If you are tired of high heating and cooling bills every year, and you have decided to make this the year to replace those ancient windows with more modern, Energy efficient vinyl windows, don't just look at the appearance as the determining factor in choosing a vinyl window brand. Since you are looking for energy savings, you should at least give some thought to the type of glass that you will choose to have in the window package.
The energy performance of a vinyl window is measured by several factors. But, the three most important are U-Value, Solar Heat Gain Coefficient (SHGC), and Visible Light Transmittance (VLT). The U-Value is the amount of energy (Heat or cold) that is able to flow through the window from inside to outside. You want this number to be as low as possible. For example, the less cool air that can escape through the window during summer, the less your air conditioner has to work. Conversely, the less heat that escapes during winter, the less wood you have to burn, or gas or propane you have to use.
SHGC measures the amount of heat that flows through a window from outside to inside. This is critical during the summer months. Again, you want the number to be as low as possible. The less heat entering the house, the less your air conditioner has to work. If you had a SHGC of zero, and a U-Value of zero, theoretically your air conditioner would come on once a day to set the thermostatic temperature, and that cool air would remain in the house all day, while no outside heat would get in. At this point in time, this is impossible to achieve.
The final factor is the VLT, which measures the amount of natural light that the window allows into the home. Ideally, you would like 100% of the light to penetrate the glass. While this is also not possible, the closer to 100% we can get, while still preventing the heat and cold from getting through, the better off we are. Architects calculate the VLT and SHGC in relation to each other to come up with the Light To Solar Gain ratio (LSG). The LSG of a window is calculated by dividing the SHGC into the VLT. For example, if a window lets 70% of the heat pass through it, while allowing 70% of the natural light to come in, you have an LSG of 1. As the amount of natural light coming through increases, so does the amount of heat. The trick is to keep the light percentage high, while lowering the heat penetration percentage.
A typical dual pane vinyl sliding window with clear glass only, has a U-value of .48, a SHGC of .63, and a VLT of .65. So, 48% of the cool air in your home during summer is able to escape through the window. At the same time, 63% of the heat is able to enter the home, and 65% of the natural light is able to come through. If you calculate the LSG of this window, you come up with 1.03. A PlyGem pro 200 sliding window with LowE glass has a U-Value of .31, a SHGC of .31, and a VLT of .57. The LSG of this window is 1.84. A pro 200 sliding window with 2 LowE surfaces has a U-value of .29, SHGC of .22, and VLT of .50. As you can see, while the Solar Cool window reduces the amount of light penetration by 15% compared to the window with clear glass, it reduces the heat gain by 26%.
When shopping for vinyl windows, use these 3 characteristics to help determine how well that particular window will perform. And don't accept verbal numbers from a salesman. Ask to see a window that has the NFRC sticker on it. The NFRC is an independent company that runs tests on the windows to determine their true values. If you live in the west, and you would like to get a quote on the Ply Gem vinyl windows with the LowE or better glass package, visit how-to-install-windows.com and fill out the quote request form.