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It’s relatively easy to replace an old window
Apr 13, 2017

Replacing an old window in your home with a double- or triple-pane unit can be relatively easy or quite demanding, depending on your new window’s dimensions.

1. Start by removing the old window. Some are nailed or screwed through the finish frame into the rough frame; others are attached to an outside wall by a screwed flange.

2. If the glass and finished frame are in reasonable shape, the unit can be donated to a Habitat for Humanity ReStore or given to a DIY friend who is upgrading a building.

3. The rough frame into which you will place your new window will be likely constructed of nominal thickness two-by-four (1 1/2 x 3 1/2 ) or two-by-six (1 1/2  x 5 1/2 )  lumber.

4. To be sure, measure the width and thickness of the frame boards; houses built before 1950 were usually framed with full-size members, meaning the boards will have actual dimensions of two-by-four inches.

5. The size of your new window determines the amount of work required to install it.

6. A custom-built unit designed to fit into the existing rough opening is the simplest to install, needing only shims to square it and a handful of nails or screws to secure it within the existing frame. (See manufacturer`s recommendations.)

7. A window unit that is smaller than the existing rough frame requires more work. In this case, a new rough opening must be constructed inside the old frame to accommodate the window`s dimensions. Keep in mind that a one-inch to two-inch gap between the rough and finished frames must be left on all sides of the window to accommodate squaring, pumbing and insulation. Use low expansion spray foam to seal the gap. Save or replace the wall insulation (usually fibreglass batts) before covering the open area between the old rough frame and the new one.

8. Replace the interior vapour barrier you removed with approved six-millimetre material and reinstall exterior house wrap (if there is any) over new OSB or plywood. Match the existing exterior siding and the interior wall covering (normally drywall), then replace the window`s exterior and interior mouldings.

9. A window unit with larger dimensions than the original rough frame requires the most effort to install because the opening must be made bigger to accept the window. Before you cut into a wall (a Sawzall is best for this job), determine whether there are live electrical wiring or junction boxes positioned inside the wall using an AC sensor, available at building stores for about $40. Mark out the new rough opening on the interior wall, drill a one-inch pilot hole for the Sawzall`s blade and cut out the opening. Mark the exterior siding and remove the same way. The Sawzall will chew through studs and nails. The existing header will need to be replaced with a longer and stronger member.  

10. Build the new header by screwing and gluing half-inch plywood between two lengths of two-by-10 for a two-by-four wall; add extra plywood to increase the header’s thickness to match a two-by-six wall. Modern engineered materials are stronger and may be preferable, though costlier. Check the Internet for Canadian building standards for wood and engineered header lengths. For example, two two-by10s will span up to six feet and two two-by-12s up to seven feet.

11. Install the new header on edge, supporting it with two vertical pillars of two-by-four or two-by-six called king posts, which are nailed to wall studs at either end of the beam. King studs sit on the sill of the rough opening. The sill is supported by cripples or short studs nailed to the wall’s base plate and spaced 16-inches OC. Replace the existing sill with a longer one, adding cripples as required.

12. With the help of a friend, lift the new window into the rough frame, using shims to square and level it. Attach the unit firmly into position, according to manufacturer’s recommendations. Spray foam the gap between the rough and finish frames. If necessary, build up the thicknesses of exterior and interior walls with matching material, so that the finish mouldings can be applied as required.