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Energy Efficiency and Glass—The Basics

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Energy efficient windows are a major focus for homeowners and builders alike when looking to install new or replacement windows. And glass plays an important role in obtaining high-energy performance levels.

In order to help consumers better understand the qualities of glass, the National Fenestration Rating Council (NFRC) developed a rating system to compare the performance of windows, doors and skylights. But what does a window’s U- or R-Value really mean? We breakdown some of the terminology behind energy efficient glass below with help from the NFRC.

U-Factor or U-Value

Designed by the NRFC, U-factor is a rating system that measures how well a product prevents heat from escaping a home or building.

The lower the U-factor, the better the product is at keeping heat inside the building – making it particularly important during the winter heating season in colder climates. Windows with a high U-factor are less efficient and have poor insulating capabilities. U-factor ratings generally fall between 0.15 and 1.20.

R- Value

R-value is a measure of resistance to the flow of heat through a given thickness of a material, such as glass, with higher numbers indicating better insulating properties.

The NFRC points out that it is often confused with U-factor, which measures the rate of heat transfer. What is important to note is that windows are generally rated using both U-factors and R-values, whereas R-value is used mainly to rate the energy efficiency of insulation in other areas of a building such as in the attic, behind walls and beneath floors. In short, the R-Value is the inverse of the U-Factor, so if you want an energy efficient window, you want one with a low U-factor and a high R-Value.

Solar Heat Gain Coefficient (SHGC)

The most appropriate windows for a region should be selected by knowing how it behaves in relation to sunlight and solar heat. Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight and is expressed as a number between 0 and 1. The lower the SHGC, the less solar heat the window transmits. A low SHGC is particularly important in hot southern climates, where homeowners are trying to keep interiors cool during long periods of time.

Visible Transmittance (VT)

Visible Transmittance measures how much light comes through a product. VT is expressed as a number between 0 and 1. The NFRC states that the higher the VT, the higher the potential for daylighting, which is the use of windows and skylights to bring sunlight into your home. The VT you need for a window, door or skylight should be determined by what your home needs and/or whether you need to reduce interior glare in a space.

Condensation Resistance

Condensation Resistance measures how well a product resists the formation of condensation. Condensation resistance is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation.

Have questions about the type of glass you need in your home and region? We’re here to help! Click here to see our different glass offerings or contact a Marvin dealer who can help you choose the best window and glass options for your home.