Best practice for retrofitting insulation in a daylight basement
I’ve read quite a number of great articles here and on FHB about insulating basements. However, it seems most of the articles are targeting full height basements.
I have a project that differs a bit in that it is a daylight basement with a transition from concrete foundation to conventional 2×6 stud wall. I’d love to get some advice on whether folks here think my plan is reasonable and what the best practice might be. Additionally, it seems like there is some detail around the transition that might be hard to manage.
42″ high concrete wall with drywall firred off the concrete. No insulation
51″ stud wall on top of the concrete foundation (2×6, fb batts), all above grade
Install 1″ foil-faced polyiso, ideally glued and sealed to the concrete foundation wall to a height of 42′
Interior non-load bearing 2×4 stud wall 24″ o.c., filled with 3.25″ mineral wool batts to the ceiling (R value overkill but whatever)
No interior vapor barrier (climate zone 5b, Colorado) at the drywall to avoid the vapor barrier sandwich (but a vb would be good above grade)
Wall will be finished with corrugated metal wainscot below grade (to 42″) and drywall above
While I’d love to do exterior excavation and apply XPS outside the building shell, the excavation costs appear to be formidable.
Some questions arise:
1. Can I get away without the vapor barrier at the interior wall? There will not be the typical vapor retarder of latex paint below 42″ since the finished surface is corrugated metal. I suspect this is not a great idea.
2. Can I install a vapor barrier behind the corrugated metal panel? I suspect that is also not a great idea as I would be creating a vapor barrier sandwich (also not desirable: see below).
3. Can I put metal directly over polyiso? I suspect not because the letter of the code would not be met in regards to fire resistance. Others have previously mentioned this problem in regards to metal over foam. I suppose Thermax could address this concern?
4. Transition: It would be nice to have a vb at the drywall above grade to protect the insulation. One option is to place that vb at the interior side of the existing stud plane and not place one behind the drywall. In that case, I’m almost building two separate walls and stacking them on top of each other.
Here’s the goal, which might help:
This basement is in the 100 year floodplain. It needs to be built with flood resistant materials. As such, choices for materials below the 42″ foundation mark (about the 100 year base flood elevation) are limited and exclude drywall. There is a GP fiberglass backed drywall type product but I haven’t laid my hands on it or anyone who has used it. Corrugated metal survives temporary wetting just fine and can be removed and reinstalled later to dry a wall. I had selected mineral wool batt for its drainability but have been discouraged from using it directly against the concrete foundation. Hence the addition of the 1″ of polyiso as a vapor barrier at the interior concrete boundary. However, polyiso supposedly will take up water in the event of a wetting so I’m not excited about its addition to the wall system.
Anyone have any suggestions for a wettable, drainable wall system that also can insulate the concrete and deal with the transition from concrete wall to above grade stud wall mid wall height?
If the answer is excavate and address insulating the concrete on the outside of the building envelope, I’m ok with that but need to hear that from the voice of experience since it is a big budget sell.
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