Flashing and exterior basement insulation detail
I am about to break ground on a basement foundation for an existing cabin that will be moved on top of it and am running into some last minute design problems. My intention was to insulate the interior walls with 2″ rigid foam and then build a 2×4 wall in front. Because of the way the existing cabin is built (the bottom of the 2×6 floor joists are higher than the bottom of the log), the 2×4 wall has to be load bearing and continue down to sit over the footer. The footer wasn’t designed wide enough for the wall to sit on it if I space the 2×4 wall 2″ from the concrete for foam. I’m supposed to start excavation mid next week. Options i’m considering:
1. build a ledger into the concrete wall and sit the 2×6 on that, continue with planned interior foam plan.
2. foam exterior walls instead. Plan is to have 9′ walls, 5′ below, 4′ above grade. In this case the foam would sit proud of the log walls and I would have to flash the top foam edge and side the foam. This solution seems like potentially the best except I can’t figure out flashing details since i have log. I could put on the exterior foam, then L-flash, then place the cabin on top perhaps? Once the cabin is placed it seems like my options for flashing get more complicated – like somehow getting a kerf cut and sliding z flashing with caulk into it? If anyone has a better idea i’d appreciate it.
3. Another interior insulation method i’m not thinking of? I’m in zone 6b, so I am pretty dry and my area is known for rocky, well draining soil. From my research on this site it seems the safest way to assure no moisture issues is to only use either rigid foam on the interior or exterior, or spray in with a gap between the 2×4 wall and concrete. Is there another way I can safely insulate with the 2×4 right up against the concrete if i don’t do exterior insulation? Thanks!
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Replies
If you have enough width on the foundation wall for the log to sit on and at least 1.5" of bearing, you can rip a 2x lumber to height as a shim under all the joists for support.
You can also install joist hangers and support the joists off the exterior log wall.
For the exterior insulation option, I think either flashing strategy you have should work well. Probably easier to do the kerf cut as it can be done near the end. Make sure this flashing slopes away from the logs, you never want water to pool there.
With something uneven like a log wall, make sure you get a good sill gasket over the foundation. This is a hard to joint to air seal afterwards. It is probably worth it to spray foam the rim area on the inside as well.