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How to Install A New Flanged Window into an Existing WRB, Part 3: Reframing and Flashing

Removing the old window frame and weight pockets, reframing the rough opening, and flashing a new-construction flanged window into the old existing WRB
February 21, 2022

OK, so the last time we surgically removed the old replacement window without disrupting the WRB behind the exterior foam insulation. AND, we learned a sweet trick for removing foam chunks.

Now it’s time to reframe and flash the opening and then install the new flanged window.

That begins with carrying the window from the barn to the house.

David is installing this window alone, and most window installation videos show two people doing it.

To make sure the window is centered in the opening without being inside to check, He makes a center mark on the outside of the window and another on the opening.

Then he marks the width of the rough opening.

David: I made my center mark, where I want the window to be centered, and then I measure from there for my rough opening.

To see how much he needs to fir each side.

Since the window is 21-¼, I want my rough opening to be 22-¼, so I measure 11-⅛ to either side, making a mark. And then I can measure from the stud to the mark to know how much to fill in the side.

I need 2 inches on this side and oddly, I need an inch and ⅞ on this side.

He pads the opening with framing lumber and various thicknesses of plywood scraps.

Inside, he measures from the paint lines so that the new interior trim will cover them, and marks the top of the window…

...about four inches; I need the top of my window to be about that height, so if I fill one inch across the top will give me a half-inch space. My window is 60-1/4 inches tall, so I want 61-1/4 for my height...

Now he measured down from there to determine where the sill should finish out.

The old sill is sloped, and so are the firring pieces he adds to get up to the line.

Making up space for the sill here...

He uses 1x stock as well as 2x stock to get to where he needs to be.

Shimming it into place...

He firs down the top and the opening is ready for what David calls subflashing. It’s the layer of flashing tape covering the framing before the window is set.

The weakest spot in a window opening is the bottom corner of the sill where water can collect if it finds its way in. Because the sill is sloped, it is vastly less vulnerable, but David also flashes the sill so that the vulnerable point is triple-protected.

It begins with little corner pieces, followed by a sill flashing folded down and sealed to the existing WRB.

David is kind of old school and cheap. He still uses a non-stretchy flashing membrane because kids these days have it too easy with their flexible stretchable flashing tapes.

So I cut my pan flashing a little long. But before I put my pan flashing in—we could use flexible things, but that's not always available to everyone, and I like to work with good old reliable materials that you can get everywhere.

So I put in the sacrificial corners first, and it is cut and stretched opposite of what we're going to do with the sill.

The little corner pieces are just to provide protection to that corner of the sill. They are cut just shy of the corner, and the flashing tape is stretched over the vulnerable point.

The sill flashing is cut about 8 inches longer than the sill, so it’ll fold up each side a bit.

I like my pan flashing to go all the way to the inside...

The sill is cut and stretched opposite how the little corner pieces were,

Now I stretch that other one with a little upcut—I just like that angle better

...and then the jamb flashing will provide triple coverage over that weak spot. This jamb flashing is not meant to integrate with the WRB, it is strictly to protect the framing, the WRB is integrated after the window is in the hole.

He cuts the jamb square to the sill and folds the flashing tape into the framing. Now the rough framing is protected from any water leaks and we can install the window.

David lines up the center marks on the window and the house wrap and reaches for his level. And his level is happy.

Usually, the bottom corners are nailed, but when working alone, it’s wiser to nail the top corners first.

Now I'm level, my sill is good

Because no one else is holding the window.

I've got my center mark, I'll put a nail in to hold it.

A little out of plumb

He pries the bottom over slightly to plumb the window and he sticks a nail in that corner, too.

I'm just sliding the window a little to the left, to plumb it up nice. Setting a nail in the bottom. I'll put a nail in each corner to hold the window in place.

Now he checks to see if the unit is square, which it is, so he nails that last corner, and goes inside to double-check that placement of the window to make sure it is in the right place. Which it is.

With the window centered, square, level, plumb, and nailed off, David seals the window to the original WRB, the Tyvek Home wrap installed under the foam.

To make the best seal, fold the tape past the flange onto the actual window frame.

Like I said, I want this stuck to the window itself, so I bend it a little. The tape is actually a little wider than the flange of the window—one-half of it peeled back—so I align it with the flange, going up above the window, so my head flashing can cover it—everything will be shingle overlapped—then I press the tape into the window onto the actual metal part of the window, and NOT just the nailing flange.

The side pieces of flashing tape should extend above the top of the window frame, but not above the top of the head flashing tape.

And that top piece of flashing tape tucks behind the Tyvek flap cut earlier so that water leaks are always kicked out and away from the building.

We want good shingling, we have our house wrap turned up and I'll stick this onto the turn because there's still a seam there.

This fully integrates the window and the WRB. Nothing’s getting in, but if it does, it can dribble out the bottom.

And he nails the bottom corners because he is a carpenter.

Tape goes over the diagonal cuts and the bottom is left open for drainage.

You wanna air seal the inside?

Hell yeah

When putting this in, we're pushing it all the way to the outside so that I keep a drainage plane.

Inside, the backer rod is pushed into the gap between the framing and the window before the canned foam is squirted in to completely seal the interior and provide some level of insulation around the perimeter.

The purpose of this backer rod is to maintain our drainage plane to the exterior. If water was to leak in behind our—very robust—flashing it can drain in that space created by the backer rod. If I were to only use spray foam, it would stick to everything and take away any drainable space.

All that’s left to do now is replace the foam pieces we surgically removed in episode 2 and trim out the window with Azek trim boards.

Other parts of this window replacement series:

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