Window U Factor
|Replacement Window U-Factor, U-Factor Rating|
|A window rating that indicates it's level of thermal insulation.|
A window's U-Factor indicates the rate of heat loss of a window unit. Window manufacturers measure a window's resistance by its insulating properties. This measurement is indicated as a U-Factor. The lower a U-Factor, the higher the level of a window's thermal insulation.
 Measuring the U-Factor
The U-Factor refers to a window's overall heat transfer coefficientcy. Using standardized conditions, the U-Factor measures the rate of heat transfer that passes through a window. Smaller U-Factors have the highest potential reduce heat transfer.
 Measuring Heat Gain & Heat Loss
The U-Factor is just one of the components that measures the rate at which a window loses and gains heat. A window's total thermal insulation is measured by the U-Factor along with Air Leakage (AL) and the Solar Heat Gain Coefficient (SHGC).
 NFRC Ratings
The U-Factor sometimes refers to just the glass or glazing alone but the National Fenestration Ratings Council, whose ratings are accepted as industry standards, assesses the entire window performance, including frame and spacer material. These ratings include the U-Factor, the Solar Heat Gain Coefficient and Air Leakage.
 Energy Star Ratings
Energy Star windows whose label is recognized for purposes of government tax credits and rebates, only bases its qualification on the U-factor and the Solar Heat Gain Coefficient ratings. The Energy Star label is granted based on ratings for the U-Factor and the Solar Heat Gain Coefficient as measured by the National Fenestration Ratings Council.
The U-factor measures the rate at which a window conducts non-solar heat flow. The lower the U-factor, the more energy-efficient the window.
 Center-of-Glass U-Factor
The National Fenestration Ratings Council offers a nationally recognized method of rating windows. The ratings provided by the NFRC assess the total window performance including the frame, spacers and glazing. The Center-of-Glass U-Factor is sometimes indicated separately. Center-of-Glass only describes the performance of the window's glazing and does not include the effects of the framing material. Homeowners should be aware that the most energy efficient windows will have a higher whole window U-factor than the center-of-glass U-factor.
 Solar Heat Gain Coefficient
The Solar Heat Gain Coefficient is the fraction of solar radiation which is admitted through a window. It is either directly transmitted and/or absorbed. The SHGF is subsequently released as heat within a building.
 Air Leakage
Air Leakage refers to the rate of air movement surrounding a window. Air Leakage is expressed in units of cubic feet per minute per square foot of frame area. A window unit with a high Air Leakage is not as tight as one with a low air leakage rating.
 U-Factor Averages
High thermal performance double-glazed windows will display a U-factor of 0.30 or lower. Some high performance triple-glazed windows can achieve a U-factor of as low as 0.15.
 U-Factors in Varying Climates
Homes located in colder climates should include windows with low U-factors.