Airtight Sheathing & Thermally Isolated Double-Stud Walls?
Has anyone out there designed / built a thermally isolated double-stud wall AND used the airtight sheathing approach in a TWO-STORY residential project?
I’m thinking of the two-story, rectangular box as the thermal envelope. It gets wrapped on all five sides with plywood sheathing, which acts as the primary air barrier. Then it gets capped with an unoccupied, fully vented, stick built roof. Make sense?
Now, combine that concept with a curtain wall—to eliminate the thermal bridges in the framing. That’s the idea anyway.
I know of examples that use airtight sheathing combined with “interior” double-stud walls, but in those houses, the exterior walls support the floor and roof frames. In these designs, the floor joists and rim joists act as thermal bridges—and they are located where they are most exposed to moisture accumulation and deterioration.
Perhaps the thermal bridges can be modeled to quantify the heat loss, but the “durability risk” is a little murky.
See this link for an example from some GBA advisors and regular forum participants (architects Jesse Thompson & Phil Kaplan, builder Dan Kolbert):
Back to a thermally isolated frame w/ airtight sheathing:
How would you treat the junction between the balloon framed curtain wall and the platform framed structure (account for the differential shrinkage in the wall heights)? How would you reliably tie the wall sheathing to the attic floor sheathing?
Is this idea worth pursuing? Or is it too much trouble for the added benefit?
This is for a mixed-humid climate zone (4A, in close proximity to zone 5).
1) Eliminates (reduces) thermal bridging.
2) Reduces the chance of wintertime moisture accumulation in critical rim joist area / frame.
3) Airtight sheathing throttles solar vapor drive in the summer.
4) Eliminates fuss of ADA methods (i.e. acoustical sealant), theoretically a lower ACH50 result.
1) New construction methodology—unintended consequences?
2) Airtight sheathing approach consumes more resources.
Thanks for the discussion!