Insulate basement wall with half wall, brick exterior, and dimple mat
First of all, this site is incredible! I’ve spent countless hours reading through threads related to my current project and many others. Thank you all for providing this amazing resource.
I live in Seattle, WA, climate zone 4 (marine, I think). My house has a brick exterior walls, and my basement has concrete pony walls with exterior framing on top. I had water in my basement last winter, so I ran the gutters/downspouts through 20′ extension drains that empty into drywells far away from the house. So far, this single step has completely eliminated the water coming in (we had record rainfall last month, and no water got inside). I will also grade the exterior ground to slope away from the house, and I am planning on installing an interior perimeter drain to runs to a sump pump. As a part of the interior drain system, I will be adding dimple mat (aka drain board, JDrain 200, ~1/2″ thick, comes in 6.5′ rolls) from the inside of the gravel trench all the way up to the top of the concrete. This system should catch any water coming through the floor or concrete walls. I’m also sealing every tiny potential crack in the concrete wall to help prevent further water infiltration.
Attached is an illustration of the existing wall structures (not exactly to scale), as well as my most recent plan for insulating the walls. The upper wall is brick veneer | air gap | paper | plywood sheathing | 2×4 framing. The concrete foundation is 58″ high (on two walls, 47″ high on the third), and the exterior framing extends the rest of the way to the ceiling, with the inside of the exterior framing plumb to the concrete foundation. Basically, I’m going to nail the top edge of the dimple mat to the bottom of the exterior studs (using capped nails), then attach 1/2″ XPS panels to bring the exterior studs plumb to the inside face of the dimple mat, then attach 2″ XPS from floor to ceiling. Then I’ll build my stud wall on the interior of the 2″ XPS. Here are my questions.
1. What do you think of my idea of using 1/2″ XPS to bring exterior studs plumb to level of dimple mat? Is 2.5″ XPS an issue in terms of vapor permeance? Should I just use some 1/2″ plywood fur strips instead? Should I make 1.5″ ‘fur strips’ out of the 1/2″ XPS!? Then I could fill the exterior cavity with unfaced R19 fiberglass compressed down to 4″.
2. I have old R13 kraft faced batts laying around from the previous (poorly installed) walls. Can I reuse these somehow, like on the interior stud wall cavities? Based on other threads, I think I *don’t* want the facing…can I just carefully pull it off and use the batts as unfaced R13? Or leave the facing on because it’s no big deal? Or junk these old batts and buy new unfaced batts?
3. Do I need a sill gasket between bottom plate of the interior stud wall and repaired concrete slab? Or just a bead of caulk or sealant (what kind)? Or is a capillary break there not needed because the dimple mat acts as a capillary break against the concrete and the interior perimeter drain should prevent ground water infiltration?
4. Do you see any other issues with my setup? Any advice or suggestions regarding R values, vapor permeance, cost-saving potential?