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Vapor Control for Exterior Insulation Retrofit

Rackliff571 | Posted in Energy Efficiency and Durability on

My question is about my planned wall assembly & vapor control.

I’m planning to add exterior wall insulation on a 1974 coastal home in Maine (6A). The house has 2×4 construction. Wall insulation appears to be R13 fiberglass batts. Sheathing is 5/8 ply. Over this are two layers of asphalt paper. Over that is T11 siding. With the exception of a few areas that we’re repairing, the siding is in good shape. Having broken into walls in a number of places, the inside of the sheathing is clean and dry.

We plan to wrap the exterior with WRB  (Henry’s VP100 blueskin). To keep air from entering through the grooves of the T11, the bottom section of the WRB will attach to butyl flashing that will be adhered to the concrete foundation wall and up over the first 4-6″ of the existing siding. We are removing the eves and wrapping up the walls and on to the roof deck (new membrane & metal roof to be installed).

We plan to add 3″ of Rockwool comfortboard80 to the exterior walls (2 layers of overlapping 1.5″ panels), secured by 1×4″ strapping. Strapping will be drilled to 2×4 studs, 16″ on center, with new cypress shiplap siding finishing the exterior. All new double pane windows are also being installed.

A new ERV will exhaust air from bathrooms, kitchen & laundry. New heat pump system will heat & cool the house (combination ducted/ductless. We have  generator power & Rinnai propane heaters for backup.)

My question is about the wall assembly and vapor control. Is this assembly and adding 12R to the exterior going to keep me warm and dry or give me potential problems with moisture and mold inside the walls?


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  1. Expert Member
    Michael Maines | | #2

    That sounds like an excellent assembly to me. I'm on the border of central and midcoast Maine; here we need at least 33% of the R-value to be on the exterior for condensation control. The more exterior insulation, the safer the assembly. With over 50% of the R-value on the exterior, your sheathing should never be cold enough to accumulate moisture via condensation. I recommend Siga Fentrim instead of butyl flashing at the foundation, as it will likely stick better to the concrete.

    One thing to watch for, especially if you're new to exterior insulation, are the windows installation details. Problems may not show up for years so it's important to be confident in your approach.

  2. Rackliff571 | | #3

    Thanks, MM - appreciate the affirmation and product tip for adhesion to the foundation. Interested in your thoughts on window installation. We're using two siding materials - Sugiban cypress & corrugated steel. Discussing two approaches with our builder- a. Build out bucks to put windows flush with new siding, or b. "Innie" windows installed on same plane as current. We'll build 4" deep frames from heavy guage roof stock. Shiplap siding will butt against outside edge of metal frames. Preference is for the latter. Thoughts?

    1. Expert Member
      Michael Maines | | #4

      You're welcome. Shou sugi ban cypress + corrugated steel sounds like a cool siding combination. I generally prefer innie windows, because they perform better again heat loss, solar overheating and rain, and because when inset they provide shadow lines that give a house character compared to most contemporary homes which have facades that are too planar for my taste. But it takes more effort to install them as innies, so especially with builders new to high performance, I often end up building out bucks. I've sometimes spec'd ThermalBuck, which seems like a good product for that situation but I think they only go up to 3 1/4" deep.

      Side note: I'm curious who your builder is. I've had a hard time finding midcoast builders to refer clients to. No worries if you'd rather not say publicly.

      1. Rackliff571 | | #5

        Thanks, Michael - on window trim, my builder isn't comfortable with innies. As much as I like the design, I want to go with his strengths so we'll build out with bucks. Thermalbucks aren't deep enough so he plans to use 2 layers of 2*4 on the flat around the rough opening. This will give us a thermal bridge, not ideal but I'm thinking this won't be a worse conductor than the window itself. Is this a reasonable way to go, or will this create cold temps and condensation inside the wall around the window? Thanks again,

        1. Expert Member
          Michael Maines | | #6

          Rackliff, that's pretty much what I'm doing on a current job as well, just with thinner bucks. I included them in my typical options here: Solid wood bucks do result in lower performance but softwood insulates at about R-1.2/in so 3" is about R-3.6, equivalent to a U-0.28 window. Not great but better than code minimum.

  3. Rackliff571 | | #7

    Thank you, Michael - very helpful. Your Fin-Jamb assembly is exactly what I had in mind and I think would be a terrific compliment to the contemporary style we're going for. Shared with my builder and he's intrigued by this innie idea. Michael (also)

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