Houseworks: Window installation best practices

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If you’re planning to have your windows replaced, you’ve got two things you need to pay attention to: window quality and installation quality. The thing is, unless you know what a good window installation looks like, you won’t know if you’re getting the most for your money. This article will change that.

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Reducing energy use is the No. 1 reason for replacing old windows, and payback period is between seven and 20 years, depending on the kind of windows you’ve got now.

Another benefit of new windows is reduced road and neighbour noise in the house. Sound transmission is measured either with the Sound Transmission Class (CTC) rating or the Outdoor/Indoor Sound Transmission Class (OITC). Either way, the higher the number, the better the sound blocking capabilities.

The first thing you need to decide on is either full replacement or retrofit replacement. Full replacement windows are just as the windows were installed when your home was being built. They require new interior trim and include some kind of exterior trim or brick mold already in place on the outside window face.

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Retrofit windows, on the other hand, are designed to make use of the existing window jambs, interior trim and exterior trim. All else being equal, replacement windows are less expensive than full replacement models, and they’re easier and quicker to install. That said, full replacement windows result in somewhat more glass area, and they deliver new exterior trim and window sills.

Another thing to consider is window performance levels. Besides the various insulation levels of double and triple pane units, the best windows also pass performance criteria as identified by independent testing. www.windowwise.ca is the best place to get independent window performance results in Canada.

White metal flashing
The white metal flashing over this window is one of the details of high-end window installation. Photo Steve Maxwell

When it comes to actual installation details, these are the points that matter most:

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  1. Caulking should be used to seal the window mounting flange to the exterior surface of the building wrap if it’s visible.
  1. Low pressure/low expansion spray foam should be used to fill the gap between windows and the surrounding rough frame. This helps ensure that window frames don’t bow inwards and fail to open because of foam expansion.
  1. Install temporary props to support window jambs against bowing inward. This is in addition to application of gentle foam. Wooden props cut to fit between opposite sides of the window jamb absolutely ensure that foam expansion poses no problem while the foam expands and cures.
  1. Always use flashing for full replacement jobs. Never rely only on the window installation flange for waterproofing a full window job. Flanges are for anchoring only, and while they do keep out some water, flanges alone are not enough for a reliably dry installation. Self-sticking or metal flashing must always be used for full window replacement. Install the flashing over the top of a window so it overlaps flashing on the sides. This side flashing must overlap the bottom flashing so the whole installation sheds water.

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  1. Install a plastic drainage pan. This is cheap insurance against window leakage. Drainage pans are sloped plastic devices made to sit underneath a window in the rough frame. If any leaks get past the windows, the sloped drainage pan directs the water out of the building harmlessly.
  1. Lap any building wrap over window openings so all the layers shed water.
  1. Install support shims at all four corners of a window frame, and also every 12” along the bottom. Shims should always be used in pairs so they provide full support under the window frame, and made of plastic or solid wood that won’t fall apart if shims get wet.

New windows are a major investment and you should do some homework to ensure you choose good windows installed well. Get things right and you’ll enjoy great window performance for a long time.

Steve Maxwell is partial to old-style windows with divided lites. Visit him online at BaileyLineRoad.com for information and inspiration on home improvements, workshop life and hands-on living.

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