Replacement Windows and Climate | Windows Replacement Climate
Now that you know all about the U-factor and other ratings for windows, it's time to look at the effect your climate has on your window choice. You might be tempted to run out and buy a window with the lowest U-factor and the lowest possible Solar Heat Gain Coefficient (SHGC), but that might not be the wisest decision. If you live in a cold climate, you'll want as much of that winter sun's heat to come in through the windows as possible! In that case, the lowest possible SHGC might not be the best option for you, after all.
Before moving on to climate-specific recommendations, here's a brief summary of the major energy efficiency factors discussed earlier:
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Window Replacements And Climate
U-Factor: The U-factor measures the rate of heat transfer, or heat loss. The lower the number, the greater the energy efficiency. U-factor ratings are typically between .20 1.20, and anything at or below .40 is considered energy efficient. Check to see if your state has a required minimum U-factor!
Visible Transmittance (VT): Visible Transmittance measures the amount of visible light that passes through a window. The VT range is 0 to 1, but most values are .3 - .8. The higher the VT value, the more daylight will enter the room.
Solar Heat Gain Coefficient (SHGC): The Solar Heat Gain Coefficient measures how well a window blocks the heat of the sun. Like VT, the SHGC value range is 0 to 1, but in this case the lower the number, the less solar heat the window transmits.
What does all this mean for that double-pane window purchase in your part of the country?
Northern Climate - In northern climates, look for windows with a U-factor of .35 or lower. If air conditioning demands are low, a U-factor of .40 is acceptable if the SHGC is .50 or higher. Some double-paned, Low-E windows have U-factors below .30.
If you want to decrease your heating costs during the winter, select the highest SHGC you can (for a cold climate) so that some solar heat transfers in and offsets your heating needs. The SHGC range for this climate is about .30-.60. If you're more concerned about your summer cooling requirements, choose a SHGC lower than .55.
North/Central Climate - Choose windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.
If you have higher-than-average (or higher than you'd like) air conditioning costs or suffer unduly from the summer heat, look for an SHGC of.40 or less. Moderate AC requirements mean you can go with an SHGC of .55 or less. Remember that windows with lower SHGC values help reduce summer cooling and overheating, but they also impede that free winter solar heat gain.
South/Central Climate - Windows with a U-factor of 0.40 or less are best. The higher your heating bill is, the more important having a low U-factor becomes.
Moderate air conditioning requirements can be met with an SHGC value of 0.55 or less. Don't go too low keep that winter solar heat gain in mind. It's free heat!
Southern Climate - In warm climates like this, a low U-factor is very important. It's useful during cold days when you'd like a little heat, and it's also helpful during hot days when you want to keep that blistering heat out. In this climate type, however, a low U-factor is slightly less important than the SHGC value. Look for windows with a U-factor lower than 0.75 preferably lower than 0.60.
The SHGC is the most important window property in warm climates. Shop for windows with a SHGC less than 0.40.
If you need more help understanding all the various options and determining which solutions are best for your climate, current energy costs, HVAC preferences and the directions your windows face, take heart. The Lawrence Berkeley National Library (part of the U.S. Department of Energy) has a program called RESFEN that will help consumers "pick the most energy-efficient and cost-effective window for a given application." You can find the program at http://windows.lbl.gov/software/resfen/resfen.html