Reading window and glass ratings are easily confusing, sometimes the high number is better and sometimes the lower number is better. Here are a few tips to help you out.
The Energy Star symbol indicates that a product meets or exceeds high efficiency standards. Currently, more than 70 product categories can qualify for the symbol and, typically, a certified model is in the top 15 to 30 percent of its class for energy performance.
Every ENERGY STAR® certified window, door and skylight is required to leave the factory with a removable label that shows:
The Energy Rating (ER) for window products is a evaluative rating made by an authorized neutral organization. All products are assessed using a strict common procedure. A window’s ER rating is a measure of its overall performance, based on the three factors below:
The higher the rating, the better the performance.
The terms refer to a measure of the heat gain or loss through glass due to the difference between indoor and outdoor air temperatures. The U-value describes how well a product prevents heat from escaping a home or building. U-value ratings generally fall between 0.2 and 1.2. U-factor is particularly important during the winter heating season. (the lower the U-value, the better the product is at keeping heat inside the home).
The R-value represents the resistance a material has to heat flow. It measures the effectiveness of insulation in stopping heat flow. There is no set rating scale (the higher the R-value, the greater the heat resistance).
The CR-value indicates how well a product resists the formation of condensation (Condensation Resistance). CR is reported on a rating scale of 1 to 100. (the higher the number, the better a product is at resisting condensation).
Visual transmittance (VT) indicates the amount of light in the visible portion of the spectrum that passes through a glazing material. (the lower the number, the less of the sun’s heat is transmitted through the glass.
The SHGC is a measurement of the heat transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. (the lower the coefficient, the less inward heat it transmits or the greater the shading ability of the glazing.)
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