You can buy the best window on the market, but if it isn’t installed properly, it will never perform up to your expectations or the manufacturer’s specifications. Thorough care must therefore be taken when putting in new windows.

According to the Siding and Window Dealers Association of Canada (SAWDAC), “Aside from seal failures, over 80% of service problems with windows result from poor installation practices.”* In the vast majority of cases, problems stem from improperly aligned windows, or poor adjustment of the shims between the window frame and the building structure.

From taking the initial measurements to applying that last bead of caulking, accuracy is a must. Certain details of the work require particular care.

Sizing it right

  • Plan for a gap of about ½ in. (13 mm) along the entire perimeter of the window, between the frame and the rough opening, to allow room for the shims and for proper insulation around the frame.
  • Check that the rough opening is square. Measure the diagonals of the opening: they should not be off my more than 1/8 in. (4 mm).

Shims and fasteners: where do they go?
Shims are the pieces of wood (usually cedar shingles) used to ensure that the window sits level, square and plumb in the window opening.

  • Insert them in pairs, one on top of the other, but reversed, so as to provide level support for the frame across the full thickness of the wall.
  • Under the windowsill: insert shims at 16-in. (400-mm) intervals.
  • On either side of the window frame: insert them near the top and bottom, and in the middle (add more as needed, to ensure that no two pairs of shims are more than 24 in. [600 mm] apart).
  • On top of the frame (the head): normally, no shims should be used here, because no structural load from the building must be transmitted to the window.
  • The window should be fastened using anodized (rust-proof) screws, driven through each shim.


  • The entire gap between the window and the opening must be filled with insulating material, but without creating any stress that could bend the frame.
  • A good choice is low-expanding polyurethane foam, which will swell to fill the slightest gaps. Note, however, that this type of insulation may not be appropriate with a frame made of vinyl or aluminum, as these two materials tend to expand and contract, which could compromise the bond between the insulation and the frame. Consult your window manufacturer.
  • If you use mineral wool insulation, make sure you do not compress it too much, otherwise it will lose its insulating property.
  • If applicable, connect the air barrier and vapour barrier to the window to ensure a continuous seal.

Check, double-check and triple-check

  • At every step in the installation process, check again that the window is level, square and plumb.

Other important aspects

  • When replacing a window, it is a much better idea to remove the old framing entirely rather than position the new frame against it. Doing so will eliminate existing sources of heat loss and air leakage, and help ensure better energy efficiency.
  • Position the new window as close as possible to the interior finish so that the glazing is aligned with the warmest part of the wall; this will reduce the risk of condensation.

There are many other factors that can have a huge impact on a window’s performance; e.g., installing flashing (waterproofing membrane) to protect the building structure, and using sealant with superior elasticity and adherence.

*SAWDAC Consumer’s Guide to Understanding Windows and Installation: