Air sealing retrofit and how to protect rigid insulation past foundation?
We are building an addition and replacing siding, roofing, and windows on the existing structure, so we have an opportunity to retrofit.
Now that we have some siding off, I can see the original wall is something like this:
1. 2×6 with R-21 batts
2. 1/2″ plywood sheathing
3. 1/2″ fiber-backed polyiso
5. Cedar Siding
#3 is flush with the outside edge of the foundation wall, which is ICF with stucco. We will be replacing from #4 onward.
The addition/retrofit (steps 6 onward) will be:
2. Certainteed MemBrain taped with Siga
3. 2×6 with dense-packed fiberglass or cellulose
4. Sheathing (OSB or plywood) – 1/2″ to match existing?
5. 1/2″ fiber-backed polyiso (to match existing)
7. 2″ GPS rigid insulation
8. 3/4″ plywood furring strips diagonal
9. EDCO board and batten vertical steel siding
First problem: air sealing. I had planned on using Siga Wigluv tape on the sheathing seams, but if we preserve the polyiso on the existing structure, we won’t be able to get to the sheathing there. Is it viable to use the polyiso layer as an air tight layer, taping its seams with Siga Wigluv?
Next: Because the polyiso is essentially flush with the foundation wall, I am less worried about protecting the bottom because we could seal the sill down to the top of the ICF foundation wall. But I am concerned about protecting the bottom edge 2″ GPS from pests. I have seen perforated J-channel but not in a 2″ depth. The furring layer will be protected with Cor-A-Vent SV-5 top and bottom.
What do you suggest to protect the 2″ bottom edge of the GPS insulation?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part