Single-glazed windows lose about 10 to 20 times as much heat as the same area of a properly insulated wall. Storm windows or double-glazed sealed units will reduce the heat loss by almost half.

Storm windows can be installed on the inside or the outside, can be permanent or seasonal, and can be made to order.

Exterior storm window systems

Exterior storm windows were once very common and continue to serve a useful role in many applications. They are usually constructed of a wood or metal frame, with glass or an acrylic sheet as glazing.

Exterior storm windows can be either seasonal (installed in the fall and removed in the spring) or permanent. Seasonal storm windows should be inspected each year before installation to ensure that the glazing, putty and weatherstripping are in good condition. A drawback of seasonal units is the labour involved in installing and removing them each year, as well as the need for storage. Permanent exterior storm windows are usually equipped with a built-in screen and a sliding sash.

When you use exterior storm windows, the main interior window must be air sealed more tightly than the storm window to prevent moist household air from entering the space between the windows and being trapped, where it can condense and cause deterioration of the sash and frame.

Interior storm window systems

Interior storm windows are generally attached directly to the window frame, which helps reduce air leakage around the window. To minimize condensation and air leakage, interior storm windows should be sealed tightly so that no warm air gets between the storm unit and the original window after installation.

Interior storm windows are lighter and more accessible than seasonal exterior storm windows especially on upper floors. A disadvantage is that blinds or other window treatments may have to be repositioned to accommodate the storm window.

Interior storm windows are typically used in the winter only and are stored for the rest of the year. However, in an air-conditioned house, they can also help keep heat out and cool air in during the hot summer months.

There are two common interior storm window system options - heat-shrink film with double-sided tape and clear rigid acrylic sheets with magnetic seals.

i) Heat-shrink film with double-sided tape

These inexpensive, temporary, do-it-yourself kits are sold at most hardware and building supply stores and include instructions for installation. With this system, two-sided tape attaches the film to the window trim, after which the film is heated with a hair dryer to shrink it tightly across the window. In most cases, the film can be used only once.

Although this system provides an excellent seal and good visibility, the two-sided tape can lift paint when it is removed. As well, once this system is installed, the window cannot be opened without removing or puncturing the film.

Points to keep in mind:

• Lightweight film may be damaged if you have young children or pets in the house.
• Plastics must be kept away from strong heat sources.
• Some people may be sensitive to plastics, which can emit odours, particularly when their surfaces are warmed by sunlight.

ii) Clear rigid acrylic sheets with magnetic seals

This seasonal system is more durable than plastic film but more expensive; plus, it may require the services of a contractor. To install a magnetic seal system, a metal strip is fastened to the window trim using double-sided tape (this strip can be painted to match the frame). A magnetic moulding is then secured to the acrylic sheet and the sheet is pressed into place on the metal strip.

When the acrylic sheets are not in use, they must be stored in a flat or vertical position (not slanted) and in a cool place protected from exposure to sunlight and excessive heat. The rigid glazing is easier to clean and has a more finished appearance than shrink film.

Some plastic supply stores sell these systems (and the appropriate cleaning products) and can cut the sheets to the size required. As well, some firms specialize in manufacturing and installing these systems.

Specialty products

There are two products that are most suitable for areas that have very high solar gains that make the room too hot: applied window films and multi-layered polyester films. Films are usually professionally applied with some precautions.

Applied window films are usually made from a clear or tinted polyester substrate with a scratch-resistant coating on one side and an adhesive with a protective liner on the other. Multi-layered polyester films are black on one side and silver on the other. The film is permanently attached to the window by removing the liner and pressing it firmly on the glass.

Window films with a solar-control coating reduce solar gain and help protect carpeting, draperies, furniture and wood from fading. They should not be used on all windows as they offer very little increased insulation and greatly cut down on solar gain, which could lead to higher energy bills during the heating season. There is also a small risk of glass breakage due to increased thermal stress and the use of these films may void the warranty issued by the original window manufacturer.

Curtains and blinds can help reduce radiant heat loss from windows during the heating season and reduce solar gain during the summer. Shutters, shades, awnings and trees can also reduce solar gains during the summer. Close window coverings during the day and open windows at night for ventilation.

During the heating season, insulated drapes offer somewhat more benefit than non-insulated drapes and blinds. However, as curtains and blinds are not airtight, they may cause window condensation problems in the winter.