Energy Efficiency

Window energy

Energy performance

The National Standards Association of Ireland has introduced an independent Window Energy Performance (WEP) Certification scheme for the Irish consumer market. Energy Efficient Windows  are windows that help to contain and conserve heat within the home by keeping out wind and rain and resisting condensation, while at the same time allowing natural ‘free’ energy – the warmth of the sun – to heat the home. Window Energy Ratings use a consumer-friendly traffic-light-style A-E ratings guide, similar to that used on ‘white’ goods such as fridges, freezers and washing machines. This ratings label can be used by you to make more informed choices about the energy efficiency of the windows you are buying.

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Energy Performance Criteria

THE THERMAL TRANSMITTANCE ( 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 (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 (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.

Triple Glazing

More expensive than double glazing, triple glazing offers an extra level of insulation and is most commonly used in countries which experience severe winters, such as Norway and Sweden.


The name of this gas derives from the ancient Greek word for inactive. Argon’s lower convection means that it is more effective as an insulator than air – hence its application in between the sheets of glass in the most energy efficient windows.

Energy Efficient Glazing

Energy-efficient glazing is the term used to describe the double glazing or triple glazing use in modern windows in homes. Unlike the original single glazing or old double glazing, energy-efficent glazing incorporates coated (low-emissivity) glass to prevent heat escaping through the windows. This makes the windows highly thermally insulating hence improving the energy efficiency of your home and helping to save money on your heating bills.