The first thing you have to remember is that your considerations for which energy efficient window to buy depend largely on your location's climate. You also have to deal with design questions, windows standards, energy codes, and tax credit considerations.
In reality, however, there are just four simple questions that you should ask. These most important things to ask when buying energy efficient windows are:1. What is its solar heat gain coefficient? This rating basically tells you how much heat goes through a window. Depending on where you live, choose a SHGC that is appropriate for you. In Texas, you would want to block out the sun's heat so get windows with a lower SHGC. 2. What is its U-factor rating? The U-factor rating is an indicator of how efficient the window is. This rating is provided by the National Fenestration Rating Council and is determined by the glass, spacer and frame used. The general rule is: the lower the rating, the better! 3. How much light goes through the window? For those who are keen on natural lighting, get a window with a high visible transmittance. Visible transmittance ratings from 0 to 1 tell you just how much light goes through a particular window. 4. Want more savings? If you want more savings from your energy efficient windows, then go for low-e windows. Low-e, shorthand for low-emissivity, basically pertains to the invisible thin coating applied to the window's glass. This coating controls how much heat goes in and out of the glass at all times. Low-e affects both the SHGC and the U-factor rating.
When looking for replacement windows in Austin, Texas, and other places with warm climates, you should opt for energy efficient windows that are low-e, a 0.30 or less U-factor and a 0.30 SHGC.