Remodel and Exterior Wall Insulation
Location: Climate Zone 6 (Cold), Traverse City, Michigan to be exact.
As part of the remodel we plan to take off the siding and sheathing to aid in electrical work and get a chance to re-insulate and tighten things up quite a bit.
For the wall cavity, my first choice is 3 1/2″ of Rockwool (R15). However, given that our studs are 3 3/4″ that leaves us with a 1/4″ gap. My thought is to add 1/2″ EPS or XPS foam (cut and cobble method) first, against the backside of the exposed drywall. This would be air-sealed with can foam. Then put in the Rockwool and compress it by 1/4″ when putting the new 1/2″ plywood sheathing back on. Siga Majvest would go over the plywood – all seams taped, etc…
My overall question… is the 1/2″ EPS or XPS acting as vapor retarder is a good idea? From my understanding, it will help stop some vapor from getting to the back of the sheathing, but it still does breathe a little (especially the EPS). However, since we’re opening up the wall from the outside, the order of rigid foam (air seal) and Rockwool seems backward — compared to doing our rim joists for example.
Would I be better off to fill the 1/2″ gap with something else that will breathe better (like the Rockwool)? Or, just leave the gap?
I understand that I’ll have thermal bridging on the 2×4 studs themselves and am not sure how that will impact things.
Overall, I’m looking to avoid air leaks and insulate the best I can with a 2×4 wall — while avoiding mold/rot issues.
Exterior insulation is not really an option due to cost and some stonework on the house. There is not much interior work that needs to be done, so removing interior drywall isn’t practical.
Any feedback would be greatly appreciated!
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Replies
Also, I see Comfortboard 80 comes in a 4" version now. Would it be crazy to cut and cobble that (pressure fit) and call it a day? I haven't touched the board version of Rockwool before, but I see the 80 is to note compression resistance. So maybe it won't compress 1/4" easily?
Warning, not an expert.
1/2" eps will allow vapor both ways, from higher to lower. Cut and cobble is harder than it sounds. I would just do the r15 rockwool flush with the outside of the studs with a slight void before the drywall.
Make sure you airseal meticulously on the outside and at all penetrations on the drywall. Putty pads, holes in top and bottom plates, etc. If possible, tape/liquid flash bottom plate, ledger board area if you can reach those.
I'm not familiar with celtoex sheathing, but I googled it. Is it true that it's essentially polyiso insulating board?
Typically, I would think removing sheathing is a lot of work just to upgrade cavity insulation and run some wire. You get a lot more benefit from wrapping continuous insulation outside the studs. But maybe with celtoex this calculus is different.
Are you planning to touch the windows?
Not sure about the 3.75" insulation, but cut and cobble seems like a lot of work just to fill .25". Could consider dense packed cellulose?