Help with air sealing our 1920s sill plate – and plans to finish the basement
We’re finalizing the plan to finish the basement of our 1920s, 1,500 sqf, 2 stories, daylight basement in zone 4a. The plan is to lower the basement slab (requiring underpinning). We’ll also waterproof the exterior walls and add exterior insulation. We haven’t decided on the details yet – party because we haven’t found a reference assembly to guide us.
There isn’t a band joist per se in the assembly – instead, there is a 4” x 6” sill plate and the joists are notched to sit on the sill, next to the balloon frame studs. The sheathing is 1” t&g, with tar paper and lap siding. Eventually, we’ll update the siding assembly with a WRB (Henry Blueskin VP100) over the t&g sheathing, new siding, and dense pack in the walls – we’re holding off on defining the wall assembly further until we decide how to handle insulating and air sealing the basement as there are some dependencies between them.
The particular construction is illustrated in the following rendering and photos that I created for this post: https://www.icloud.com/sharedalbum/#B0y5O3DjH8AcFa
We’re looking for help on two major questions, but open to any thoughts you have.
1) The CMU below grade is a smooth finished cinder block. ~6’ below grade assembly will probably get damp proofing, dimple mat, and 2.5″ of EPS. But we would like to preserve the ~3’ of split-faced cinderblock above grade – albeit with industrial epoxy paint to seal it.
Questions: Anything we should be concerned about if we don’t insulate the top ~3 feet of the basement wall? Any ideas on sealing the top of the edge of this assembly at grade?
2) I’d like to reduce the stack effect and air leakage in the basement. I was thinking of using 2” THERMAX against the sill and firestop block with beads of canned foam. But even though I’ve seen 100’s of assembly illustrations I can’t confirm that this is going to work in our particular situation.
Questions: Is there any harm in sealing the inside of our sill with THERMAX before the new siding and WRB over the next year? Can we air seal around the sill and firestop from the inside, and fill the CMU cell voids? Or maybe wait to fill the CMU voids until the damp proofing is done? I’m concerned that we’re eliminating the ability for the wall to dry out – which could create issues we don’t have at this point.
Notes on the foundation and framing:
– 10 ½” cinderblock CMU foundation wall, cells are empty
– Finished depth of the foundation wall (after underpinning) will be ~6’
– New basement slab will have a VB and be insulated to R10
– Adding interior and replacing exterior drainage. Adding a sump pump
thanks in advance for your thoughts!
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