WINDOWS 101

Energy Performance

Figure 1 shows an example of an NFRC label. The National Fenestration Rating Council (NFRC) is a non-profit, public/private organization created by the window, door, and skylight industry. The NFRC has developed a window energy rating system based on whole product performance. The NFRC label provides the only reliable way to compare the thermal properties of different products. Each performance rating provided on the label, which accompanies any product sold, is explained in more detail in the following section.


Figure 1: Example of NFRC label


Energy Performance Ratings

U-Factor: When there is a temperature difference between the inside and outside, heat flows from the warmer to the colder side. The ability of a window to resist this heat transfer is its' insulating value. To quantify the insulating value, the industry utilizes a measurement called the U-Factor. The U-Factor is a coefficient from 0 to 1.25, which indicates the rate of heat flow through the window system. Specifically, it is a measurement of heat flow per hour (in Btus per hour or watts) through each square foot of window for a 1 degree F temperature difference between the indoor and outdoor air temperature (Btu/hr-sq ft-Degree F). The lower the U-Factor, the less heat transfers through the window per hour. In other words, the lower the U-Factor, the less heat escapes out, the more energy efficient the window is, and the more comfortable you will feel in the winter.

Solar Heat Gain Coefficient (SHGC): Solar heat gain is the direct and diffuse radiation coming from the sun or reflected from the ground and transmitted through the glazing. Solar heat gain from windows is the most significant factor in air conditioning loads in residential buildings. The solar heat gain coefficient (SHGC) is a measure of the fraction of solar energy admitted into a building through the window assembly. The coefficient is expressed as a number from 0 to 1. A high coefficient means high heat gain, while a low coefficient means low heat gain. The lower the SHGC, the less heat enters the building, the more energy efficient the window is, and the more comfortable you will feel in the summer.

Air Leakage (AL): Air Leakage is a measure of the rate at which air passes through cracks and crevices in the window assembly. AL leads to increased heating or cooling loads when the outside air entering the building needs to be cooled or heated. AL is measured in cubic feet of air passing through one square foot of window area per minute. Operable windows with low air-leakage rates are well designed and have high-quality construction and weather-stripping. The lower the AL value, the less air leakage, the more efficient the window is, and the less drafts you will feel.

Visible Transmittance (VT): Visible Transmittance is a measure of the amount of light in the visible portion of the spectrum that passes through a glazing material. VT does not affect cooling or heating loads at all. Yet, it is a very important factor to consider since many windows with outdated Low E coating technology will produce low VT while lowering the SHGC (see glossary at end of section for a discussion of Low-E coatings). That means that while they are creating energy efficiency, they are also making your home darker. New high performance low-solar-gain low-E coatings have made it possible to reduce solar heat gain with little reduction in VT. You want an energy efficient window, and you want to allow as much visible light as possible as well.

Condensation Resistance: In cold climates, single glazed windows suffer from water condensation and frost formation on the inside surface of the glass in the winter. This occurs because the surface temperature of the glass drops below either the dew point or frost point of the inside room temperature. This condensation can contribute to the growth of mold and mildew, damage painted surfaces, and rot wood trim. Condensation Resistance is a measure of how well the window resists water build-up. The higher the condensation resistance factor, the less build-up the window allows. Condensation Resistance is scored on a scale from 0 to 100. CR is a function of many factors, including frame material, spacer, and glazing type. CR is often confused with resistance to seal failure, a situation that allows build-up of moisture inside the two panes of glass. That subject will be discussed in the Durability section later.