|Windows and doors are among the weakest components in your home’s insulation. PG&E states that the areas with the largest return for dollars spent on insulation are (in order of importance):
The U-Value is a rating of a window’s insulation quality that takes everything in that window into consideration. The lower the U-Value the better the insulation properties. Coating or tints on the glass surface further increases the insulation factor.
BOTTOM LINE, THE U-VALUE IS THE SUMMATION OF ALL THE FACTORS THAT EFFECT INSULATION.
Windows that have good insulating values will make your home more comfortable, particularly in winter. This is partly because they allow less heat to pass through, but there’s another reason – the inside surface of a better insulated window will be warmer. When you stand or sit by it, your body won’t lose as much heat to the window as it would to a colder surface. Sometimes the draftiness that people feel from windows isn’t due to air movement at all, but rather to the fact that we radiate body heat to the cold window surface.
In addition to improving comfort, windows with high insulating values are less likely to have problems with condensation. Condensation occurs when warm, moist indoor air comes in contact with a cold surface, such as a poorly insulated window. Better insulating values are most valuable in cold climates. The bigger the difference in temperature between outside and inside, the faster heat will move through the window. You’ll notice the difference if you replace an old window with a better-insulating one. For instance, when it is 0°F outside, the inside surface temperature of a double-pane glass window is about 44°F, but for a high-performance window it jumps to about 56°F. High-performance windows will help in the summer as well, particularly if you are trying to cool the house to 78°F as the outside temperature climbs toward 100°F.
ou can tell how much heat a window allows through by its U-factor, which measures thermal conductivity. A lower U-factor means a better-insulating window. The more common term R-value refers to the resistance of the window to heat conduction, and it is the inverse of the U-factor (that is, R-value = 1/U-factor). Better windows have high R-values and low U-factors (see Table 1).
Since the different parts of a window all have different U-factors, you should look at the U-factor for the whole window. The frame and the edge of the glass usually have higher U-factors than the center of the glass. If they don’t specify-and they often do not-manufacturers or dealers may refer to a window’s center-of-glass U-factor, which is almost always lower than the U-factor for the window as a whole.
Improved Frames and Spacers
Window frames are made from aluminum, wood, vinyl (polyvinyl chloride), or fiberglass. There are also composites of two materials (for instance, vinyl and wood) mixed together and formed or extruded like plastic. To achieve a certain look, manufacturers also offer vinyl or fiberglass frames with a thin veneer of wood on the inside (wood-clad vinyl). Others offer wood frames with a cladding of vinyl or aluminum on the outside for increased durability. The frame can account for about 15% of the energy loss through a window. Aluminum frames have high U-factors, unless they include a thermal break-a strip of urethane that interrupts the transfer of heat through the metal.
Wood, vinyl, and fiberglass are much better insulators than standard aluminum frames (without the thermal break). Of these, fiberglass performs slightly better than the rest and is also the most durable. You’ll find that vinyl and wood frames generally have similar U-factors. Some very expensive vinyl frames are filled with urethane foam insulation.
In double- and triple-pane windows, the panes of glass are separated by spacers. The spacers are traditionally made of aluminum, even in wood, vinyl, or fiberglass frames, creating greater conductivity around the window edges. This makes the windows colder at the edges in winter, and water vapor may condense there as it hits the cold surface. New warm-edge spacers are made from better-insulating materials, and are recommended for cold climates. The biggest advantage to warm edge spacers is that they reduce condensation around the edge of the window.
Many people put up storm windows in the winter, and they do help. But storm windows are typically fairly leaky. If you are deciding whether to buy storm windows or replace your existing ones, you’re probably better off putting your money into new double-pane windows.