Replacement window ratings are helpful in choosing the right new windows for your home. Window ratings can be broken down into two categories: 1) technical ratings, and 2) consumer ratings. Technical ratings are based on the specifications of a window and its ability to minimize heat transfer, solar gain, air leakage, condensation and other factors. Consumer ratings are based on real life applications and include such things as a window’s durability and expected performance over time, not just when brand new.
It is important to note that no window will perform up to its ratings or expectations if it is not installed properly or is used in an application not intended by the manufacturer. Additionally, improper installation can void manufacturer warranties.
Buying replacement windows
When choosing a replacement window, consumers have a lot of information to use to help choose the right window for their home. With the availability and ease of use of the replacement window ratings and information that is available, all consumers should do their research before buying new windows or doors.Working with a knowledgeable window sales or installation professional can be a tremendous help.
In choosing replacement windows greater emphasis should be placed on different ratings based on the climate the home is located in. For instance, in a warm, sunny climate like Phoenix the Solar Heat Gain Coefficient is probably the most important rating to consider however other things should be factored in such as the orientation of a window to the sun and any shade that may be present.
Consumer window ratings
Probably the best known consumer ratings of windows is done by Consumer Reports. Consumer reports tests windows by subjecting to real life situations such as extreme temperature changes then testing for any increase in air and water leakage.
Recent Consumer Reports testing revealed that, while popular for their low price and low maintenance, popular Polyvinyl Chloride (PVC), better know as vinyl replacement windows tended to leak more air in cold climates when compared to wood and fiberglass clad windows. Consumer reports window ratings are a paid service.
Technical window ratings
NFRC rating label - click to enlarge
The most well known window ratings are the NFRC ratings which are the ratings used by the Energy Star Window Program. NFRC ratings assign a number to each rating category making it easy to compare various window styles and manufacturers.
A windows’ energy efficiency ratings are affected by the glass, the and the frame. How a window is installed will greatly impact its energy efficiency because if it does not fit properly in its track it will let in air and water. Additionally, a windows energy efficiency will be affected if replacement inserts are installed as opposed to installing completing replacing the window frame and the window frame.
NFRC window ratings
The National Fenestration Rating Council (NFRC) is a non-profit organization that administers the only independent rating rating system for windows and doors. The goal of the NFRC is to provide energy information so consumers, architects and contractors have reliable and consistent data to use when comparing windows and doors.
The NFRC mission statement is:
NFRC develops and administers energy-related rating and certification programs that serve the public by providing fair, accurate, and credible information on fenestration performance.
AAMA window ratings
The American Architectural Manufacturers Association (AAMA) also rates windows and doors. The AAMA Certification Program requires that components used in finished windows and doors pass the AAMA’s own set of performance tests (they don’t base their ratings on NFRC or any other organization’s testing). The AAMA does not perform the tests themselves, but rather requires all tests be performed in AAMA accredited labs. The AAMA also requires that manufactures certify that units produced during a period of time where built using the same methods and specifications as the units that were tested and manufacturing facilities are subject to two surprise inspections each year.
The AAMA markets their certification program to window manufacturers as a way to differential their product from competitors products, not necessarily as a way to identify the best windows. Unlike NFRC ratings which any decent window will have, AAMA ratings are not available for many windows making comparing different windows difficult and therefore not very useful
Energy Star ratings
The U.S. Environmental Protection Agency established the Energy Star Program in 1972 to assist consumers in identifying energy efficient electronic products. The program has since been expanded to other products including the Energy Star Window Program.
Energy Star Window Program
All Energy Star replacement windows are rated in five categories but Energy Star qualification is based only on the U-Factor and Solar Heat Gain Coefficient. The U-Factor and Solar Heat Gain Coefficient are also the categories used to determine if a window qualify for the IRS Energy Tax Credit.
The Energy Star Window Program further simplifies the window selection process by showing windows which are Energy Star Qualified by region. The regional differentiation takes into account the importance of a particular rating category based on the region of the United States the window will be used in.
NFRC rating categories
U-Factor
Measures the rate of heat transfer and indicates how well the window insulates. U-Factor values generally range from .25 to 1.25 and are measured in Btu/h·ft²·°F. The lower the U-Factor, the better the window insulates. A rating of .40 or below is considered energy efficient. The window U-Factor incorporates the thermal properties of the frame as well as the glazing.
Solar Heat Gain Coefficient (SHGC)
Measures the solar energy transmitted by the window and indicates how well the window blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1. Values typically range from .25 to .80. A rating of .5 means the window allows 50% of the sun’s heat to pass through. The lower the SHGC, the less solar heat the window transmits.
Visible Transmittance (VT)
Measures the amount of light a window lets through. Visible Transmittance is measured on a scale of 0 to 1. Values generally range from .20 to .80. The higher the Visible Transmittance, the more light you see.
Air Leakage (AL)
Measures the rate at which air passes through joints in the window. Air Leakage is measured in cubic feet of air passing through one square foot of window area per minute. The lower the Air Leakage value, the less air leakage.
Condensation Resistance
Measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the Condensation Resistance factor, the less condensation build-up the window allows.
Other replacement window ratings
There are some other ratings that are not included on the NFRC Energy Star label but they are helpful and appear on some windows.
Light to Solar Gain
The ratio of Visible Transmittance (see above) to Solar Heat Gain Coefficient (see above) measures the amount of light that is transmitted without adding heat. A higher number means that more light is transmitted without adding heat.
R-Value
R-Value was created as a way to rate insulation. It measures conductivity and indicates the resistance to heat flow. A high R-Value means heat is being transmitted (in or out of a window). A high R-Value indicates a better insulated window. Most windows today are measured using U-Factor (see above).