Technical Glazing Terms

What Are the Differences Between WER Ratings and Uw Values?

An Oriel box window is reminiscent of the bay window in a historic scullery

Differences Between WER Ratings and Uw Values

Building regulations and building standards are evolving and changing as our building standards are rising. With a large focus on thermal efficiency and performance it is important to understand what ratings your existing, and new glazing systems will achieve. The most common ratings used are WER ratings and Uw values which are calculated and used in different ways… 

What Is a WER Rating? 

Windows with the WER rainbow mark, (which is akin to the labels on high-quality white goods, appliances, and lightbulbs), indicate that their thermal efficiency has been assessed. The British Fenestration Rating Council (BFRC) introduced window energy ratings in March 2004. These ratings are based on a scale from G to A++, with A++ being the most energy efficient. Since October 2010, legislation has required replacement windows to have a minimum Window Energy Rating of band C in order to comply with English Building Regulations. WER Ratings are helpful for general glazing systems such as UPVC windows and non-bespoke aluminium systems.  

How Is a WER Rating Calculated?  

The material, frame design, and glazing are all taken into account when calculating the ratings for the window or door as a whole. The efficiency of any product is determined by measuring the quantity of air leakage, solar gain and the thermal insulation. 

What Is a Uw Value?

The phrase "U value" is commonly employed to depict the thermal efficiency of materials. There are many different aspects of thermal performance that can be described by the general term "u-value," sometimes known as "U factor." BS EN 673:1998 is the standard test technique for evaluating thermal transmittance in glass and glazing and is typically used to compute all aspects of U factors. 

A computed U value, expressed in W/m2K units, shows the amount of heat lost via a material or system. 

The greater the thermal performance, the lower the "U value," which indicates that less energy can pass through the material. 

A Uw Value refers to the overall thermal insulation of the windows (including the frame). 

What Are the Differences Between WER Ratings and Uw Values?  

The primary difference between WER Ratings and Uw Values is what factors contribute to each rating. WER Ratings take into account the U-Value, Solar factor (G-Value) and Air Leakage (L-Factor). This provides a full overview of the whole glazing product and is presented in an easy-to-read G-A++ scale similar to school grades. On the other hand, A Uw-Value measures the thermal transmittance of a material. This provides a very simple way to determine if a system is compliant with building regulations for architects, contractors, designers and building owners.  

If you have questions for the team regarding either values, contact us here today!