Replacement Windows
by Robert Green, All-Tex Exteriors
When shopping for replacement windows for your home, there are several choices to make. This article serves to answer some of those questions.
Energy Efficient Windows
There are a lot of claims being made about how much energy you can save by replacing your windows. Claims of between 25% up to 45% can be true but, it really depends on other factors besides just replacing your windows. Replacement windows are still a critical step to making your home efficient but certainly not the only thing for many homes. You must also take into account the amount of insulation in your attic and walls, your weatherstripping on the doors, cracks in the walls and open chimney and stove flues for example.
In the Southern Region of the U.S. we have many more months of what we call "cooling days" than "heating days". We run our air conditioner much more often than our heater. When we pay to cool our air, we must do everything we can to keep that air inside the home. Air can escape (air leakage) through the many gaps, cracks, open flues, inadequate weather stripping and other passages. Quite a bit of the surface of our homes is made up of windows. Single pane glass is a perfect path for cool air to dissipate/leak from the house. That's "money out the window". Replacing your windows will seal most of that leak and will contribute a great deal to reducing you energy bill.
Solar Heat Gain
When sunlight strikes a window the solar energy passes right throught the glass and lights up the room but also heats up the room. Single pane windows are almost completely ineffective at stopping solar heat gain. Your air conditioner must work extremely hard to keep the air in the room cool when all your windows do is let the cooled air out.
There are several ways to help stop solar heat gain by blocking the sun light before it strikes the window or at the point of striking the window. A shade tree helps. Solar screens are also very helpful. Even films applied to the glass to filter or reflect the sun are somewhat effective (when applied to the exterior). The best solution is to get replacement windows with a low Solar Heat Gain Coefficient.
The National Fenestration Rating Council (NFRC) has come up with a rating system and standards for measuring the Solar Heat Gain Coefficient (SHGC). The SHGC is a number between 0 and 1. The lower the solar heat gain coefficient, the less solar heat is transmitted into the house. A SHGC rating of 1 would indicate that all solar heat is allowed through and a rating of .40 would mean that only 40% of the solar heat is passed into the room. Most modern replacement windows will have an NFRC lable affixed to it that displays each of the window's specifications.
Low-Emittance (Low-E)
Low-e is a metal oxide coating that is chemically applied to the glass at the window manufacturer. This coating filters out and or reflects back a certain spectrum of of Ultra Violet light that is responsible for solar heat gain and contributes to the color fade of your carpet and furniture. Having a low-e coating improves efficiency of the windows and makes your home more comfortable.
Spacer, Air Gap and Gas Filling
These three elements are used in combination and are important to the performance of the windows. Since we are mostly talking about at least 2 panes of glass, there is a space between the glass of usually around 3/4 of a inch. The space is created as an insulating layer between the glass panes with the spacer being the component that maintains the space or air gap while preventing the panes from touching and eliminating the potential (by insulating) of heat to transfer from the outer pane to the inner pane of glass. If it were just an air gap in a sealed unit of glass panels, the heat would rise to the top of the unit and cold air would sink to the bottom of the unit. In order to equalize the effect, gas is used in the air gap. Argon gas is the most popular and is the least expensive of the gases. Krypton gas is also popular but is more expensive and is more effective when the gaps are much narrower. Krypton is more popular on close air gaps of 3/8 of an inch or less.
U-Factor
The U-Factor is the rating used to describe the window's ability to keep heat inside. It is the reverse of the SHGC and is must less important in the South as the SHGC. You will normally select windows based on the U-Factor in our Northern States.
Conclusion
There are number of very reputable window manufacturers available. Some specialize in new construction and some specialize in the Replacement Window market. There are various frame materials available(Vinyl, Aluminum, Wood, Vinyl clad wood) and their ability to assist in the resistance of solar heat gain through transference needs to be considered. The combination of a low SHGC number and the use of Vinyl frames are generally considered to be excellent choices for replacement windows in the Southern Region. All-Tex only sells and installs windows with a low SHGC and an Energy Star rating.