The flow of heat through a window is the result of the differences in temperature between the inside of the home and the outdoors. Windows can transfer heat to the outdoors during the cold season, and transfer it inside during the warm season. The ways that windows transfer heat are:

  • Conduction: This is the way that heat moves through a solid material. Highly conductive materials are conducive to heat transfer, less conductive materials impede it.
  • Convection: When the air in a heated room comes into contact with a glass windowpane, it cools and falls. The cycle of heated air cooling and falling is called a convective loop and you will feel it as a cold draft.
  • Radiation (Radiant Transfer): This is the movement of heat energy from a warmer body to a cooler body. The clear glass in a window will absorb the heat from a warm room and radiate it outside.
  • Air Leakage: Air leaks transfer about half of a home’s heated or cooled air to the outdoors. Windows that are not properly weatherstripped and that lack efficient closing mechanisms are the source of much of this lost heat.

Heat transfer is usually more pronounced during the cold season than in the warm season. The reason is that the difference between outdoor and indoor temperature tends to be greater in the colder part of the year than in the warmer part. The greater the difference in temperatures, the greater the flow of heat; this is so for all types of windows. In the winter, a home can lose as much as 30 percent of its heat through its windows alone; this means that energy efficient windows can save you money each month.

U-Factor and R-Value

These are two ways to determine the insulating properties of windows. U-Factor is the measure of non-solar heat flow through a window. R-Value is used to measure the window’s resistance to that flow of heat. This means that a window with better insulating properties will have a lower U-factor; that window will also have a higher R-value. Those two measurements should be kept in mind when choosing a window for you home.

The glazing in a single pane window offers relatively little insulation value. Any insulation comes from the thin films of still air on the interior and exterior surfaces of the pane. Double-paned windows offer still air spaces between the panes, which means that they provide better insulation. Other methods that window manufacturers use to minimize heat transfer include low-emittance (or “Low-E”) coatings and gas-fills.

Low-E Coatings and Gas-Fills

These extremely thin metallic coatings that are used to cover one or more of the pane’s surfaces or the surfaces of a sheet of plastic that lies between the panes. The coating reduces the transfer of heat and thus provides additional insulation benefits. These coatings reflect heat energy while letting in light; this has the effect of keeping homes cool in the summer and warm in the winter. The space between the panes in a multi-pane window may also be filled with a gas that provides superior insulation when compared to air. The gases used for this are heavier than air and thus reduce the rate at which heat passes through. Those gases include argon and krypton. The greatest insulation benefits are seen when both gas fills and Low-E coatings are used together.

Many factors go into the choice of a window aside from the effects of temperature. However, it is important that you keep in mind the energy efficiency that newer window technology can provide.