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Community and Q&A

Rainscreen detail with brick mould window

kszach | Posted in Green Building Techniques on

I’m getting ready to reinstall the windows on my exterior foam retrofit project. I installed two 1″ layers of PolyIso foam. I plan to side the house with Hardi Lap siding. I had planned on using a rainscreen between the PolyIso (foil surface is my WRB, all seams taped for each layer). I’m hung up on how to install the windows. They are brick mould Marvin casement windows that were on the house prior. I am installing new PVC brick mould and PVC stool to the windows before re-installation. How do I incorporate the rainscreen into the window installation? I’m afraid that I may cover up much of the brick mould if I install the windows to the PolyIso with the rainscreen and Hardie. I had planned on using 1x4s for the furring strips, maybe I should run a thinner material but then will I have enough material to “bite” into for the Hardie? My house is 2×6 construction but there is horizontal boards notched into the 2x6s which would allow me to run furring strips ever 1′ if I wanted. So, back to my main concern… Do I fur out the windows and cover the furring strips with the flexible rubber window flashing thus moving the WRB at the windows to the outside of the furring strips? Or do I install the windows to the WRB and fur out only the siding (brick mould is 1.25″ thick, so I’d have about .125″ after the Hardie which is close to the look that I had with the previous Cedar siding).

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  1. GBA Editor
    Martin Holladay | | #1

    There are several ways to do this. It's usually harder to explain these details in words than with a detail sketch.

    Many builders prefer to install a plywood box to hold each window. The box sits in the window rough opening and extends as far as the outermost layer of foam. This box needs to receive sill pan flashing and needs to be flashed to the WRB in a water-tight manner.

    Once the box is watertight, the window can be inserted in the box. Of course, you'll need to install metal Z-flashing to protect the brickmold flashing at the window head.

    If you wait until the last minute to determine your window flashing plan, you may run out of options. After all, the method I describe requires oversized rough openings (1.5 inch oversized in both directions to accommodate the box made of 3/4-inch plywood).

    There are other ways to flash such a window.

    Here is a a detail for "innie" windows from the GBA detail library:

    Here is a detail for "outie" windows from the GBA detail library:

  2. kszach | | #2

    This is a retrofit project. I am stuck with the rough openings and windows that I had. I think that I have two options 1) build up the brick mould with another layer of 1" PVC trim either between window frame and brick mould or atop the brick mould. The rough opening will be flashed with flexible flashing. The windows were caulked to the previous sheathing (1" EPS) which leads me to believe that is how they should install into the new setup. They were nailed but I intend to use masonry clips in the jambs.
    or 2) use thinner furring strips of 3/8." The stack-up of furring strips, and siding would leave the brick mould about 1/4" proud which is better than what I had.

    Do you see a problem with either option?

    I plan to flash the window per Dupont's recommendation,

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