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Window head flashings with exterior insulation and rain screen

RussellO | Posted in General Questions on

Good Tuesday GBA folks!

We are in the process of doing a full renovation on a 4200 sq.ft. 1890’s Victorian boarding house and turning it into 5 apartments. The building is located in Montana. We are taking the entire building down to its shell. The only thing left is the granite block foundation, balloon framed walls (2.5 stories), ship lap sheathing, and the framed roof (4 gables, 1-2 ft. overhangs).

The plan is to do a VOAT wall using Henry BlueSkin VP100 over the shiplap, 3 inches of mineral wool, a 1×4″ rainscreen, and lap siding for our cladding. We are going to use ThermalBuck to bump out our rough openings for our windows and doors to accommodate the exterior insulation. The problem I’m wrestling with today is how to properly sequence the window flashing.

We will be following the ThermalBuck manufacturer installation instructions here:

Windows will then be installed and flashed to the ThermalBuck using butyl tape.
Exterior insulation will be installed on all exterior walls and rain-screen over the top.

Once that is done, there will be a wood 1×4 rainscreen batten next to the windows and along the head/sill so that we can fasten exterior window casing and a somewhat decorative head casing above the window.

What I’m struggling with is if this assembly needs an additional metal flashing to cover the head casing. If it does, where does it go and how do I get it there? The sequencing seems difficult enough that maybe it isn’t worth messing with? I think we will be purchasing pre-manufactured composite casing, so it likely wont be very susceptible to water damage. But, it would be nice to get the details right if possible.

What do you all think?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Russel.

    It's not necessarily the casing that you are trying to protect with the head flashing, it's the window. From the conversations I've had with many builders, it seems like most window leaks happen at the upper corners. So when a head flashing is installed and the wall leg is tucked under the WRB, that's to direct water from the WRB out to the face of the trim and window to prevent leaks (see drawing below).

    In an assembly like yours that means custom bent flashing and a sequencing issue, for sure. I have seen builders (and I've done this myself) install the head flashing just back to the furring strips as a way to keep any water that lands on the head flashing from dripping back behind it. The former is the right way to do it, the latter is cheap, minimal protection.

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