The NFRC label is applied to the glass of every window and door when it leaves the factory. In order to evaluate competing window brands on an even basis, you must obtain a copy of the NFRC label for all of the windows which you are considering. This may prove to be fairly difficult, since the vast majority of window salespeople do not provide the label, and do not even understand the ratings which are listed on the label anyway. You should not even consider a window for which you are unable to obtain a NFRC label.

U-Factor - measures how well a product prevents heat from escaping. The lower the U-Factor, the better a product's resistance to heat flow and the better it's insulating value.  A new single pane aluminum window has a U-Factor of approximately 1.2.

SHGC (Solar Heat Gain Coefficient) - measures how well a product blocks heat from the sun. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better a product is at blocking unwanted solar heat gain. A new single pane aluminum window has a SHGC of approximately .95.

Air Leakage - measures how much outside air comes through a window when it is closed and locked. The lower the AL, the less air will pass through cracks in the window assembly.  The AL is expressed in cubic feet per minute, per square foot (cfm/sq. ft.). Heat loss and gain occur by infiltration through cracks in the window assembly. This is an optional rating, which manufacturers often choose not to include on their label. It should not be optional for your evaluation. This rating is every bit as important as the U-Value and the SHGC.  The highest AL rating in the industry is a .30 and the lowest is a.01.