Window Air Leakage
|Window (AL), Replacement Window Air Leakage, Air Leakage Rating|
|A ratings provided by the NFRC to determine the percentage of air that will leak into a house through a replacement window.|
Air leakage ratings provide an assessment of the rate at which air leaks into or out of a home due to gaps surrounding a window. When replacing windows homeowners should review the air leakage rating to ensure that their window choice offers them the lowest possible amount of air leakage.
 Measuring Air Leakage
Rating a replacement window's air leakage refers to measuring the rate of air that moves around the window in the presence of a specific pressure difference. An air leakage rating is expressed in units of cubic feet per minute. A window product which displays a low air leakage rating is tighter than a window which displays a high air leakage rating. Industry observers suggest that when purchasing a window the homeowner should select a window with an Air Leakage rating of 0.30 or less.
 Positive Air Leakage
Positive air leakage refers to the pressure that occurs when moist indoor air leaks into cavities which surround a window. This can happen through the exterior siding, the sheathing, the outlet box or through insulation. This air leakage causes moisture to condense on cold surfaces leads to a build-up of condensation on the window's interior.
 Negative Air Leakage
Negative air leakage occurs when dry outdoor air leaks into the house and prevents moist indoor air from leaking out. Negative air leakage prevents moisture and condensation problems.
 Heat Loss and Heat Gain
Heat loss and heat gain occur when air infiltrates through cracks in the window assembly. The rate at which air filters into a house through the window unit is expressed as the Air Leakage (AL) rating. It measures in cubic feet of air as the air passes through a square foot of window area. The lower the Air Leakage rating, the less air will pass through cracks in the window assembly.
At this time, the AL is an optional National Fenestration Ratings Council rating and is not included in the Energy Star Program. The NFRC often tests and lists the AL on a window's label for code compliance purposes and to provide the customer with data that he will need to make an informed decision when he is preparing to purchase a replacement window.
Air leakage rates for a replacement window vary, depending on environmental factors, humidity, temperature, pressure, barometric, quality control, operating style differences, usage-stresses, long term durability, methods of installation, and design/material selection. Air leakage measurements are primarily intended for purposes of product comparison.
 National Fenestration Ratings Council
The National Fenestration Ratings Council (NFRC) provides Air Leakage ratings for replacement windows as well as for new construction windows. The NFRC is a non-profit agency which rates windows according to a window's ability to offer protection from weather elements as well as to provide thermal insulation and reduce solar heat gain. The NFRC rates windows and lists their ratings on the windows' labels. It performs this service for participant window manufacturers so that these manufacturers can alert consumers regarding the energy performance of their window products. These labels also assist homeowners who want to compare the performance of various brands of replacement windows. The Energy Star program uses NFRC ratings to identify windows which may be eligible for rebates and tax credit on national, state and local levels.
The NFRC rates a window's U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage and Condensation Resistance.
The Energy Star window program utilizes the U-Factor rating and the Solar Heat Gain Coefficient rating which are provided by the NFRC when identifying eligible Energy Star program windows.
The U-Factor indicates a window's ability to prevent heat from escaping from a home. Lower U-Factors are better at maintaining a building's heat than high U-Factors. The recommended window U-Factor for most climates is 0.30.
 Solar Heat Gain Coefficient
A window's solar heat gain coefficient measures the rate at which a window is able to block the sun's heat. Lower SHGCs are better at blocking unwanted heat gain than high SHGCs.
 Visible Transmittance
A window's Visible Transmittance rate indicates the amount of light that comes through a window. Windows with higher visible transmittance rates have more potential to use the sun's light in an energy efficient manner.
 Air Leakage
Air Leakage measures the rate at which air enters into a home through the window by way of cracks or other openings -- generally in spaces where the glazing is held in place within the window frame or through gaps between the wall's rough opening and the window frame. AL rates are rated as numbers between 0.1 and 0.3. Windows with lower AL ratings are better able to prevent air leakage than windows with higher AL ratings.
 Condensation Resistance
Condensation Resistance rates a window's ability to resist the formation of condensation along the window's glass or frame. Condensation Resistance measurements are expressed as a number between 1 and 100. Higher numbers indicate that the window product is better able to resist condensation.
 Energy Star Windows
The NFRC's ratings are used by the Energy Star Windows program when indicating the windows which are eligible for tax breaks and credits. The United States Department of Energy relies on these measurements when advising homeowners regarding which replacement windows will provide them with an energy efficient window solution.
The criteria for tax credit-eligible windows varies according to the homeowner's climate. The Energy Star program identifies Energy Star windows according to the NFRC's U-Factor and Solar Heat Gain Coefficient ratings. Energy Star does not include the ratings for Air Leakage, Visible Transmittance or Condensation Resistance.