WEP (Window Energy Performance) Scheme

Introduction

The WEP rating is provided for a standard window to represent typical window sizes. This enables a comparison between different products. 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.

Energy Performance Criteria

• The termal transmittance (4) (Uwindow 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) (gwindow) 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) (Lfactor) 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 600 Pa 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.

• NSAI Window Energy Peformance (WEP) Scheme Booklet
• Manufacturer's application form and guidance for Window Energy Performance Sheet

Apply for WEP

Manufacturers who wish to have a NSAI Agrément Window Energy Performance certification their products should review the application process which outlines the terms and conditions of the WEP scheme and WEP application forms

Click on window type links below to view energy rating label of each certified window.