Flanged Window Replacement in a House with Wood Siding (5/5)

Step 5: Flash, Trim, and Seal the Window

Watch remodeler Bill Robinson as he shows how to use a combination of materials to flash over the flanges, and install a pre-built casing over the new window.

Bill Robinson: Now that the window is installed, I'm ready to flash it. If you remember, I cut back the siding a little over three inches. I'm using a 4-in. self-adhesive butyl flashing tape. I'm using my cutting table to cut my tape to 48 inches. I always start by pulling back the edge of the release paper enough so I can work with it when I get it into position.

Because this window has an applied flange, I need be sure to roll the flashing tape about 1/2-in. up onto the window frame itself to make sure we have a water-tight seal here. I'm going to use my speed square to to push the tape up against the frame and get the overlap tight into the corner. Then I'm going to cut where the tape runs past the corners of the window frame so the tape can lay flat. And I'll just go to the other side of the window and repeat these steps. I also use my J-roller to make sure all tape is tight into the corners.

Next, Bill cuts small pieces of flexible corner-flashing tape to seal off the corners at the top of the window. He also cuts a piece for the joint where the two windows are mulled together.

Bill: Now the rigid metal flashing will go over the top of the window. I'm going to apply a bead of sealant to the back of that, then center it over the top of the window. You can see how the flashing will take any rain that gets under the eaves and deflect it out away from the window. I'll put a couple of fasteners through the flashing to hold it in place.

The final step for flashing is to put a strip of flashing tape over the head of the window.

Because of the narrow space between the window and soffit, Bill will use his shoot-board to trim the top of the casing to fit the opening.

Bill: Now that I have placed the pre-built casing around the window, I'm going to use plastic shims to space it. Once I have the gap where I want it, I'll go ahead and fasten it. I'm using corrosion-resistant trim-head screws with small heads that won't leave a large hole or dimple in the face of the material.

Bill is careful to set the screws far enough away from the window so that he doesn't drive one through the mounting flange.

Bill: I left an intentional gap around the window. Now I'm going to put some backer rod in that gap and then fill it with sealant. Once we've caulked the window to the casing I'll seal the casing to the siding.

I like to push the caulking tube rather than pull it so that I can see the sealant coming out of the front and know that I'm filling the joint better that way.

That finishes up the exterior install; we're sealed up watertight. Now it's time to go inside and do the air-sealing.

The best way to air-seal from the inside is with a low-expanding polyurethane spray foam. I want to get a nice even bead between the framing and the window to prevent air from moving through the opening. I didn't need to do that on the bottom of this window because the sill pan I installed earlier has a foam gasket to provide the needed air seal. If I didn't have that, I would have to put spray foam across the bottom.

That's it. the outside is watertight. The inside is air-sealed. We're ready for drywall and trim. We're done.

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