Window Ratings

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Window Ratings
Replacement Windows Ratings
Other Names:

Replacement Windows Ratings,

Energy Efficiency Rating
Various ratings used to determine the energy efficiency of replacement windows.

Energy performance for replacement windows is rated by a non-governmental body, the National Fenestration Ratings Council, which assesses a window's properties and its energy saving performance. These window ratings were established to provide potential buyers with information about a particular window's propensity for gaining or losing heat or transmitting sunlight into the home, and at what rate. Initial costs of an energy efficient windows are offset, over the course of the window's life, by a better insulated home and lower energy bills.


[edit] Window Ratings

Homeowners who want to purchase energy efficient replacement windows can assess the window's energy efficiency by reviewing the window's ratings. The United States Department of Energy and the Environment Protection Agency accept the ratings that are provided by the National Fenestration Ratings Council.

The NFRC's ratings appear on the Energy Star label. Energy Star windows are frequently eligible for tax credits and other rebates, depending on the region in which the windows are to be installed. The NFRC also rates components of the window that are not included in Energy Star qualifications.

[edit] Reliance on Ratings

Homeowners, window manufacturers and governmental agencies rely on these ratings to determine a window's thermal performance. The ratings give window manufacturers an opportunity to review their own windows' energy efficiency comparison with other window products on the market. Homeowners can determine which window will offer them the best value for their money, both for the short-term purchase and relating to long-term energy savings. Governmental agencies which encourage energy savings and reduced energy usage are able to access these ratings to make recommendations and encourage consumers to make energy efficient choices.

[edit] NFRC Ratings

The National Fenestration Ratings Council (NFRC) is a non-profit, third-party agency which rates, tests, certifies and labels windows according to their ability to provide thermal insulation and protection from the weather. The NFRC provides these ratings in order to assist consumers compare replacement window performance and are used in the Energy Star program.

The NFRC rates a window's U-Factor, its solar heat gain coefficient, its visible transmittance, its air leakage and its light to solar gain but ENERGY STAR windows® are based only on a window's U-Factor and SHGF ratings.

[edit] Energy Star Ratings

The Energy Star certification provides assurance that a window is able to limit heat transfer and control the amount of solar gain that enters into a home through the window. Energy Star qualified windows lower energy consumption and reduce greenhouse gas emissions through their ability to control weather penetration and heat transfer. Energy Star windows are certified based on the ratings of the National Fenestration Ratings Council but they do not include all the NFRC's component ratings.

Windows which receive the Energy Star label are assessed according to the NFRC's ratings for U-Factor and solar heat gain coefficient but do not take into account the additional ratings -- Air Leakage, Visible Transmittance and Light to Solar Gain. ENERGY STAR® windows are accepted by many governments as windows products that make the buyer eligible for tax credits and rebates.

[edit] Heat Loss and Heat Gain

Ratings for energy efficient windows concentrate on how the window gains or loses heat. This occurs in any of three different ways:

  1. Windows gain or lose heat through direct conduction through its frame or glazing.
  2. Windows gain or lose heat through the sun's radiation
  3. Windows gain or lose heat through air leakage.

Ratings measure and rate these different elements.

[edit] Direct Conduction

Conductance refers to the movement of heat through a solid material. As heat conducts through a window's glass panes it transfers heat. Window manufacturers can reduce this conductance by building windows which include multiple panes. Another method for reducing conductance involves adding gas fills which are inserted between the panes. These methods create added thermal barriers between the outside and the inside of a home and help to reduce heat transfer.

Window efficiency ratings assess the effectiveness of these barriers in reducing conductance.

[edit] Radiative Heat Flow

Radiative heat flow encompasses the heat which is caused by the sun's radiation. These solar rays -- also called "solar gain" can cause heat to transfer into a room which then warms the room. Many window manufacturers add a Low Emissivity (Low E) coating to the window's glazing to suppress radiative heat flow and heat transfer as well as to reduce the infrared radiation that is transferred from the warmer pane of glass to the cooler pane. Low E is applied to the window's glass surface and blocks a significant amount of radiant heat transfer, lowering the total heat flow that enters through the window. Low-E coatings are virtually invisible.

Window ratings measure the energy efficiency of coatings that block radiative heat flow and the extent to which they reduce the heat flow, and thus the window's U-Factor.

[edit] Sunlight Transmittance

The transmittance of sunlight is measured and rated according to visible transmittance and light to solar gain.

[edit] Rating Component

Replacement window ratings measure the window's energy efficiency according to energy performance characteristics. These characteristics include U-Factor, the Solar Heat Gain Coefficient, Air Leakage, Visible Transmittance and Light to Solar Gain.

