Purchasing energy-efficient windows becomes relatively easy if you are aware of what rating you should be looking for. This article will explain the concepts involved in establishing these ratings and will also identify which rating is right for you.
Who Establishes the Ratings?
The National Fenestration Rating Council (NFRC) is a non-profit organization which is responsible for providing ratings for windows based on performance. This organization has varied members -- from window manufacturers, suppliers, builders, architects and designers, specifiers, code officials, utilities, to government agencies.
The NFRC Label
The performance of any energy efficient window can be rated based on the NFRC label. The Label gives information about the type of window the product is and then the label rates the energy performance. Energy performance is divided into four coefficients, each of which are discussed below. The label also shows the ratings of each window for residential and non-residential applications.
The Coefficients
This section will help you in understanding how ratings work and can be applied to any climatic region. To understand the values shown on the ratings label, the basic terms and concepts need to be explained:
- Solar Heat Gain Coefficient (SHGC)- Solar heat gain occurs through a window by transmission and absorption. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.
- U-Value and R-Value - U-Value is the rate of heat loss and R-Value is its inverse. The lower the U-value (or higher the R-Value), the greater a window's resistance to heat flow and the better its insulating value.
- Visible Transmittance (VT) - is the amount of light being transmitted through a window. This is a concern when daylighting is a priority. VT is expressed as a number between 0 and 1. The higher the VT, the more daylight is transmitted. A high VT is desirable to maximize daylight. Air Leakage (AL) - is the heat loss and gain that occurs through cracks around windows.
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