How Windows Transfer Heat

Heat moves through windows through three mechanisms: conduction through the glass and frame materials, convection of air between panes or within the room air near cold glass surfaces, and radiation of infrared energy through the glazing. Each mechanism is addressed differently by modern window design.

A single-pane window offers very little resistance to any of these. The RSI value of 3 mm glass alone is negligible. Adding an air space between panes—as in a double-glazed unit—creates a buffer that reduces both conduction and convection. Filling that space with argon or krypton gas instead of air reduces convection further, since these dense gases move less freely than air.

Double-Glazed vs. Triple-Glazed Units

Double-glazed windows, also called insulated glass units (IGUs), consist of two panes separated by a spacer and sealed at the edges. The space is typically filled with argon. This construction has been standard in Canadian residential construction for decades and represents a significant improvement over single panes.

Triple-glazed windows add a third pane and a second gas-filled space. The additional pane and space improve thermal performance and reduce the likelihood of condensation on the interior pane surface in very cold weather, which can be a visible indicator of heat loss and moisture movement through the window assembly. Triple glazing is more common in new construction in colder provinces and is standard practice in passive house construction.

Canadian Standard

ENERGY STAR Canada classifies windows into climate zones (Southern, Central, Northern, Most Northern). A window's zone rating indicates the minimum climate conditions for which it meets ENERGY STAR criteria. Northern and Most Northern ratings are relevant for the majority of Canadian provinces and territories.

Low-Emissivity Coatings

Low-emissivity (low-e) coatings are thin metallic or metal oxide layers applied to one or more glass surfaces within an IGU. They work by reflecting long-wave infrared radiation—the kind emitted by warm objects in a room—back into the interior, while allowing visible light to pass through.

Hard-Coat vs. Soft-Coat

Hard-coat low-e glass is produced by applying a coating during the float glass manufacturing process, making it durable and suitable for use on exposed surfaces. Soft-coat (or sputter-coated) low-e has better optical performance but must be used on interior surfaces of sealed glazing units, protected from air and moisture contact.

Solar Gain Considerations

Low-e coatings are also characterized by their Solar Heat Gain Coefficient (SHGC), which describes how much solar radiation passes through the glass. In Canada, south-facing windows in colder climates can benefit from higher SHGC values that admit winter sun, while overheating risk in summer is less of a concern than in warmer climates. The optimal SHGC depends on orientation, climate zone, and shading conditions.

The Energy Rating System

The Canadian Standards Association (CSA) and the Fenestration Canada organization developed the Window and Door Energy Rating (ER) system. The ER number combines U-factor (heat transmission), solar heat gain, and air leakage into a single value. A higher ER indicates better overall energy performance for Canadian heating-dominated climates.

ENERGY STAR Canada uses minimum ER values as part of its certification criteria, with the required minimum varying by climate zone. Replacement windows sold in Canada that carry an ENERGY STAR label must meet these zone-specific thresholds.

Frame Materials

The window frame conducts heat independently of the glazing. Wood frames have moderate thermal conductivity and have been used historically, but require maintenance to prevent rot and paint failure. Vinyl (PVC) frames became prevalent in Canadian residential construction over the past few decades due to their low maintenance requirements and reasonable thermal performance. Fibreglass frames offer better dimensional stability in extreme temperature swings, which is relevant in Canada's climate, and can achieve lower U-factors than standard vinyl.

Aluminum frames, without a thermal break, conduct heat readily and are generally not suitable for exterior applications in cold Canadian climates. Aluminum with a thermal break—a layer of non-conductive material through the frame cross-section—performs better but still typically trails vinyl and fibreglass in cold-climate residential applications.

Installation Quality

The performance of a window in the field depends heavily on installation. Air sealing at the rough opening—between the window frame and the framing of the wall—is important because air leakage around a window can move more heat than conduction through the glazing itself. Using low-expansion foam and proper flashing tape at the rough opening, followed by interior sealing with a backer rod and sealant, prevents this air bypass.

Flashings at the sill and around the frame also determine whether bulk water from rain is managed correctly, which affects the durability of both the window and the surrounding wall assembly.

Retrofit Consideration

When replacing windows in an existing wall, the position of the new window within the wall assembly matters for condensation and moisture management. Window placement relative to the thermal mass of the wall affects where the dew point occurs within the assembly during cold weather.

Comparing Single, Double, and Triple Glazing

  • Single pane: Very low thermal resistance. Interior surface is cold in winter, leading to drafts from convection near the glass and risk of condensation. Found in older Canadian homes and some older commercial buildings.
  • Double pane, argon-filled, low-e: Standard for new residential construction. Significant improvement over single pane. Suitable for most Canadian climate zones at the required ENERGY STAR threshold.
  • Triple pane, argon or krypton, low-e: Best performance for cold Canadian winters. Interior pane surface remains warmer in cold conditions, improving comfort near windows and reducing heat demand. Cost premium over double-pane units is recovered more quickly in colder climate zones.

When Replacement Makes Sense

Replacing windows is expensive compared to most insulation upgrades. Single-pane windows in a climate zone 6 or colder location are generally good candidates for replacement given the performance gap. Double-pane windows that are sealed-unit failures—visible as fogging between the panes from moisture infiltration—have lost most of their thermal performance and are worth replacing. Double-pane windows in good condition in a mild climate zone may not recover their replacement cost through energy savings alone, though comfort and condensation reduction are additional factors to consider.