DIY window replacement:
where people lose money

Replacing windows is one of the most-pitched projects in any Ottawa home. Old single-pane or fogged double-pane units leak heat, frost up in winter, and contribute to high gas bills. New windows are an obvious upgrade.

What's not obvious is that "window replacement" is actually two completely different jobs. One of them is approachable for a careful DIYer. The other is full-frame finish work that requires opening the wall, fixing flashing, and re-detailing the exterior. Confusing the two is where homeowners lose the most money.

Here's the honest split.

Insert vs full-frame: the only question that matters

Every window replacement falls into one of two categories.

Insert replacement (also called pocket replacement or retrofit). The new window slides into the existing wood frame. The old sashes come out, the old jambs and exterior trim stay in place. The new window sits inside the existing rough opening like a smaller picture inside a frame. Only the glass and the moving parts get replaced.

Full-frame replacement. The entire window assembly comes out, including the wood jambs, the brick mould trim, and the interior casing. The rough opening in the wall is exposed. New flashing goes in, the new window's nailing fin attaches to the sheathing, the new exterior trim and interior casing get installed.

The decision is usually made by the condition of the existing wood frame. If the existing jambs are sound (no rot, no soft wood, no daylight visible at corners), an insert is the right call. If the existing wood frame has rotted at the sill or shows water damage anywhere, a full-frame is the only correct fix.

The cost difference: an insert install carries meaningfully less labour per window than a full-frame. The materials cost is similar, but full-frame requires re-trimming inside and out, which is real time and the main driver of the gap between the two scopes.

The brick mould detail (and why DIYers miss it)

Brick mould is the wood or PVC trim around the outside of a window in a brick or stucco veneer wall (most Ottawa homes built between 1960 and 2000). The brick mould bridges the gap between the window frame and the brick veneer, sheds water away from the rough opening, and supports the window in the opening.

The detail that determines whether the window leaks: the flashing behind the brick mould.

What goes there in a properly built window:

1. The rough opening is wrapped with self-adhered membrane (Blueskin, Grace Vycor, or equivalent) on the sill and up the jambs at least 9 inches.
2. The window's nailing fin is taped to the sheathing all four sides with sheathing tape.
3. Z-flashing or step flashing wraps over the top of the window head, tucked behind the housewrap above and lapping over the front of the brick mould.
4. The brick mould is installed and caulked at the perimeter only on the top and sides, NOT the bottom.
5. The bottom of the brick mould stays open to drain. Any water that gets behind the brick veneer runs down the housewrap, hits the flashing under the window, and weeps out at the bottom.

The DIY mistake: caulking the brick mould all four sides including the bottom. Now any water that gets behind the brick (and water always gets behind brick eventually) is trapped at the sill, can't drain, and rots the framing. We've torn out 15-year-old "newly replaced" windows in Ottawa where the entire sill is gone because the bottom of the brick mould was caulked shut.

Insulation behind the brick mould

Between the window's outer frame and the rough opening in the wall, there's a 1/4 to 1/2 inch gap that has to be filled with insulation. The wrong material here is what makes most DIY window installs leak air for the life of the window.

The wrong choices:

• Fibreglass batt jammed into the gap. Looks like insulation, isn't an air seal. Air leaks straight through fibreglass. The window feels drafty for the next 25 years.
• Standard expanding spray foam (Great Stuff yellow). Expands too aggressively. The pressure of the foam expanding can bow the window's jambs inward, which jams the sashes. By the time you realize, the window won't close properly.
• No insulation at all. The cavity stays empty. Air moves freely between inside and outside through the gap.

The right choice: low-expansion window-and-door spray foam (Great Stuff Window & Door, blue can, or equivalent). Specifically formulated to expand minimally and not bow the frame. Apply in thin passes, fill the gap from inside, let cure 4 hours, trim the excess flush with a knife.

The foam is the air barrier. Backer rod and a continuous bead of high-quality exterior caulk on the outside of the brick mould-to-veneer joint complete the air seal. Two layers, both important.

Caulk and spray foam, used right

The right products in the right places:

Inside the wall cavity (between window frame and rough opening). Low-expansion polyurethane spray foam. One product. Don't substitute.

