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Innie window with 1.5″ Rockwool: details and advice

pbout | Posted in Green Building Techniques on

I start framing this week on a remodel, CZ03c. 2×4 walls with Blueskin Air/WRB, 1.5″ continuous exterior Rockwool insulation, 1×4 rain screen straps. Exterior cladding is 3-coat stucco downstairs, and fiber-cement siding on second floor. House is now gutted down to studs and sheathing. 

We are doing innie windows. I had planned on using flanged windows, but I’m concerned about what it takes to replace that window in the future. Would using masonry window straps make more sense? If I did that, should I build a plywood box to accept the clips? 

Second part of the question is about how to trim out the exterior of the window; how to attach the trim inside the window opening? Do I pre-assemble trim that turns in? 

Any details appreciated. I’ve been referencing detail 5.3.3 of the FP Innovations Guide for the Designing Energy Efficient Building Enclosures: https://rdh.com/wp-content/uploads/2014/07/Guide-for-Designing-Energy-Efficient-Building-Enclosures.pdf (starting on page 5-68, or page 219 of that link)

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Phil,
    If you haven't read it yet, you should check out this article: "Installing Windows In a Foam-Sheathed Wall."

    Q. "I’m concerned about what it takes to replace that window in the future. Would using masonry window straps make more sense?"

    A. I imagine that it would be easier to replace a window with masonry clips than one with flanges. That said, most innie windows are flanged windows.

    Q. "If I did that, should I build a plywood box to accept the clips?"

    A. Just because you have masonry clips, doesn't mean you need a plywood box. Masonry clips can easily be fastened directly to the jack studs. The main reasons to install a plywood window buck are (a) you have outie windows, or (b) your exterior insulation is particularly thick. In your case, 1.5 inch of continuous exterior insulation isn't particularly thick.

    Q. "How do I trim out the exterior of the window? How do I attach the trim inside the window opening? Do I pre-assemble trim that turns in?"

    A. My article notes, "Trim out the exterior of the window with water-resistant jamb extensions and a water-resistant sloped sill made of cedar, cellular PVC, or copper flashing. These exterior jamb extension details don’t really have to be watertight — just resistant to weathering, durable, and attractive."

    Here are links to three useful resources:

    "Windows Can Be A Pain"

    "Mass Save Deep Energy Retrofit Builder Guide"

    "Installing Exterior Insulation in Cold Climates"

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