How to Replace a Window (Part 2)

In Part 1 of “How to Replace Window … Again” (PR, May/June 2022), we followed David Joyce’s careful process of removing a double-glazed tilt-in replacement window while preserving both the plaster on the inside and the housewrap and insulation layers on the outside, then prepping the opening for a new, slightly larger, flanged triple-glazed window.

Here in Part 2, we’ll look at what it takes to install the new window, tie it back into the housewrap and insulation, and retrim both sides of the opening.

Sealing the Window to the WRB

Compared to everything Joyce has done to this point, installing the window is a piece of cake, even working solo. He lines up the center marks on the window and the housewrap, checks the head for level, and, because there’s no one inside holding the window, pops a nail into the nailing flange at the upper corner. After checking the jamb for plumb, he pops a second nail into the same flange at the bottom corner.

how to replace a window part 1 — After installing the new window, Joyce begins flashing at the top of one jamb, using split-paper tape that is twice the width of the window flange. Peeling only a few feet of the paper from the window-side of the tape, he starts a few inches above the top of the window and positions the tape so it adheres not just to the flange but runs up the side of the exterior window jamb **[1A]**. After both jambs are sealed, he applies a strip of tape at the top flange, making sure it slides underneath the existing housewrap [1B]. This strip also overlaps the jamb tape at the upper corners, where diagonal cuts in the housewrap are also taped. The bottom flange, however, is not taped, which will allow any water that gets in to drain out.

Then, after measuring diagonally to confirm the window is square, he fastens the rest of the window flange. After heading inside to make sure the window is where he wants it (it is), Joyce heads back outside to seal the window to the flap of housewrap he was careful to preserve when he cut out the old window [1].

Then he heads back inside to prep the window for trim. The first step is to press a thick strip of backer rod into the airspace around the window frame, followed by spray for insulating and air sealing [2].

how to replace a window — Before sealing around the window, Joyce presses backer rod into the air space, making sure it bottoms out against the flange **[2A]**. Without the backer rod, the spray foam would adhere to the back of the flange, blocking the path of escape for any water that leaked in. The final step before casing the window is to fill the void at the perimeter with low-expansion foam [2B].

“The purpose of this backer rod is to maintain our drainage plane for the exterior,” he explains. “If water was to leak in behind our robust flashing, it can drain in the space created by the backer rod.”

Otherwise, the spray foam he uses to fill the voids around the window will stick to the back of the window flange, blocking the path for water to drain out the bottom.

Putting the Puzzle Back Together

With the interior side ready for trim, Joyce heads back outside to button up the exterior. The first step is to replace the foam jigsaw puzzle pieces that he surgically removed when he removed the old window [3].

replacing a window — Because the new window is bigger than the old one, the salvaged pieces of foam need to be reduced in width. Joyce dry fits the foam and marks the cut [3A], trims away the excess from the inside edge with a long-blade utility knife **[3B],** pieces the parts back together, and tapes the seams **[3C].**

installing a new window

Next, he cuts back the clapboards [4] because the new AZEK trim is the same width as the old wood trim, but the new window is bigger than the old one. Next, he measures for the PVC extension jamb, cuts and assembles the pieces, and dry fits it at the window to mark the siding for one last cut at the sill horns.

installing a new window

how to install a window — After cutting and assembling the AZEK extension jamb, he dry fits it **[4D]** and scribes around the sill horns **[4E]**, then makes the cuts with a multitool **[4F].**

Before fastening the extension jamb in place, he sprays the newly exposed edges of the wood siding with primer.

The last step is to install the casings, which are screwed in place and finished with Cortex plugs. Before installing the head casing, Joyce cuts a piece of metal flashing to size and slides it behind the furring strips [5].

replacing window

“You can’t really tape the top of the flashing to seal it to the foam, which is the drainage plane,” Joyce explains. “But most of the water that might leak into this wall will be directed out with this flashing.”

And because he just opened the wall and inspected the inner layers, he’s pretty confident that there’s not much water leaking in anyway. And the little bit that does quickly finds its way out.

One down, 37 to go.