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WEP Terms Explained

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


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 ‘A1’ 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 and both the window assembly name (1) and the manufacturer (2) will appear on the WEP Certificate. The WEP Certificate Energy Index (3) combines the following characteristics to allow consumers to determine how well each window assembly will perform.



THE THERMAL TRANSMITTANCE (4) (U window or Uw) is a measure of the insulation properties of the window assembly and allows the consumer to compare how effective each window assembly is at containing and conserving heat within a building in the winter. The lower the U-value of the window assembly, the greater the thermal performance of the window. The published U-value combines the thermal transmittance for both the glazing and the window frame.

THE SOLAR FACTOR (5) (G window) or Solar Heat Gain Coefficient (SHGC) 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 (6) (L factor) is a measure of the air tightness of a specific window assembly. The weakest point in any window arrangement will tend to be around the seals on an opening section of window. To qualify for the NSAI WEP Scheme, manufacturers must first demonstrate that their window arrangements achieve a Class 4 air tightness rating when tested at 600Pa to I.S. EN 12207:1999 Windows and doors - Air permeability - Classification. As a result well-made windows should have little or no air leakage. The lower the air leakage value of the window assembly, the greater the overall efficiency of the window assembly.


(7) The certificate identifies if the window assembly has a double or triple glazed unit.
(8) The thermal transmittance of the glazing unit (Ug) is published on the certificate.
(9) The material from which the window assembly is manufactured is identified on the certificate i.e. uPVC, Timber or Aluminium.
(10) The Solar Energy Transmittance (g┴) is measured perpendicular to the glazing units. The solar energy transmittance (g┴) is then converted to the Solar Factor (5) (g window) in order to make allowances for overshading and inclination of the sun.

The Rating Index for each window assembly is derived by combining U window, g window and the L factor. The WEP Rating Band is based on the Rating Index.

For example:-

  • An A1 rated window has a rating index greater than +20 kWh/m²/year.
  • An A3 rated window has a rating index in the range of 0 to +10 kWh/m²/year.
  • A C rated window has a rating index in the range of -30 to -20 kWh/m2/year.

A positive (+) WEP rating represents a better energy performing window.

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