Energy Efficient Window Ratings

With May being National Home Remodeling Month (who knew?) and adding new energy efficient windows topping the remodeling project list, (according to a survey done by the National Association of Home Builders) I thought it a perfect time to clarify some of the number associated with those energy efficient windows.  What exactly are those ratings found on the stickers of the new windows?

As explained in previous blogs on glass types, there are different efficiencies in glass types alone.  You're most efficient glass type is going to be tripane.  As far as the entire window, there is a National Fenestration Ratings Council (NFRC) that  is an independent, non-profit organization which rates and certifies the performance of windows and doors.  The NFRC label lists how a product performs in several efficiency categories.  You'll see their sunshine logo emblem on these labels.

In addition to this association, there is a Window and Door Manufacturers Association (WDMA) with a certification program designed to provide code to officials, builders, architects, and consumers with an easily recognizable means of identifying products manufactured with the appropriate WDMA and other referenced performance standards.  The WDMA Hallmark is considered a mark of excellence among architects and contractors and is accepted industry-wide.

The first form of rating a window is it's U-Factor.  (A) Usually expressed in units of Btu/hr-ft²-^oF, U-Factor correlates to the rate of heat transfer. The lower the number, the better a window is at keeping heat inside a building. The U-Factor is key in regions with cold winters. Just remember: the lower the U-Factor, the more energy-efficient the window, door, or skylight. 

Solar Heat Gain Coefficient (B) (SHGC) may also be identified on a window.  (SHGC) 
is the fraction of solar radiation admitted through a window, door, or skylight -- either transmitted directly and/or absorbed, and subsequently released as heat inside a home. In warm climates, the lower the number, the better. Here you want to keep heat out by choosing windows that reflect solar radiation. Less heat coming into the home means lower air-conditioning costs and a reduced carbon footprint. In cold regions, your windows can also help you take advantage of solar radiation, which is free heat that eases the workload of your furnace or other energy-powered heat source. A higher solar heat gain coefficient means a window will allow more heat to pass through.

Visible Transmittance (C) is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window, door, or skylight. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home's daylight requirements and/or whether you need to reduce interior glare in a space.

Condensation Resistance (CR) (D) measures how well a window resists the formation of condensation on the inside surface. CR is expressed as a number between 1 and 100. The rating value is based on interior surface temperatures at 30%, 50%, and 70% indoor relative humidity for a given outside air temperature of 0° Fahrenheit under 15 mph wind conditions. The higher the number, the better a product is able to resist condensation. CR is meant to compare products and their potential for condensation formation. CR is an optional rating on the NFRC label.

The U.S. Environmental Protection Agency launched the ENERGY STAR Most Efficient Program for the fenestration industry on January 1, 2013. The program is designed to recognize products that meet the highest energy efficiency guidelines.  A federal tax credit is available for homeowners who make qualifying energy improvements. Learn more about the tax credit and how it can apply to your home.