Level of Difficulty: Intermediate Do-It-Yourselfer – Moderate
Completion Time: 3 Hours
Installing a window is actually a fairly simple project. Although the fit may vary according to the manufacturer, the installation process is basically the same. There are basically two types of windows: windows with a nailing fin and windows without a nailing fin.
This project outlines the installation of a nail fin window, which means you’ll need to install flashing to ensure the opening is watertight. Installing a nail fin window is more involved than installing a window without a nailing fin. Finless windows include exterior decorative moulding, which fills the gap between the window and exterior cladding.


  • Stapler
  • Mitre Box
  • All-Purpose Knife
  • Caulking Gun
  • Hammer
  • Carpenter’s Level
  • Drill
  • Punch


  • Wood shims
  • Finishing nails
  • Galvanized framing nails
  • Trim and moulding
  • Fibreglass wool
  • Drip cap
  • Self-adhesive waterproofing membrane/Primer aerosol
  • Self-adhesive flashing
  • Silicone-based caulking
  • 2 ½” or 3 ½” No.8 Galvanized screws



For both renovation projects and new construction, correct measurements are essential.
For new construction:
Measure the window opening at three different places for width, height and depth, and keep the smallest measurement for each; the window will adjust in the opening.
For interior or exterior renovation:
For an interior renovation project, remove mouldings to expose wall studs. Take three measurements of the interior width, and three of the interior height. To measure depth, remove fiberglass insulating wool and measure between the drywall and exterior frame. For exterior renovation projects, measurements are taken from cladding to cladding, either vinyl or brick. You need to take three measurements each for the height and width, and retain the smallest.


The window casing is not always big enough to fill the total thickness of the wall. You must therefore measure for interior and exterior additions, commonly referred to as face flange, to obtain a good finish on both sides of the wall. Once the window is in place, it will be possible to determine what is required.
Face flange on the exterior will be covered with J moulding if the exterior cladding is vinyl, and with brick moulding for brick cladding.
In the interests of insulation, a window should always be installed on the warm side of the wall. There is less risk of condensation on the inside of the wall because the glass is swept dry by the warm air in the room.
This project outlines the installation of a nail fin window, which means you’ll need to install flashing to ensure the opening is watertight.
Finless windows include exterior decorative moulding to fill the gap between the window and exterior cladding. Before installing the window, the existing frame must be watertight with the wall structure. It’s not necessary to remove the exterior cladding. Make sure, however, that the opening is free and clear. Simply add an adhesive flashing on the sill and bottom of the studs and insert the window from the inside of the house. Adjust with shims, then screw securely. Insulate the interior and exterior perimeter and run a bead of caulk.


1.1. The wall studs on either side of the door are double studs.

1.2. Verify the condition of the door frame and threshold, and replace or repair if needed.
Make sure the threshold is water and moisture tight.

1.3. Make sure the threshold is level; if not, level it with wood shims.

1.4. Place the door frame in the opening making sure you leave sufficient space around the door (3/8” on either side and on top).

1.5. Remove the door.

2.1. From the outside, place the window into the rough opening so that the nail fin is well-supported.

2.2. From the inside, use (cedar) wood shims to secure the window; shims should be doubled up (one shim enters from the exterior side of the window, and the other from the interior) so as to form a rectangular block.

2.3. Insert shims all around the window at 12″ intervals, using a hammer.

2.4. Use single shims for the bottom of the window; you’ll adjust them later to square the window.

2.5. Use a carpenter’s square to check that the casing is plumb and level.

2.6. Fasten the window frame to the frame of the house with long galvanized screws (2 ½” or 3 ½” No.8 screws), pre-drilling the holes.
Apply a small amount of caulk to the screw threads, then drive the screws through the window frame, through the shims, and into the frame of the house. If your window comes with metal fasteners, drive screws through the openings. It won’t be necessary to screw through the casing.

2.7. Cut the ends of protruding shims with an all-purpose knife.

2.8. On the outside, nail the nailing fin to the window perimeter, taking care not to dent the fin.

To insulate the space between the casing and the frame of the house, there are essentially two options: cushioned insulation strips, or polyurethane foam. For PVC windows, the best insulation material is mineral wool; avoid using compressed foam, because it does not adhere well to PVC. A badly insulated window will not be energy efficient.

3.1. From the inside of the house, insert fibreglass insulating wool all around the window casing using a flat instrument, like a shim; do not over-compact, to the point that it loses its insulating properties.

3.2. Put low-expansion foam (compressed neoprene) around the windows for optimum energy efficiency.

3.3. Add silicone caulk.

4.1. Protect the window, glazing and frame, then apply an aerosol adhesive to the perimeter of the window.

4.2. Cover the perimeter with self-adhesive flashing, starting at the bottom and continuing with the sides, as close as possible to the window frame.

4.3. Measure the length of drip cap you need and install on the upper part of the opening.

4.4. Cover with self-adhesive flashing and lower the waterproofing. Glue.

4.5. Run a bead of caulk around the perimeter of the window.

The J-mouldings line the windows and prevent water from seeping in behind the vinyl facing.

5.1. Measure the outline of the first window.

5.2. Using metal snips, cut the moulding 2″ longer than the length of the bottom of the window.

5.3. Cut a 1″ notch on each side of the nailing flange, on the part that rests on the window sill, leaving the front intact.

5.4. Nail the moulding under each overhang, with an extra 1″ on each side.

5.5. Use metal snips to cut two lengths of J-trim, 2″ longer than the sides of the window.
Nail to secure.

5.6. Cut the J-trim for the upper part of the window, 2″ longer than the sides of the window.
At each end, cut a 1″ notch in both corners at the bottom of the J-trim and fold back the strip at a right angle.

5.7. Make a 45° mitre cut towards the roof line in the part before the J-trim. Nail to secure.

5.8. Fit the upper moulding strips into the notches of the two lateral mouldings to make a rain water run-off.

5.9. Apply silicone-based caulk to the perimeter of the exterior frame as well as on nail heads.

5.10. Install vinyl siding.