Tuesday, February 22, 2011

Guest Columnist, Dowd, Explains Window Ratings

Dean Dowd is the Chief Technology Officer for NFRC member company, CalFinder. This month, Dowd offers the first part in a series dedicated to understanding energy efficiency.

By Dean Dowd

Understanding Energy Efficient Windows

Energy efficiency ratings reveal the differences between high performing and low performing windows, and yet few consumers actually understand them. Let’s break it down, once and for all, and help you choose the best windows or doors for your money. In general, energy efficient products are those that consume less energy but offer an equal (or even better) level of usefulness. Many items around the home can be either energy efficient or energy draining.

Examples include the following:

• Windows

• Skylights
• Doors
• General appliances
• Electronics and more

What Do The Numbers on the NFRC Label Stand For?

One of the most common questions consumers ask is, "What is fenestration?" Fenestration is defined as any opening in a building's envelope, including windows, doors, and skylights.

Take a look at this sample NFRC label (left). Notice the visible transmittance rating (VT), wind permeability (AL), U-Factor, and SHGC ratings. Not all manufacturers use all of these ratings supplied by the NFRC. But many of them, especially ENERGY-STAR-labeled products, do.

• The U-factor: this is a measure of how well a product prevents heat from escaping from a home or building. The U-Factor starts at around 0.19 (the most insulating) and tops out around 1.25 (the least insulating). The lower the number, the better the product retains heat.

• Air Leakage: AL measures how much outside air comes into a home or building through a fenestration product. The measure of air is expressed in cubic feet of air per every square foot (cfm/sq ft). Old, outdated windows typically have high air leakage. Therefore, old windows have a very high AL value, while new, energy-efficient ones have low and ultra-low AL ratings (0.2-0.4 is ideal).

• Visible Transmittance: VT measures how much visible light comes through a product. In other words, it is a central measurement of how much light permeates a material, or in this case, glass. Ratings are between 0 and 1, and of course, the lower the number, the less light that enters the home.

• SHGC: The Solar Heat Gain Coefficient (0 to 1) rates how much radiant heat from sunlight is allowed to pass through your window. The lower the number, the less sunlight entering your home. A lower number will be great in the summer, but on the flip side might prove undesirable in especially-frigid winters and climates.

• CR: Condensation Residence is how well a window guards against condensation in the cracks and sills. A window that blocks the most moisture buildup will have a high CR rating, and those with less protection will maintain a lower CR.

Which Kind of Window is Most Efficient for My Home?

A double-pane, argon-filled, Low-E (emissivity) window is an example of a cost-efficient way to provide improved energy efficiency. Although they’re present in most new homes, ultra energy-efficient replacement windows (and other products) can equally benefit older homes in terms of efficiency and monthly utility bills.

In Summary

Energy efficient appliances and fixtures:

• Reduce energy costs

• Help the environment and lessen the greenhouse gas problem

• Can work side-by-side with renewable energy sources (solar, wind, etc.)• Can be found in new/replacement windows

What Exactly Does NFRC Do?

It's important to know that NFRC certifies the energy performance of fenestration products. NFRC does not sell windows nor does it endorse any manufacturer, retailer, or installer. As a third-party, independent certification organization, NFRC provides the energy performance information consumers need to make better decisions about the products they purchase.

NFRC's certification and labeling program is voluntary, so you may not see the NFRC label on every product.

So What do You Think?

Are you in the market for new windows, doors, or skylights? Have you seen the NFRC label? Did you understand how to read it and what the values meant? How can NFRC help make you a more informed consumer?

Get in touch with us in the comments below.

If you are interested in becoming a guest blogger, please contact Tom Herron, NFRC's Communications and Marketing Manager.


  1. Has the NFRC ever analyzed older wood windows as a comparative analysis to newer windows for all factors?

  2. NFRC has not conducted study on this subject specifically, but you can compare the energy performance of many products by using our Certified Products Directory, which is accessible from our landing page.