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Installing outie windows with 2-inch exterior Roxul board

user-954155 | Posted in Green Building Techniques on

Since we originally planned to install 2″ of foam board over the exterior sheathing with house wrap over that. Our architect specified outie windows mounted on a plywood box extension (2″ out from wall).

Now that the Roxul Comfortboard IS is available we would like to use that instead of foam, but it can’t be taped and sealed like foam (making window sealing problematic at that layer) and since the compressive strength is less I imagine the window flanges should not be attached through it anyway.

Should we just put foam around the windows for ease of installation and just bend the house wrap to the outside of the foam at that point? I understand it is recommended to put the house wrap under the Roxul. Does anyone have any recommended window details for this kind of situation?

I read the details on GBA about installing windows over exterior foam. The Gienow vinyl windows have been ordered and the rough window openings are there, but not the window boxes yet. You can follow our progress here



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  2. user-1061844 | | #2

    Patrick, there is no harm in putting a good vapor open WRB over the mineral wool - see our 475 blogpost where Ryall Porter Sheridan used this approach.

    You can then simply use TESCON Vana tape to connect your flanged windows to the WRB.

  3. albertrooks | | #3


    I like Martins response here: It's all about airsealing the interior and weather sealing the rough opening.

    At 2 inches of MW, I'd still consider making a plywood buck for the windows and flashing it well so it will drain out and over the MW. For flashing the buck, SIGA just came out with Wigluv 230 which is a 9" wide permeable flashing tape for windows. I also like Prosoco Fast Flash liquid applied membrane for an R/O. Both look "bullet proof" (in a manner of speaking) to me but will still let the substrate dry through if & when needed.

    Here is the Prosoco from us:

    But you can likely find it locally too. Good stuff.

    With all that Mineral Wool, you'll have a nice wall after you get the battens to "plane out". Plan on that taking some serious time. With such a good wall, treat your window R/O's well. That's the most likely spot to have moisture issues.

  4. user-757117 | | #4

    Putting aside the pertinent issues Martin raised regarding fastening of the Comfortboard...

    I don't think you will have any problems putting the WRB over mineral wool sheathing.
    The WRB should integrate with the plywood bucks in the openings without any trouble.
    After installation, the windows flash to the WRB - I'm assuming there's no rainscreen to worry about.

    One difficulty you might have (depending on how thick the plywood is for the bucks) is whether or not you have enough solid lumber to secure the flanges to.

    Your flashing details don't sound entirely dissimilar to my own.
    Notice the 2x4s around the exterior of the plywood bucks.
    Without that additional lumber, it would have been hard or impossible to properly fasten the flange.
    After installation, flashing the windows to the WRB wasn't difficult.
    I was a little concerned that it would be a bit of a challenge getting the flashing tape to stick properly without some kind of rigid sheathing to support the WRB from behind - but the 3M tape is so eager to stick to things that there were no issues.

    Shortly after flashing was complete, the east wall windows got a good test - strong east wind and driving rain but no leaks.

  5. user-954155 | | #5

    Thanks for all of your helpful advice, product suggestions and especiaI photos Lucas! While I never seem to have quite enough time and a finite amount of money for this build, I can manage to squeeze more of my labour into this project. So I don't mind fiddling a bit with the fastening of the strapping (yes we will have a rainscreen) to try for a more aeshetically pleasing siding result. I recall John Straube wrote somewhere about setting the drill ratchet consistently and using a long level to plumb the strapping and check for compression consistency. The North Vancouver house builders that used Roxul nailed the strapping first with air guns and then srcrewed it. Others used plastic pipe or foam squash blocks. I figured that the windows could not be installed using the rigid foam technique due to the different characteristics of the roxul board, but I appreciate the ideas suggested.

  6. propeller | | #6

    Here's an interesting way to quickly install spacer block in roxul. I saw it here on GBA earlier but can't remember where exactly. I remember Albert Rooks developed a special screw to install furring strips over roxul also. Anyway good luck with your project, I love what you're doing.
    Marc from across the pond in New-Brunswick.

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