Researching all about windows? Seeing a bunch of numbers and not sure what they mean?
Let Floyd simplify things for you.
Who Determines These Numbers?
The National Fenestration Rating Council (NFRC) is an organization created by the companies within the window, door and skylight community. The council relies on input from suppliers, builders, architects, manufacturers, government agencies, and many other entities to help in the window ratings creation process. The replacement window ratings system developed by the NFRC is based on total product performance.
Watch this video from the NFRC to learn more.
This means that the ratings you see on any NFRC window sticker are calculations of how the entire window unit performs under the testing guidelines determined by the NFRC. These total replacement window ratings will differ from center glass ratings. It is important that when you are comparing replacement window ratings you insure that you are using the NFRC’s total unit window ratings.
replacement window ratings Every window that is certified to the NFRC standards will include an NFRC label on the product. This label provides the only certain way to determine a window’s energy properties and make product comparisons between windows. The NFRC label will also be found on all products which are part of the ENERGY STAR program.
When you are shopping for replacement windows be sure to have the salesperson provide you with the NFRC window ratings for the windows they sell. This will insure your windows are tested and will perform according to the replacement window ratings assigned to it.
There are four primary replacement window ratings that the NFRC uses to determine the window performance, U-factor, Solar Heat Gain Coefficient, Visible Light Transmittance, and Air Leakage. In the near future a fifth window rating will be included: Condensation Resistance.
U-Factor
The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. Because it is a measure of heat loss through the window, the lower the U-value, the better the window will perform. When you are shopping for replacement windows be sure to talk in terms of the U-Value and not the R- Value of the windows.
R-Values are a measure of how well something insulates and is typically used to judge the performance of insulation in your walls. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value.
Solar Heat Gain Coefficient (SHGC)
The official definition of the Solar Heat Gain Coefficient is as follows: The SHGC is the fraction of incident solar radiation admitted through a window, both admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it 
transmits.
While that is a very detailed definition, you are probably sitting there wondering what the heck it means! In layman’s terms solar heat gain is the same feeling you get when you stand in the sun for an extended period of time. The suns radiant heat hits your body and begins to warm your skin. After time your body has absorbed the sun’s radiant heat and you have in essence regained the sun’s heat. This results in your body temperature rising and you get hot and want to get out of the sun.
The same principle applies to the windows in your house. As the sun beats down on your windows, the windows will begin to absorb heat gain. If the SHGC is high on your window, the heat passes right on through and starts to raise the body temperature of your home.
By having a window with a low SHGC, you prevent the radiant heat from being able to pass through the window keeping the inside of the house cooler in the warm summer months. SHGC is the more important in Southern climates than it is in Northern because of the sun’s brutal heat.
Visible Transmittance (VT)
The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. The NFRC’s VT is a whole window rating and includes the impact of the frame which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.
Be careful about getting window tint on your replacement windows because it will reduce your visible light coming into the house. If you are simply trying to increase the energy efficiency of the house, use the window’s low-e and gas filled airspace to provide the performance. A window tint will help to reduce the amount of heat transferred across the window, but it is not worth the loss of daylight and view inside the house.
Select windows with a higher VT to maximize daylight and view.
Air Leakage (AL)
Air Leakage measures how much outside air comes into a home or building through a product. Air leakage rates typically fall in a range between 0.1 and 0.3. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of window area.
The lower the air leakage, the better a product is at keeping air out. Air leakage is an optional rating, and manufacturers can choose not to include it on their labels. Windows with a higher air leakage window rating will let the heating or cooling out of the house. This will result in a drafty window and less energy efficiency.
Best Rated Window?
Which window has the best air infiltration rating? The Casement window wins this battle. The Picture window comes in a close second, but a casement window offers the best seal against the air. A Casement window has a fantastic locking mechanism that secures it to the frame in three different locations
Who wins the battle between single and double hung? Single hung window edge out double hungs every so slightly. A Single hung only has one sash that moves, so there is less area for the air to infiltrate
A great resource is the NFRC (National Fenestration Rating Council). Also read: What The Numbers Mean to lean more.