all of them.
Hi VTLiving!
I think what I would do instead if you want to try & do both (full concrete & partial concrete & stud wall) is to switch to a product like "Envirostrap" from Amvic & install the embedded strapping horizontally.
Where it is concrete "glue" it according to spec & where it is a stud wall - mechanically fasten to the stud thru the strapping
I used a different manufacturer on my own house for inside concrete walls (Dow "Wallmate") but I like this product too because of the built in "rainscreen" in an exterior application.
https://alleguard.com/wp-content/uploads/2023/06/Alleguard-Envirostrap-Product-Data-Sheet.pdf