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Window Installation Checklist: Flashing Order, Sill Pans, Foam, Sealant, Shims and Screws, What to Look at on Install Day, and the Faults That Show Up in Year Three
Windows & Doors
What a Proper Window Installation Looks Like, and the Ten Minutes on Install Day That Decide the Next Twenty-Five Years
The window is a sealed box. The leaks are at the edges of the box: where it meets the wall, where water running down the wall meets the top of it, where the sill sits. Every one of those joints is a decision the installer makes in about thirty seconds, with tape, foam or caulk, and you can see most of them if you know when to look. Here is the order, the checklist, and what to check before the trim goes on.
The numbers
6steps in the flashing sequence, in order, every time
1/4 in.shim gap, each side, filled with low-expansion foam
$0extra for foam and tape; it is the job
Year 3when the stain shows up from a skipped step
Full-frame or new construction: the sequence
- The opening. Square within 1/4 in., sill level, sheathing sound, the house wrap or barrier cut and folded in at the jambs and sill, and the head flap folded up and taped out of the way. Sized right.
- The sill. A sill pan (formed, or flashing membrane with the corners patched) covering the sill and turning up the jambs 6 in., over the house wrap at the front edge so water drains out. A sloped sill or a back dam at the inside edge. Nothing seals the bottom of the window to the pan; the sill drains.
- The window in. A continuous bead of sealant on the back of the flange at the head and jambs only. The unit set in the pan, shimmed plumb, level and square at the hinge points and the lock, and checked by operating the sash. Screwed through the frame (not just the flange) at the shims. Check the diagonals again.
- Jamb tape. Self-adhered flashing over the side flanges, onto the house wrap, running from above the head to below the sill pan, lapping over the pan’s upturned ends.
- Head tape, then the wrap flap down. Flashing over the head flange and over the top of the jamb tapes. Then the house wrap flap folded back down over the head tape and taped at its cut corners. Shingle fashion, always: everything above laps over everything below.
- Inside: foam, then the air seal. Low-expansion window foam in the shim gap, full depth, from inside; trimmed flush when cured. Then backer rod and sealant, or an interior air-seal tape, between the frame and the drywall side of the opening. Outside, backer rod and sealant between the cladding and the frame, with the head left to drain if the cladding detail wants it.
Retrofit: the shorter version
The old frame stays, so the flashing is whatever it was. What the installer controls: the old sashes, stops and weatherstrip out and the old frame cleaned; a bead of sealant on the old frame where the new unit lands; the new unit shimmed and screwed through its frame into the old one; low-expansion foam in the gap all round; the exterior joint between new and old capped with bent aluminum sealed at the laps, or trimmed and caulked over backer rod, with the head arranged so water runs off and not in; the interior stop or casing back on. The one thing to insist on is the foam. Fibreglass stuffed in the gap is the 1988 method and it is why your old windows were drafty.
What to look at on install day
- Before the first window goes in: is there a roll of flashing tape and cans of low-expansion foam on the truck? If not, ask what they use. “Caulk” is the wrong answer.
- After the first window is in, before the trim: look at the sill. Pan or tape turned up the jambs? Look at the head: tape over the flange, wrap folded over it? Look inside: foam in the gap, all round, not just the top?
- Operate every sash. Casements crank smoothly and lock without lifting. Sliders roll without binding. A sash that catches means the frame is racked by a shim; it is a two-minute fix now and a callback later.
- Check plumb with a level on the jamb and look at the reveal between sash and frame: even all round. Uneven means out of square.
- Outside, after the trim: sealant joint continuous, tooled, over backer rod where the gap is wide; no sealant across the bottom of a drained sill; head flashing or drip cap present on anything with trim.
- Count the screws. Through the frame at each shim point, at least two per jamb on a small window and more on a tall one.
Ask before the trim goes on. After it is on you are trusting. Most installers are glad to show you; the ones who are not are the ones to watch.
The faults that show up in year three
| What was skipped | What you see |
|---|---|
| Sill pan | Stain on the drywall under the window; soft sill; sometimes nothing until the trim comes off |
| Head tape, or wrap not lapped over it | Stain at the top corners inside after a wind-driven rain |
| Foam | Draft at the casing; frost on the frame in January; dust streaks at the trim |
| Sealant on the sill flange | Water trapped in the pan; the sill rots from below |
| Screws through the frame | Sash that goes out of adjustment every season; a frame that bows |
| Shims at the lock point | Casement that will not lock in winter |
| Expanding foam instead of low-expansion | Frame bowed in; sash binds; glass stressed |
Where homeowners go wrong
- Judging the job by the trim. Trim covers everything. Look before it goes on.
- Accepting “we caulk it well”. Caulk is the last line, not the first. Tape, pan, foam, then caulk.
- Not operating the sashes before paying. The adjustment is free on install day.
- Paying the balance before the exterior is finished. The sealant and the capping are the last day’s work and the most skipped.
Questions people ask
Should windows be foamed or caulked?
Both, in different places. Low-expansion foam fills the gap between frame and opening for air and thermal sealing. Sealant closes the exterior joint against water and the interior joint against air. Caulk alone is a draft with a nice bead on it.
Should a window sill be caulked?
The exterior joint between the sill nosing and the cladding, yes, except where the detail is meant to drain. The underside of the window flange at the sill, no; it sits in a pan that drains out.
Can windows be installed in winter?
Yes, with foam and sealant rated for the temperature and tapes that stick in cold (or a primer). The opening is open for an hour per window; plan the order so the house is never open on two sides.
Put the checklist on the quote
The scope the calculator writes names the foam, the sealant and the flashing so every installer quotes the same job. If the quotes you have do not mention any of it, $99 gets them read for what is missing.
Sources and further reading
- Keeping the Heat In: Natural Resources Canada. The free guide to windows, doors, air sealing and insulation written for Canadian houses.
- Ontario’s Building Code: the official page, with the current regulation. Egress is 9.9.10, glazing and windows 9.6 and 9.7, energy is Supplementary Standard SB-12.
