What are U-Values?

Every time you check out a window or door, you will probably see a mention of U-Values. But what are U-Values and why are they important when you’re choosing new windows and doors for your home improvement or self-build project? 

What is a U-Value – the definition 

U-Values are also known as ‘thermal transmittance’. Put simply, it is the measurements that we use to measure the rate of heat transfer through the main elements that make up the ‘fabric’ of your home such as the roof, walls, windows and doors.  

The U-value itself is a number followed by W/m2K, which stands for watts per square metre kelvin. It represents the amount of heat that passes through 1 square metre of material per hour, for every one degree Kelvin temperature difference between inside and outside. 

U-Values Measure Energy Efficiency 

As a general rule, the lower the U-Value of a material, the better the energy efficiency or thermal performance. 

When it comes to windows, doors and other glazed products, lower U-Values indicate better energy efficiency. Products with low U-Values will prevent heat loss from your home during the colder months and heat gain in the summer when it’s warmer. 

Minimal heat transfer helps to maintain a consistent temperature inside your home, to keep you and your family comfortable all year round. 

U-Values in Building Regulations 

U-Values of products are an important part of achieving building regulation compliance and energy performance standards.  

These regulations and performance requirements are put in place by Government to ensure the safety and security of new buildings, renovations and home improvements. Energy efficiency performance requirements continue to be tightened as the Government works towards Net Zero targets by improving the efficiency of UK homes. 

U-Values are specified in Building Regulation documents such as Approved Document L and set out the limiting (or maximum) U-Values that building products can have in order to be used in compliant building projects. 

What Influences U-Values in Windows? 

• Glass Units – glass is the biggest element of a window, so the performance of the glass unit has a significant impact on the U-Value of the whole window. Double and triple glazed units feature multiple panes of glass, separated by insulating gaps to improve efficiency. 
• Frame Insulation – aïr aluminium frames feature multiple chambers and insulating thermal breaks to reduce heat transmittance through the frame. 
• Gas Fills and Coatings – argon or krypton gas fills between the glass panes, and low-emissivity (low-E) coatings can enhance the thermal performance of windows, resulting in lower U-Values. 

• Seals – properly sealed windows minimise air leakage and heat transfer, contributing to lower U-Values. Our aïrtight seals offer optimum protection against air leakage. 
• Good Installation – even the best performing window products can lose their thermal benefits if they are installed badly, so always choose a professional and competent installer, such as an aïr Accredited Retailer. 

If you’re looking for windows and doors with the best energy efficiency, pay attention to U-Values! The lower the value, the less heat the window or door will transmit from outdoors to indoors (and vice-versa!), creating a consistently comfortable temperature inside your home.  

aïr Aluminium U-Values 

• Aluminium Bifolding Doors – from 1.0 W/m2K 

• Aluminium Lift & Slide Doors – from 0.8 W/m2K 
• MOD Contemporary, MOD Heritage and MOD Flush Windows – from 1.0 W/m2K 
• MOD Contemporary Doors – from 1.5 W/m2K 
• MOD Heritage Doors – from 1.4 W/m2K 
• MOD Tilt & Turn Windows – from 1.2 W/m2K