Window installation, and how to tell it was done right

What is window installation?

Window installation is the physical work of fitting a window into an opening: preparing and flashing the opening, setting the unit level, square and plumb, insulating the perimeter gap, sealing the exterior, and trimming inside and out. A typical opening takes a competent two-person crew under two hours. Almost every complaint homeowners have about new windows - drafts, leaks, sashes that bind - is an installation fault, not a product fault.

Signs you need window installation

• Your new windows are in and you have a draft you did not have before, which usually means the perimeter gap was packed with fibreglass rather than sealed with low-expansion foam
• A sash binds or the lock does not meet, which means the unit was fastened out of square and the frame is twisted
• Water appears on the interior sill during driving rain but not in ordinary rain, the classic signature of a missing or reversed flashing lap
• You can see daylight or feel air at the corners of the interior trim
• The exterior sealant has cracked or pulled away within the first year, which means the wrong sealant, a dirty substrate, or a joint too wide to hold
• The reveal - the gap between sash and frame - is visibly wider at one end than the other

Seeing something not on this list? Browse windows problems by symptom →

How window installation actually works

1. 1

   Site measure, weeks before anyone installs anything

   Replacement windows are manufactured to your openings, so the measure is the single point where a job is made or lost. Each opening is measured at three heights and three widths, and the smallest figure governs.

   Watch forOne quick measurement per opening with a tape held at eye level. Old houses are rarely square, and a unit ordered to the widest dimension will not go in.

2. 2

   Protection and removal

   Floors and furniture are covered, the interior stops or trim come off, and the old sashes are taken out before the frame. On an insert installation the sound original frame stays; on a full-frame the whole assembly comes out to the rough opening.

   Watch forOld paint being sanded or scraped dry in a home built before 1978. That is lead-safe-work-practice territory and there are federal rules about how it is handled.

3. 3

   Inspecting the opening once it is open

   This is the only moment anyone can see the condition of the sill, jambs and sheathing. Rot, insect damage and a failed water barrier are all found here, and all of them change the job.

   Watch forBad news delivered as a verbal change order. Ask for a photo of what was found before it is covered up again. This is also the honest argument against inserts: an insert leaves the old frame in place, so if water in that frame is why the window failed, an insert buries the problem instead of fixing it.

4. 4

   Flashing and the sill pan

   Water that gets past the exterior sealant has to be led back out. On a full-frame installation that means a sloped or pre-formed sill pan, with the jamb and head flashing lapped over it in the right order - bottom piece first, each piece above overlapping the one below.

   Watch forFlashing tape run in the wrong sequence so an upper piece tucks behind a lower one. It looks finished and it funnels water into the wall. This is the most common invisible defect in the trade.

5. 5

   Setting the unit level, plumb and square

   The window is shimmed at the fastening points until the frame is square and the sash operates freely, then fastened. Shims go where the screws go, so the frame is not drawn out of shape when it is tightened.

   Watch forA crew that fastens first and checks operation afterwards. Every sash should be opened, closed and locked while the unit can still be adjusted.

6. 6

   Insulating and air-sealing the perimeter gap

   The gap between the window frame and the rough opening gets low-expansion foam rated for windows and doors, or is carefully packed and sealed. It is a small detail that decides whether the window feels cold.

   Watch forFibreglass batt stuffed into the gap. It fills the space and stops almost no air, and it is the single most common reason a brand-new window is draughty. Standard-expansion foam is the other failure - it can bow the frame and jam the sash.

7. 7

   Exterior sealing and trim

   The exterior joint is sealed with a suitable sealant and backer where the joint is wide, and exterior trim, capping or brickmould is fitted. Interior stops, casing and sill are refitted or replaced.

   Watch forA continuous bead of sealant sealing the bottom of the window all the way across. Many systems are designed to drain at the sill, and sealing that path shut traps water inside the frame.

8. 8

   Cleanup, labels and handover

   Old sashes, packaging and offcuts leave with the crew. The NFRC and energy labels should stay on the glass until any inspection is complete, then come off cleanly.

   Watch forLabels scraped off with a blade, and debris left in the garden bed. Also confirm what was actually installed matches what was ordered - the product line is printed on the label you are about to throw away.

What does window installation cost?

Installation labour is usually bundled into the per-window price rather than quoted separately, and the spread comes mostly from insert versus full-frame, opening size and access, and whether any rot repair is found once the old unit is out. The cost guide breaks each of those out.

See the full window installation cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →