How to Evaluate the Energy Performance of Windows

When shopping around for windows, you want to think about more than a nice view and safety. These are important but there’s another factor to consider – energy efficiency. If you are considering home window replacement Cape Cod can be a hostile environment if you make the wrong choice. The weather changes greatly throughout the year. The marine moisture can reduce a window’s longevity if it’s not the optimal choice. In terms of efficiency, you want something that will keep the monthly energy bills down whether it is hot outside or the cold of winter has set in.

Look at ENERGY STAR® Qualifications

ENERGY STAR qualifies windows based on testing by the National Fenestration Rating Council. Evaluating windows, doors, and skylights, the program also factors in solar heat gain and loss. This involves the conduction of heat through glass or glazing, or the frame, in addition to radiation of heat into the house and air leakage.

A window’s U-factor measures the rate of non-solar heat flow. The lower its number is, the more energy efficient the product. Glass, glazing, frames, and spacer materials are included in the rating.

Another way to gauge a product’s efficiency is its solar heat gain coefficient, which measures the fraction of solar radiation the window transmits, absorbs, and releases into the home. If it is a low number, little heat is being transmitted inside and the window is providing adequate shading. Higher ratings are often better for the winter, because more natural heat will be let inside, but a low rating will reduce the load on your cooling system in the summer.

Additional Energy Performance Variables

Air leakage is measured in units of cubic feet per minute, per square foot, of frame area. With this in mind, one can identify the rate of air movement around the window. A high air leakage rating means the product is not relatively air tight compared to one with a low rating.

You also want to look at sun transmittance. Measured as a fraction of the visible light spectrum, visible transmittance, or VT, gauges the product in terms of the human eye’s sensitivity. High VT means a lot of visible light is getting in, and interior glare can make things uncomfortable not only visually, but in terms of excess heat production and energy efficiency.

Another energy performance rating is light-to-solar gain; this is the ratio between solar heat gain and visible transmittance. With this, you can gauge the efficiency of a product relative to another type of glass or glazing in how it transmits sunlight and avoids unnecessary heat gain. A high number means a particular window transmits more light, yet adds a more minimal amount of heat, compared to another product.

There is a lot to think about when buying new windows. Fortunately, there are a few ways you can compare the energy performance of different ones. Depending on where you live, and what direction the window faces, you’ll want to factor in the most optimal U-factor, solar heat gain, transmittance, and light-to-solar gain before making a purchase decision.