[edit] U-Factor

The U-Factor measurement assesses the rate at which a window is able to conduct non-solar heat. This rating represents an entire window's performance including the window's glazing, frame and spacer material. The lower the U-Factor rating, the more energy efficiency the window provides.

[edit] Center of Glass U-Factor

The Center-of-Glass U-Factor is sometimes rated separately. The U-Factor for the Center-of-Glass measures the performance of the window's glazing. It does not measure the effects of the framing material. Most energy efficient windows will have a higher U-factor for the whole window than the U-Factor for the Center of Glass.

[edit] Solar Heat Gain Coefficient

The solar heat gain coefficient (SHGF) relates to a percentage of solar radiation which is either absorbed or directly transmitted through a window, and is subsequently released, creating warmth in the room. When less solar heat is transmitted there is greater shading ability which results in a lower solar heat gain coefficient. Windows with high SHGF ratings transmit more solar heat gain during the winter months while windows with a low SHGF rating reduce cooling loads more effectively during the summer months when they block heat from the sun. Homeowners will consider the climate in which they live when they decide whether they want to purchase a replacement window with a high or low SHGF.

[edit] Air Leakage

Air Leakage ratings rate how air infiltrates into a room from the area surrounding a window . Window spacers, sealants and other weatherstripping products impact on a window's air leakage. A low air leakage rating is better and indicates that the window has a low percentage of air leakage.

[edit] Light-to-Solar Gain

Light-to-Solar Gain (LSG)is the ratio between the visible transmittance and the solar heat gain coefficient. The measurement for light to solar gain assesses the relative efficiency of different types of glass as they transmit daylight while they block heat gains. A high LSG rating indicates that a higher ratio of light is being transmitted without adding significantly higher amounts of heat.

[edit] Visible Transmittance

Visible Transmittance (VT) gauges the percentage of sunlight that is transmitted through a window's glass. A high VT measurement indicates that a higher rate of visible light is being transmitted. Energy efficiency ratings for windows express the VT of a window as a number between 0 and 1. Homeowners may prefer different levels of VT depending on the home's lighting requirements and whether the homeowner wants to increase or reduce interior glare.

[edit] Regions

The United States offers tax credits to citizens who purchase energy efficient windows with sufficient Energy Star ratings. However, the criteria for receiving these tax credits differs for taxpayers from region to region, depending on the climate in which they live.

The Department of Energy offers recommendations to residents of the four main climate areas of the United States, the Northern Zone, the North/Central Zone, the South/Central Zone and the Southern Zone regarding recommended ratings for replacement windows installed in those zones.

[edit] Labels

There are two Energy Star labels. One label indicates that the window product is eligible for a tax credit in all areas of the country and a second label indicates that the window product is eligible for a tax credit in specified areas of the country. In these cases the Energy Star label will indicate exactly in which areas of the country the window is an eligible tax-credit window.

Eligible windows must have a U-factor and Solar Heat Gain Coefficient (SHGC) rating of less than or equal to 0.30. The tax credit equals 30 percent of the product price.

[edit] Recommendations

In addition to the tax credit guidelines, the Department of Energy offers other ratings recommendations for the Northern Zone, the North/Central Zone, the Southern Zone and the South/Central Zone.

[edit] Northern Zone

For the Northern Zone, windows should provide sufficient access to winter solar heat gain. The rating will depend on the air conditioning needs -- .40 or higher for minimal air conditioning requirements or .40 or less for homes with high air conditioning requirements. Visible Transmittance should be high and the Air Leakage rating should not be higher than 0.30.

[edit] North/Central Zone

The U-Factor in the North/Central zone should not be higher than 0.30 and the SHGC should be 0.30 or less if the home experiences high energy bills. If energy bills have been moderate in the past a window of 0.40 or less is sufficient. These windows will reduce summer cooling costs and prevent overheating but they impact on winter solar heat gain by reducing winter solar gain. A high Visible Transmittance will maximize daylight. Air Leakage should not exceed 0.30.

[edit] South/Central Zone

Both the SHGC and the U-Factor of a window in the South/Central Zone should be 0.30 or less. Homeowners should remember that while these windows will reduce summer cooling needs and overheating, they also reduce free winter solar heat gain. Windows should have a high VT to maximize daylight and an Air Leakage rating of no more than 0.30.

[edit] Southern Zone

A low U-factor of 0.30 or less is useful for both hot and cold days in the Southern zone. The SHGC in the Southern Zone should also be low -- 0.30 or less. The windows should have a high VT to maximize both the daylight and the view along with an air leakage rating of 0.30 or less.

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