Outside, brick mould to brick/siding joint, top and sides. Polyurethane sealant (e.g. Sikaflex 1A, Quad Max). NOT silicone, NOT acrylic latex. Polyurethane bonds to mineral surfaces (brick, stucco) better than other sealants and stays flexible for 20+ years.

Inside, casing-to-drywall joint. Paintable acrylic latex caulk. This is interior cosmetic, not air-sealing. The air seal is the spray foam in the cavity.

The mistakes I see most:

• Silicone caulk on the exterior brick mould joint. Silicone doesn't bond well to brick or stucco and won't take paint. Within 5 years it shrinks, peels, and looks bad.
• Acrylic latex on the exterior. Acrylic isn't UV-stable enough for exterior exposure. Cracks within 2-3 years.
• Foam left untrimmed on the exterior. Spray foam degrades in UV light. Always trim flush and cover with trim or caulk.

The DIY install, step by step

For an insert window replacement on a single-hung or slider where the existing frame is sound:

1. Measure the existing frame inside dimensions. Order the new window 1/4 inch smaller than the rough opening width and height. (The 1/4 inch gap allows for shimming and squaring.)
2. Remove the old window's stops (the trim pieces that hold the sashes in place). Pry the bottom sash out, pry the top sash out. Inspect the inside of the existing frame for rot. If you find any, stop and reassess.
3. Test-fit the new window in the opening. It should drop in with 1/8 inch gap on each side.
4. Set the window in place. Shim the bottom level (use composite shims, not wood, in case of any future water exposure).
5. Plumb the jambs. Shim where needed.
6. Square the window using diagonal measurements. Diagonals should match within 1/8 inch.
7. Install screws through the new window's frame into the existing wood frame at the manufacturer's specified locations.
8. From inside, fill the gap between the new window and the existing frame with low-expansion window foam.
9. From outside, fill any remaining gap between the new window and the existing brick mould with backer rod and polyurethane caulk on the top and sides only.
10. Re-install interior stops or new casing.

For an experienced DIYer, an insert replacement runs 2 to 4 hours per window. For a first-timer, plan 6 hours on the first one and 3 hours per window after that.

Why DIY full-frame is where the money is lost

Full-frame replacement is where homeowners get into trouble. The job looks similar from the inside (cut out the old window, put in the new one). What's different is everything between the wall sheathing and the brick veneer.

The failures we get called to repair:

1. No flashing under the sill. DIYer popped the old window out, slid the new one in, foamed and caulked. No self-adhered membrane wrapping the sill. Within 3 years, water has tracked down the back of the new window, into the rough opening, and rotted the wall framing below.
2. Housewrap not lapped over the nailing fin correctly. Housewrap above the window should lap over the top of the window's flashing tape. If it tucks under instead, water runs down the housewrap, into the rough opening, and rots the framing.
3. Brick mould caulked at the bottom. Water trapped, sill rot, full-wall rebuild required.
4. Out-of-square install. The new window twisted in the opening because shimming wasn't done methodically. The window appears to work but the seal between sashes isn't tight, the window leaks air, and the warranty is void because it was installed out of plumb.

A full-frame window install in a typical Ottawa wall has 6 to 8 distinct flashing details. Each one has to be done in the right sequence, with the right materials, and lapped correctly. A single missed step means water gets behind the wall and rots framing for the next decade.

Insert window replacement is a finish carpentry job. Full-frame replacement is a building envelope job. The difference between them is whether the wall stays dry for the next thirty years.

The honest call

For an insert replacement on a basement, garage, or non-critical window where the existing frame is in good shape, DIY is reasonable. The labour savings are real and the failure modes are mostly cosmetic.

For full-frame replacement, anywhere there's existing rot, anywhere on the second floor or above (where leaks travel inside walls before showing up below), or anywhere the exterior is something other than vinyl siding (brick, stucco, stone), this is professional work. The labor savings are not worth the hidden water damage risk.

For larger window-replacement projects across multiple openings, especially in older Ottawa homes where window sizes vary and rough openings need adjustment, this gets folded into broader home renovation scopes where the windows, exterior siding, and air-barrier work happen together. That's the cost-effective approach for whole-house window replacements anyway, since the labor to set up scaffolding and tarp the work area is shared across all the openings.