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Liquid Flashing a Window Opening in a Double-Stud Wall

A monolithic layer of protection for holes in walls
February 22, 2020

Last time, we were bucking out the window openings in a double-wall house. While we were at it, we cut holes where the windows go.

Step 1: Slope the sill

Now that the openings are open, we can get ready to flash them, beginning with a sloped sill. And because we’re going to flash and film four windows today, Ben cuts them all at once and then installs each one too.

Rather than using tape to flash this window, Ben’s going to demonstrate how to do it with liquid flashing. He gathers hit little kit and walks us through the process and his thinking.

Mask the opening with painter's tape

"I previously worked out where the window locations are in this wall, they're kind of an innie-window. They're set in about three inches into the opening.

So I worked out where I want the waterproofing to come to. Right now, I'm giving myself some marks to mask off the opening. It's not totally neccesarry to mask off the opening, but I tend to be a little bit OCD, and I like the look of a clean line.

Doesn't make it perform any better, but when the homeowner sees it and other contractors see it, they see that you're giving a level of care to your installation. And it'll drive other subs on the job to follow suit as well as give the homeowner and client confidence that they're really getting a nice install."

He uses painters tape to mask the inner edge — inside of the interior window line a little way.

"That gives me a place—when I foam or tape the interior of the windows I know that if there is ever to be any water to travel back in behind the face of the window, it's still going to be protected until it hits foam or tape."

Before masking off the outer perimeter, he makes sure the sheathing is as tight as possible to the framing. Running nails around the perimeter and whacking them all home with his hammer.

He masks off the outside and cuts the existing flashing tape out of the way because it just introduces topography that is not needed and will not make it easier to get a tight seal.

Now, we’re ready to break open some tubes. Ben begins at the weak spots: corners and seams.

To cover the sill, he tries out a duck-bill nozzle for the sausage tube.

Seems to work.

After a little while, you will get a feel for how much you need to apply in order to spread it to the correct thickness. Inevitably, you’ll have a reserve pile on the next surface in front of you that hasn’t been sealed yet.

"This stuff is also very forgiving. you can lay it onto cold wet substrates and it will stick to them. Not like tapes, where if you're trying to flash pans first thing in the morning and it's wet, or you've got dew, you've got to wait for that to dry off with tape. This stuff, as long as you clear the standing bulk water off of it, you're good to go."

Once it is spread out, he smoothes over the edges and looks for weak spots.

"OK, so I’m shooting for 12 mils, and I've got material on my 12 mil line, and that means I'm at my minimum thickness. Installed thickness, they want between 12 and 15 mils."

Work your way down the sides and across the top of the opening, and then you can finish up on the outer surface of wall sheathing.

Out here, he adds a dab every couple of inches to spread an even surface.

First, it is about spreading the membrane over the entire surface around the window opening.

"The other benefit of a liquid flashing product is that you don't have any lap situations, so if we were trying to do this whole rough opening perimeter in tape, we'd end up with areas where we'd have three-layers thick, and a number of edges and folds. In theory, those should be impermeable, but if the tape were improperly installed, it could then funnel water right into our enclosure.

With the liquid product, it becomes a monolithic barrier, and we don't have any concerns over that happening."

The next step is about a smooth consistent surface.

After I get the inside of the opening done and the outside perimeter done, I pay close attention to making sure that the corners are good, because that's the easiest place for it to get too thin.

The tape comes off right away, and that’s pretty much all there is to it except wait for it to dry.

—This is the sixth part of a seven-part series on framing, flashing, and sealing a double-wall house.

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