WEP Terms Explained

NSAI Agrément has introduced an independent Window Energy Performance (WEP) Certification scheme for the Irish consumer market.

THE WEP RATING
The WEP rating is provided for a standard window to represent typical window sizes. This enables a comparison between different products. The WEP Certificate will display an overall energy rating on a scale from ‘A’ to ‘G’. Each WEP rating label is specific to a unique window frame and glazing assembly from a single manufacturer. This WEP label is non-transferable. The WEP Certificate value combines the following characteristics to allow consumers to determine how well each window will perform.

THE THERMAL TRANSMITTANCE (U window) is a measure of the insulation properties of the window and allows the consumer to compare how effective each window is at containing and conserving heat within a building in the winter. The lower the U-value, the greater the thermal performance of the window THE SOLAR FACTOR (g window) measures how well a product blocks heat caused by sunlight. Heat gain can be beneficial in winter months but can also present consumers with additional cooling loads in summer months. The Solar Factor is expressed as a number between 0 and 1. A lower Solar Factor means less heat gain. THE AIR LEAKAGE (L factor) is a measure of the air tightness of a specific window. Good quality windows tested to the appropriate standards should have no air leakage and therefore, makes little difference to energy performance however, for leaky windows, the impact is significant. The lower the air leakage value, the greater the air tightness of the window.

THE WEP INDEX
A positive (+) WEP Index represents a better energy performing window. For example a passive standard window could achieve a WEP rating index of +30 kWh/m²/year.

In order to assess the financial benefits of installing a passive window for example, a calculation based on floor area and unit cost of electricity must be performed.