Windows and doors can account for up to 25 percent of total house heat loss. There are a number of options for upgrading the energy efficiency of your windows. Windows can be repaired by servicing hardware such as latches, cranks and locks or retrofitted with caulking and weather stripping or adding glazing and storm windows. At times the best choice is total window and frame replacement with new, high- performance ENERGY STAR® certified windows or inserts. If the frames are still in good condition, inserts (i.e. new sash and glazing units) can be a good option, especially for homes with heritage status.

Casement window showing parts and hardware
Casement window showing parts and hardware

Replacing Glazing, Sashes and Windows

Properly installed energy-efficient windows make homes more comfortable by reducing drafts and increasing the temperature of the interior side of the window, reducing condensation. Energy-efficient windows will have many of the following features:

  • double-, triple- or even quadruple-glazing
  • low-emissivity (low-E) glass
  • inert gas, such argon or krypton in the sealed unit
  • low conductivity or warm-edge spacer bars
  • insulated frames and sashes
  • good air tightness

If your inspection has revealed serious problems with a window’s glazing, sash or the entire unit, your best option will be to replace all or part of the window.

For example, if the glazing is only a single pane of glass or is in poor condition, you can buy a new sealed glazing window insert. If the frame is in poor condition, it may be time to replace the unit.

Double-hung window showing parts and air-leakage paths
Double-hung window showing parts and air-leakage paths

Taking stock

Check each window for signs of damage: rot, mould and/or staining on or around the window, the condition of the glass, putty and paint, weather stripping and the operation and condition of the hardware. Some windows may need only minor air sealing work, while others require major upgrading or even replacement. Check for air leakage around the frame and at all movable joints. Combine a visual inspection with a test using a leak detector.

Condensation problems

Interior surface condensation and frosting are common complaints. Sometimes the problem is light fogging on some windows; at other times, there may be persistent and heavy frost covering the glass. Many homeowners buy new windows only to find that the problem becomes worse because the old, leaky windows actually helped to reduce humidity. The new windows seal the house more tightly, causing a rise in humidity. One solution is to reduce humidity levels in the house.

Alternatively, you can increase the surface temperature of the window and frame by adding another layer of glazing. New energy-efficient windows are the best solution.

When condensation forms between panes on non-sealed glazing units or storm windows, moist house air has leaked past the inner pane and condensed on the outer pane. Even dry houses can suffer from this type of condensation problem. This problem is common on second storeys where there is more air being pushed out the window because of the stack effect. The solution is to weather strip the inner sash to prevent air leakage; make sure that the weep holes on the storm windows, which allow water to escape, are open to the outside.

If condensation occurs inside a sealed double-glazed unit, the problem is best corrected by replacing the glazing unit. Although some specialty companies can refurbish sealed glazing units that have failed, this is considered a temporary fix that will not offer the same original energy efficiency. Check to see if the window is still under warranty.

Interior caulking

Window air leakage can be reduced by applying a continuous bead of caulk around the window trim where it meets the wall, at the mitred joints of the trim, and between the trim and the frame. Make sure the caulk is intended for indoor use (do not use exterior caulking indoors), can be painted and is of good quality.

Where to caulk the joints of a fixed window
Where to caulk the joints of a fixed window

If a window is particularly leaky and the trim can be easily removed and re-installed, remove the trim, add insulation and seal the gap before reapplying the trim. If the gap is small, 6 mm (¼ in.) or less, insulating the gap followed by caulking may suffice. Larger gaps may require either a backer rod with caulking or low-expansion foam (see below image).

How to seal behind the window trim
How to seal behind the window trim

To further reduce air leakage, apply a layer of red technical tape to cover the joint between the wall and window frame. Ensure that the tape will be hidden by the trim as it cannot be painted and red adhesive may remain after excess tape is removed.

Exterior caulking

Exterior caulking is the last and weakest defence against rain entering a wall from the outside. The best defence against window and door wall leakage includes the following two items:

  • properly applied flashing (i.e. top window flashing is underneath the air barrier, while side and bottom flashings are on top of the air barrier)
  • a properly detailed drainage plane

Caulking on the outside of a window should be done only after interior sealing is complete. If the exterior is caulked first, it can trap warm, moist air in the wall, which over time, can damage the wall.

Click here for a complete copy of Natural Resources Canada’s Keeping the Heat In guide.

Natural Resources Canada’s Office of Energy Efficiency offers this guide to educate on basic principles of building science and to provide guidance in home retrofit projects such as insulation and air sealing improvements.

Natural Resources Canada (NRCan) seeks to enhance the responsible development and use of Canada’s natural resources and the competitiveness of Canada’s natural resources products.

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