A common scene in new Canadian homes: two winters after move-in, a stain shows up on the drywall under a bedroom window, or a cold draft runs along the stool on a January night. The window is usually blamed, and the window is usually fine. The fault is almost always in the few inches around the frame: a flat sill with no pan, a membrane lapped the wrong way, foam that bowed the frame, or a gap in the interior air seal. All of these are hidden once the siding and drywall go on.
The reference for doing it right is CSA A440.4, the Canadian standard for window, door and skylight installation, together with the installation instructions of the window manufacturer. Your provincial code and your municipality may add requirements, and the BC Energy Step Code or a high-performance build in Ontario or Quebec will test airtightness with a blower door. Check the current rules with your building department or designer. What follows is the sequence, stage by stage, with what an owner can verify on site.
Stage 1: Rough opening preparation
- Size: the rough opening matches the size the manufacturer specifies, typically about 1/2 to 3/4 inch larger than the frame on each side, enough for shims and foam but not so much that the gap cannot be sealed properly.
- Level and square: the sill is level and the diagonals of the opening are close to equal. A twisted opening means a window that will bind or leak.
- Sheathing membrane: the housewrap or sheathing membrane is cut so the top flap can be folded up and taped back temporarily, ready to lap over the head flashing later.
- Clean, dry substrate: no sawdust, frost or wet sheathing where tapes and membranes will go. Many self-adhered products need a primer below certain temperatures.
On a late-fall or winter build, ask what the crew does about cold-weather adhesion. Self-adhered membranes applied to frosty plywood can look fine on the day and peel in spring. A temporary heat source or a primer rated for low temperatures costs little compared to removing siding later.
Stage 2: Sloped sill pan and side flashing
- Slope: the sill slopes toward the outside, using a pre-made sloped sill block, a bevelled wood shim or a formed pan. Water that gets past the frame must have a way out.
- Back dam: the inside edge of the pan is turned up, so water cannot be driven inward by wind pressure.
- Corners: the corners of the pan are fully sealed, with a corner patch or a stretchable membrane, not two cut pieces butted together.
- Lap onto the wall: the pan membrane runs down over the sheathing membrane below the opening, never tucked behind it.
- Drainage path: the bottom of the window is not sealed solidly to the pan at the exterior, so the pan can drain. Follow the manufacturer on where continuous sealant goes and where gaps are left.
The single most useful idea for an owner is shingle lapping: every upper layer overlaps the layer below it, like shingles on a roof. Take a photo of each window sill before the unit goes in. If you cannot see a slope and a back dam, ask before the window is set, because after that the pan is gone from view for good.
Stage 3: Setting the window, foam and the interior air seal
- Shims: the window sits on shims at the points the manufacturer shows, under the vertical members of the frame, not in the middle of a long sill.
- Fastening: nailing fins or clips are fastened at the spacing given in the instructions, without crushing the fin or distorting the frame.
- Operation: each operable sash opens, closes and locks smoothly once fastened. Check this again after foaming.
- Low-expansion foam: only foam labelled for windows and doors is used, and the gap is not overfilled. Ordinary expanding foam can bow jambs.
- Air seal: the continuous interior seal ties the window frame to the air barrier of the wall, usually with a backer rod and sealant or a tape, connected to the polyethylene or the airtight drywall approach used on the house.
Foam alone is insulation, not a reliable air seal: it shrinks, cracks and pulls away over the years. Ask your builder where the air barrier line runs around each window and how it connects to the rest of the wall. On a house that will be blower-door tested, a leaky window perimeter is one of the first places a technician finds air movement, and it is far cheaper to fix while the jambs are still open.
Stage 4: Head flashing and the details for exterior insulation and rainscreen
- Head flashing: a membrane or metal flashing is applied over the head, then the top flap of sheathing membrane is folded down and taped over it.
- End dams: metal head flashing is turned up at its ends, so water does not run off the corners and into the jamb.
- Window position: with exterior rigid or mineral wool insulation, the drawings say whether the window sits at the sheathing plane, within the insulation, or out at the face of the insulation.
- Bucks or brackets: if the window moves outward, a plywood box (a buck) or engineered brackets support it, and the waterproofing wraps the buck back to the sheathing membrane.
- Rainscreen cavity: where strapping creates an air gap behind the siding, the trim and flashings are detailed so the cavity drains at the sill and head and is screened against insects.
Exterior insulation changes the geometry, not the rules. A window left at the sheathing creates a deep exterior return that must be flashed and sloped, but keeps the drainage plane simple. A window moved out to the insulation face reduces thermal bridging and condensation on the frame, yet the weight and the waterproofing depend on the buck or bracket detail. Neither is wrong; what matters is that the drawings show one clear detail and the crew builds it. Homes with many large openings, like a contemporary two-storey with 3,625 sq ft and walls of glass or a modern farmhouse of 3,371 sq ft with tall gable windows, deserve a dedicated window section in the construction set. If big glazing is what draws you, the plans with large windows are worth reviewing with this detail in mind.
Stage 5: Owner walk-through before the siding goes on
- Photos: every opening photographed at pan, set and head stage, so you have a record once it is covered.
- Laps: at every window, the membrane below overlaps out, the membrane above overlaps down. No reverse laps and no taped "fixes" over reversed layers.
- Taped seams: tapes pressed flat with no fishmouths or wrinkles that channel water.
- Garden-level and basement windows: on homes with a full basement, the windows near grade have the same pan and flashing, and window wells drain.
- Labels and instructions: the manufacturer's instructions are on site and the labels on the units match the windows specified, including the performance ratings required for your region.
Plan this walk-through with the builder and do it room by room. A single-storey home such as this 1,745 sq ft Northwest-style bungalow with three bedrooms and a basement can be checked in an hour from the ground. A three-storey Craftsman of 2,244 sq ft with five bedrooms or a 2,750 sq ft farmhouse over three levels has upper windows you will only reach from scaffolding, so book the visit while it is still up. If something looks wrong, ask for an explanation in writing referencing the manufacturer's instructions or CSA A440.4, rather than an argument on the spot.
The most common mistakes
- Flat sill, no pan: water that gets past the frame has nowhere to go but into the wall.
- Reverse laps: the head flashing tucked over the housewrap instead of under it.
- Sealed bottom fin: continuous sealant across the bottom that traps water in the pan.
- Wrong foam: high-expansion foam that bows the frame and jams the sashes.
- No air seal: relying on foam alone, with no connection to the wall's air barrier.
- Improvised exterior insulation detail: windows in thick walls with no drawn buck or bracket detail, decided on the day.
