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Community and Q&A

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

user-2069108 | Posted in Plans Review on

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 2×4 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.

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  1. GBA Editor
    Martin Holladay | | #1

    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.

    1. Colin63 | | #2

      Unless the framing sill plate is pitched to the exterior intentionally and flashed accordingly, don't wrap the tyvek in to the opening, don't use butyl tape as a sill pan and for God's sake, stop using butyl tape on top of tyvek paper to seal the window flanges... Tyvek tape would serve the same purpose if that's your method. Cheapest method, tyvek below window opening leaving it 2 inches below sill framing.install butyl tape flush with top sill plate of rough opening.Set window into continuous bead of silicone, plumb window, set the sash reveal and nail. Apply butyl tape to side flanges (EPDM roller should always be used on 45 angle)install aluminum header pan flashing, butyl tape above aluminum to plywood transition. now you can install tyvek wrap and if you still want to butyl tape to the flange go for it. Tyvek tape works fine. The windows should be 100 percent waterproof before any house wrap gets installed excluding under the window sill where the rubber overlaps the tyvek.**** Wrapping the sill opening on a conventional install is no good, you have to cut and fold the corners leaving an in for water, and if we transfer water to our sill rough opening intentionally,we are asking for problems. water likes to travel inward on a level 2*6 sill you got a 30/ 70 chance it will run outside, insulation will absorb it, cedar shims will direct it inward or wick or find that roofing nail you used. Fool proof but more costly method is same steps above but using EPDM cover tape and primer, rolled over.** Note EPDM and asphalt products don't work together** EPDM cover tape doesnt melt over time and bleed and run over time and is a perfect solution for all you OSB plywood fans lol.

      Colin rappa construction inc.

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