When it’s time to shop for windows, there can be a lot of confusion about their insulating properties. When building a new home or renovating an older one, heating and cooling costs are an important consideration these days.
What is U-factor?
Most people are aware of something called an R-value, which is a measure of insulation, but don’t know that windows are rated differently and are measured by U-factor. U-factor was invented by engineers and scientists to measure the rate at which heat flows through any material. The main difference between U-factor and R-value (or R-factor) is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss. So these two ratings are, in fact, measuring opposite things.
The direct opposite of an R-value, a lower U-factor rating means that less heat will be lost. U-factor ratings for windows generally fall between 0.20 and 1.20. A window that has a U-factor of 0.40 or less is considered energy efficient.
There Is More To Consider
U-factor is only one part of the heat transfer equation. Windows should also be air-tight, as a leaky window let’s in winter’s cold and allows warm interior air to escape. In summer, a leaky window may let your home warm up to an uncomfortable level and increase costs for air conditioning.
The next part of the window equation is low emissivity (a.k.a. low-e). This refers to a surface’s tendency to emit radiant thermal energy (heat).
Window glass, by nature, is highly emissive, meaning it lets a lot of radiant heat pass through. In Low-e windows, this is overcome by adding a thin film to the inside surface of the glass, which keeps radiant heat from moving through the glass while still letting light through. There are various methods and substances used to reduce a window’s emissivity.
Also, the space between layers of glass can be filled with argon gas, which further reduces the transfer of heat.
Another part of the equation is solar gain. The more a climate requires a building to be heated , the greater the importance of south-facing (in the northern hemisphere) windows to help heat your home during winter. Even though at night the windows have a negative impact on the home, overall, the solar gain during daytime more than makes up for it. This is a form of passive solar energy.
Other Fuel Saving Options
Another way to reduce the loss of heated or cooled air through windows is by installing double-cell cellular shades with side seals. For a 40″x60″ window, these shades cost about $130 if you install them yourself.
In extreme climates, where outside temperatures make interior heating or air conditioning a necessity, you might also consider installing triple-glazed windows instead of the more common double-glazed kind.
What Will It Cost?
You have homework to do. As window manufacturers use different frame and seal materials, and sometimes different thicknesses of glass, you will need to get estimates from several manufacturers in order to choose the windows that best suit your needs and budget.