Why Windows Lose Heat

Before you learn how energy efficient windows can save money on energy bills, it's good to understand exactly how your old windows are losing heat energy. There are four ways that heat energy escapes out the window:
1. Conduction
When warm inside air hits the glass and frame of the window, heat energy is transferred molecule-to-molecule through the window material, gradually warming it up. When cold air from the outside touches the warm material, some heat is transferred to the air and carried away on the wind like soup getting cooler when you blow on it.
2. Convection
Warm air rises and cool air falls and this is as true in the space between the panes of glass in a double paned window as it is everywhere else. This transfer of heat from the bottom of the window to the top of the window means the bottom of the window is constantly cooler and is therefore drawing more heat from the air inside the house.
3. Air Leakage
Ill-fitting windows with bad seals and cracked caulking allow air to pass between the window frame, the glass and the wall. Since warm air will always flow into a region with cooler air, windows that leak will allow warm air from inside the house to flow out and be lost.
4. Radiation
Heat travels through the air from any hot source in the form of infra-red and ultraviolet radiation. This radiation passes through regular glass, heating air and objects on the other side. This means that windows act as large portals through which heat inside the house can radiate out.

When shopping for new energy efficient windows it is important to find out how they address all four methods of heat loss. The very best products can reduce your heating costs by hundreds of dollars a year!

Canadian Window and Door Manufacturers Association.