Visible Transmittance

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VT Window Rating
Block Glass Windows
Other Names:
Window Visible transmittance (VT)
Rating measuring the amount of daylight entering in through a window.

Homeowners can rely on the ratings for a replacement window's visible transmittance to ensure that the daylighting provided is integrated with lighting controls. Such integration will reduce dependence on artificial lighting and, in many cases, lower heat transfer.


[edit] Visible Transmittance

A window's visible transmittance (VT) indicates the amount of visible light that is transmitted into a room. The VT is a whole window rating which includes the impact of the window frame and glazing, even though the frame does not transfer any visible light. The VT can vary between 0 and 1 but most double- and triple-pane windows VT values are between 0.30 and 0.70. Windows with higher VT values transmit more light. Homeowners who want to maximize daylight will wish to select a replacement window with a high VT rating.

[edit] Ratings

A home's potential for daylighting is related to the number of windows that it contains. The National Fenestration Ratings Council employs the "NFRC 200 – Procedure for Determining Fenestration Product Solar Heat Gain Coefficients and Visible Transmittance at Normal Incidents" as the industry standard for ranking, comparing and rating the Visible Transmittance of all fenestration products.

[edit] Technology

In the past windows were often manufactured using reflective or tinted glazed products to reduce solar heat gain through the windows. However, using these types of glazing also reduced visible transmittance which increased the need for artificial lighting. Spectrally selective glass is a growing solution to this problem. Spectrally selective glass blocks or re-radiates the infrared energy from the sun. This succeeds in maintaining a high level of visible light transmittance while reducing the solar gain that comes in through the windows.

Spectrally selective glass includes glazing with an invisible Low-E coating which is coated on the interior surface of insulating glass.

[edit] Energy Efficiency

When an architectural design integrates daylighting needs, including windows with high visible transmittance ratings, with appropriate lighting controls, energy usage and expenses can be reduced anywhere from 30% to 60%.

[edit] National Fenestration Ratings Council

The National Fenestration Ratings Council (NFRC) provides the Visible Transmission ratings for new construction and replacement windows. The NFRC is a non-profit, third-party agency which provides window ratings according to a window's ability to offer thermal insulation, reduce solar heat gain and offer protection from weather elements. The NFRC provides these ratings, which appear on the labels of windows manufactured by participant window manufacturers, in order to assist consumers who want to compare the performance of different brands of replacement windows. The ratings provided by the NFRC are used by the Energy Star program to identify windows which may be eligible for tax credits and other rebates.

The NFRC rates windows according to a window's U-Factor, its Solar Heat Gain Coefficient, its Visible Transmittance, its Air Leakage and its Condensation Resistance.

Energy Star windows utilize the U-Factor and the Solar Heat Gain Coefficient ratings provided by the NFRC when they identify Energy Star program windows.

[edit] U-Factor

The NFRC rates a window's ability to prevent heat from escaping from a home or building as its U-Factor. Lower U-Factors are better at maintaining a building's heat.

[edit] Solar Heat Gain Coefficient

The solar heat gain coefficient measures a window's ability to block the sun's heat. Lower SHGCs are better at blocking unwanted heat gain.

[edit] Visible Transmittance

Visible Transmittance rates the amount of light that comes through a product. Higher VTs present higher potentials for using the sun's light in an energy efficient manner.

[edit] Air Leakage

Air Leakage measures the rate at which outside air enters into a home through the window. AL rates are measured as numbers between 0.1 and 0.3 with lower ALs better able to prevent air leakage.

[edit] Condensation Resistance

Condensation Resistance measures the ability of a window to resist the formation of condensation. Condensation Resistance measurements range between 1 and 100 with higher numbers indicating better resistance to condensation.

[edit] Energy Star Windows

Energy Star Windows use the NFCR's ratings to indicate selected windows which are available to homeowners as energy efficient window alternatives. The Energy Star program is used by the United States Department of Energy to provide data to homeowners who wish to find windows which offer quality energy efficient window performance. In addition Energy Star windows are often eligible for tax credits and rebates, depending on the climate in which the windows are to be installed. Different energy efficiency ratings are appropriate for windows in different climates.

The Energy Star Window program includes U-Factor ratings and Solar Heat Gain Coefficient ratings in their assessment of a window's eligibility for tax credits. They do not include the ratings for Visible Transmittance, nor for Air Leakage or Condensation Resistance.

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