How to Read a Home Window Label

This is the beginning of our series called Energy-Efficient Windows 101 made possible by Marvin Windows and Doors.  For this first article, I want to talk about the various acronyms you might see on a home window label, or in window specification materials, so you’ll know more about what you’re reading.  When evaluating energy-efficient window options, Marvin suggests that you understand the basics of the following words and acronyms:

AL – the Air Leakage rating expresses the cubic feet of air which passes through a square foot of window area by way of leaks or cracks in the window assembly.  The lower the AL number, the less air will pass through the window assembly.

CR – Condensation Resistance is expressed in a number from 0 to 100 to indicate the ability of the window to potentially resist the formation of condensation on the interior surface.  The higher the CR number, the better the product will be at resisting interior condensation.

Energy Staris a joint program administered by the Environmental Protection Agency (EPA) and Department of Energy (DOE) which proscribes energy performance standards for products, including appliances, light bulbs, windows, and even homes.

Energy Star zones – in order to qualify as an Energy Star product, windows and skylights must meet certain U-factor and, if applicable, SHGC requirements based on a climate zone.  There are four zones in the United States: northern, north-central, south-central, and southern.  You can find out the Energy Star requirements for your climate zone on the Energy Star website.

Fenestration – this is an opening in the envelope of a building.  It’s a fancy word that may be used to reference the design or placement of windows and other openings in a structure.

NFRC – the National Fenestration Rating Council is a non-profit organization that administers a rating and labeling system for the energy performance of windows, doors, skylights, and attachment products.  NFRC certifies data and testing of windows, so it’s easy to compare two window products with NFRC values.

SHGC – the Solar Heat Gain Coefficient ranges from 0 to 1 and indicates how well a window blocks heat caused by sunlight.  The lower the SHGC, the less solar heat transmits through the window; the higher the SHGC, the more solar heat will transmit through the window.

U-factor – this is a measure of heat transfer through the whole window (including the frame and glass).  Think about U-factor as the opposite of insulation R-values where the higher the R-value, the better the insulation.  The lower the U-factor, the better the window is at preventing heat transfer through the window.

VT – Visible Transmittance also ranges from 0 to 1 and indicates the amount of total visible light that the window will transmit.  The higher the VT, the more visible light will pass through the window.

Tripane glazing, available with argon or kryton gas, provides excellent energy performance. It’s available with LoĒ2-272 on both panes, with LoĒ-179® on both panes for high SHGC, or with LoĒ3-366® on the exterior pane and LoĒ-179 on the interior pane for low SHGC.

Some window manufacturers use their own ratings based on their own testing standards, and their information is hard to judge or compare.  Marvin uses third-party rating information from NFRC, so homeowners can perform an apples-to-apples comparison of what’s available on the market.

Also, meeting the Energy Star threshold with energy-efficient windows may reduce energy consumption by about 7-15%, according to the Energy Star website.  Marvin has more than 150,000 options for meeting Energy Star requirements, and, if a project is pursuing higher performance territory, Marvin can exceed Energy Star requirements, too.

By the way, right now Marvin is kicking off a fall energy efficiency program with the Smart Performance Promotion giving one lucky homeowner $5,000 toward the purchase of new Marvin windows and doors.  In connection with the promotion, Marvin has a collection of energy efficiency and other home improvement tips from Lou Manfredini, a homebuilder, contributor to the Today Show, and host of HouseSmarts TV.


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  • ChKen

    I looked into windows a couple years ago, Marvin, Pella, Andersen, and priced out my new passive solar house. I was looking for high-R, low-U triple-glazed windows, aluminum clad, doug fir frame, with high solar gain, as I’m in the northeast. Ultimately I chose Loewen windows from Manitoba, Canada.

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