Logic of flashing & air sealing new construction window.


Old 08-14-18, 12:34 PM
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Logic of flashing & air sealing new construction window.

As in the included image, typical recommendations for flashing tell us that the bottom flange covers the flashing (which covers the WRB). The head flange is flashed and then covered by the WRB.

Supposedly, this allows water to drain from behind the WRB at the head. It also allows moisture from inside the window to drain out the bottom.

But, what happens when the window is trimmed? Doesn't the trim trap that moisture?

And, when weather sealing from the inside, a common recommendation is to use low expanding foam.
But, how can anything drain out of the bottom of the window if the space at the bottom is filled with foam?

I wonder about these things. Can you please explain the reasoning to me?

Thanks in advance.
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Old 08-14-18, 02:48 PM
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Don't think of it as trapping water. Think of it as keeping the water on the outside of the WRB where it is not going to rot your structural components... the framing and sheathing. Siding and trim are expendable in a sense... they will eventually get replaced. But more importantly you want to protect the structural components of the building that shouldn't need to be replaced (if you flash things correctly... and if your WRB holds up.

Almost all types of siding rely on the WRB to be the primary barrier if any water gets behind the siding. And any water behind the siding (heaven forbid) will leak out the horizontal laps of the siding or find it's way down to the bottom of the WRB and drip out.

This is one reason the "rainscreen" method of siding installation is often preferred in wet climates... because it provides an actual drainage plain behind the siding and the space also encourages drying.

Regarding foam on the bottom, most mfg's show you where to foam and instruct you to leave a space between the foam and the nail fin. This is where moisture is most likely to occur and the space allows it to weep out or at least breathe. Also why most mfg's don't recommend caulking or taping the bottom fin shut.

The shiplap method of flashing and taping encourages water to get to the outside of the WRB as soon as possible... and you don't for example, get water leaking in a 2nd floor window, then leaking in on the top of the first floor windows below.
Old 08-15-18, 03:09 AM
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I can personally attest to what happens when nothing is done. THe builder of my moms house installed all the wood windows without a single piece of flashing or sealing tape.

Five years later several windows were rotting and had to be replaced along with the wall of tile below the bathroom window!

I replaced several installing upper flashing and tape and 14 years later no issues!

It does some good!
Old 08-15-18, 08:04 AM
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Thanks @XSleeper. I'll keep what you said in mind (makes it, at least, a manageable goal).

Thanks @Marq1. Oh, I do intend to do something. I'm just hoping to do the best thing I can do with the materials I have to work with. My first encounter with one of the windows I am dealing with was, at least, 15 years ago--before I cared enough to research anything about renovation or good building practices. Even then, I knew something was wrong, so I removed some of the cedar shakes around the window and found the Homasote sheathing to be teaming with carpenter ants. At that time, I replaced the Homasote around the window with plywood, fastened with (gasp) drywall screws. This go-round, I plan to do better :-)
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