Before installing a new window, it is important to understand the different energy ratings and what they mean. The "U-Factor" of a window refers to the rate at which it conducts non-solar heat flow. The lower the U-factor number the higher the window energy efficiency level. The "Solar Heat Gain Coefficient" (SHGC) refers to the level of solar radiation absorbed. A window with a high SHGC rating will more efficiently collect and transmit solar heat into a building during the winter. The Visible Transmittance (VT) window rating corresponds to the amount of visible light that is permitted to enter a building or home.
Energy Efficiency Technology
To increase energy efficiency, windows can be filled between the glass panes with an inert gas, such as argon or krypton. Inert gasses do not conduct heat transfer as much as air. Argon is a third less conductive of heat transfer than air, and krypton, though more expensive than argon, is even better. Special heat absorbing window-glazing tints can also be used. In cold climates, a special black-tint should be used to absorb and produce more solar heat within a building. Insulated windows, or storm windows, can reduce the loss of heat from within a building. Spectrally selective window glazing reflects unwanted wavelengths while permitting others; this glazing can be customized to fit specific climates or aesthetic needs. Also, low-emissivity (Low-E) window glazing can reduce solar heat loss in cold climates. Low-E window glazing can reduce energy loss by 50 percent. This glazing is extremely thin. In a cold climate, the Low-E glazing should be applied to the inside window pane.
Aluminum or metal window frames have a high conduction rate and therefore are not good at insulating. Composite frames are made from composite wood products. Perhaps slightly better than regular wood frames, composite frames are more stable and they resist moisture and decay. Fiberglass frames have air cavities. These air cavities can be filled with an insulation product to enhance insulating capabilities. Vinyl frames are typically made from PVC and this material offers good insulation. However, at exceptionally cold temperatures, vinyl frames have the potential to crack. Insulated vinyl frames can also be purchased. These frames are similar to fiberglass frames, in that they have a small air cavity that is filled with insulation to improve insulating efficiency. Wood window frames offer decent insulation capabilities, however wood frames can warp with shifting temperature and humidity.
Desired Windows in Cold Climates
An effective window in a cold climate would be south-facing to absorb the sun's heat and would have an SHGC rating equal to or greater than .6 to allow solar heat gain to enter the building. A U-factor of .35 or less to negate heat transfer is desired while a VT level to permit the entrance of visible light is also preferred. Using a window with argon or krypton between two panes of glass would greatly reduce heat loss during cold months. A customized spectrally selective window glazing could be installed to fit a specific cold climate and a Low-E glazing could also be used to further reduce heat loss. A good window frame in cold climates would be an insulated vinyl or fiberglass frame that can be filled with an insulator.