The strategy for testing windows as performed by Energy Star was put forth by the National Fenestration Rating Council (NFRC) - a not-for-profit organization. Reading the Energy Star rating stickers on windows, however, can be tricky for homeowners. This article is going to help you make sense of Energy Star's ratings in terms of the U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT), air leakage (AL), and condensation resistance.
Making sense of an Energy Star rating example
Refer to the example image on the right for each of the measurements to learn how to understand a window's insulating performance.
The U-factor: A measure of a window's winter heat loss, a U-factor rating tells you how resistant to transmitting heat a window is. Most windows' U-factors are usually between 0.25 and 1.25. The higher the U-factor, the poorer the window's insulating performance.
Solar heat gain coefficient (SHGC): A high SHGC value indicates that the window allows that much solar radiation to pass into your home. If you use air conditioning in the summer, a low SHGC will help you save on energy costs. The SHGC value is a number between 0 and 1.
Visible transmittance (VT): Also a value between 0 and 1, VT quantifies how much light is allowed to pass through your window's glass panes. The lower your VT value, the less the amount of natural light that could pass through. VT is basically the visibility quality of your window.
Air leakage (AL): AL is the measure of the rate of air that could pass through your window's frame, sash interlocks, and sill joints. The higher the AL value, the more air that could infiltrate into your home.
Condensation resistance: Scored from 0 to 100, condensation resistance is the measure of moisture that can collect in between the double or triple-glazed panels of glass. The lower the value of condensation resistance, the more moisture that could gather inside your window. You want this number to be accordingly high for optimal home insulation.
The advantages of Energy Star ratings
Reading the Energy Star ratings of a window is the most credible way of knowing and comparing the insulating properties of home windows. Windows that do not include an Energy Star rating lose credibility and this gives manufacturers the incentive to produce a quality product while you reap the benefits of more energy-efficient technologies such as quality frame materials, better low-emissivity (low-E) glass, and better glazing.
More on higher quality insulating components:
- Quality frame materials: From wood to vinyl, to fiberglass, to composite, to aluminum window frames, manufacturers are always racing to improve the insulating performance of their windows in order to compete.
- Low-E glass: A major contributor to quality home insulation is the low-E coating found on the glass panels. These help in trapping heat during the winter and keeping it out during the summer.
- Better glazing: Double or triple-glazed windows use multiple panes of glass with gasses (argon, krypton or others) filling the space in between. The gas of choice has a higher density than normal air and this practically helps in the insulation process of your home. The warm edge spacer (the gray rubber block in between the glass panels) works to keep the glasses in position while allowing enough room for the glass to contract and expand without breaking between changing temperatures.
