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Are we asking for trouble with our window/flashing/insulation method?

I help to build houses with a Habitat for Humanity affiliate south of Houston, TX (Zone 2A) and I would like your thoughts on how we weatherproof the homes.
We use standard 2x4 construction, with OSB sheathing. (Cavities and attic floor are filled with blown in rock wool)
Flanged vinyl single hung windows are installed as per Texas Windstorm Regulations. We do not put any window pan or any other drainage plane/ water resistant barrier in before installing window.
We do use Dow Weathermate 4" Flashing Tape over all four flanges (working upward).
Styrofoam is then applied over the sheathing, up to the window frame, covering the taped flanges.
The rigid foam is then taped with Dow Weatermate 2 7/8" clear tape. Using the same tape, the joint between the foam and side of the window is sealed.
A pressure treated 1 x 4 is framed around the windows, and then Vinyl siding is applied.
The 20” wide soffit is just 4” above the top of the window. I believe that the Flashing Tape would run above the bottom of the soffit
I am thinking that if the tape does not fail, the only way water can get behind the Styrofoam is through the window. In particular the “well” where the moving part of the window sits - water could accumulate in there and potentially leak out a corner weld on the vinyl.
I really want to keep that OSB sheathing dry, and prevent any chance for water getting access to it.

PS I read the recent article “Using Rigid Foam As a Water-Resistive Barrier” and the referenced articles, and although very interesting, don’t quite cover our situation down here.

Asked by Steve Young
Posted Sun, 02/16/2014 - 12:41


1 Answer

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The best practice when installing windows requires the rough opening to be flashed so that it can handle any water that leaks through the window. My window installation mantra is, "Flash the rough opening, not the window."

You guessed correctly: the way that water can reach your rough sill is by leaking through the window (usually between the sash and the window frame). Joe Lstiburek likes to say, "All windows leak. Some windows leak right away; others leak after a few years."

You need to begin by flashing your rough opening. This type of flashing includes a sill pan -- either a site-built sill pan or a commercially purchased sill pan. Then you flash the rough jambs so that they direct water to the sill pan.

Here is a link to a series of GBA videos that explains what you need to do: Window Sills That Won't Rot.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 02/17/2014 - 07:52

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