Understanding U-values for windows and doors in the UK

What is a U-value?

A U-value, also known as thermal transmittance, measures how effective a material is as an insulator. It denotes the rate at which heat transfers through a structure, such as a window or door. The value is expressed in Watts per square meter Kelvin (W/m²K). The lower the U-value, the better the material is at keeping heat in or out, making it a better insulator.

Why are U-values important?

• Energy Efficiency

Windows and doors with lower U-values will be more effective in preventing heat loss during colder months and heat gain during warmer months. This means you will be less reliant on heating and cooling systems, resulting in energy savings.

• Comfort

Homes with windows and doors that have optimal U-values maintain a more stable internal temperature, making living spaces more comfortable throughout the year.

• Building Regulations

In the UK, Building Regulations set standards for the energy efficiency of new buildings and renovations. Meeting specified U-value targets is essential for compliance.

How to understand U-Values for windows and doors:

1. Single vs Double vs Triple Glazing

• Single Glazing has the highest U-value (poorest insulation) and is rarely used in modern UK homes

• Double Glazing comprises of two panes of glass with a gap filled with air or inert gas. It offers better insulation that single glazing, with typical U-values ranging from 1.2 0 3.0 W/m²K

• Triple Glazing has three panes and provides superior insulation with U-values often below 1.0 W/m²K

2. Gas Fills

The spaces between double and triple glazing panes can be filled with air or inert gases like argon or xenon. Gases have lower thermal conductivity that air, thus lowering the U-value.

3. Low E-coatings

Low emissivity (Low-E) coatings are thin metallic layers applied to gas panes, reducing the amount of UV and IR radiation passing through. This also helps to reduce the U-value.

Aluminium U-values

Aluminium as a material, is highly durable, corrosion-resistant, and offers a sleek and modern aesthetic making it a popular choice for windows and doors. Aluminium is a good conductor of heat, which is why we create thermally broken profiles to improve the U-values of our windows and doors.

What is a "thermally broken" profile?

A thermally broken aluminium profile refers to a design where the aluminium frame is split into two parts, with a non-metal "thermal break" placed in between. This break disrupts the thermal conductivity of the aluminium, significantly reducing the heat that can pass through the frame.

How thermally broken aluminium profiles improves U-values:

• Interrupting Heat Transfer

The non-metallic insulator, which we make with polyamide, acts as a barrier. It hinders the rapid transfer of heat, which would normally occur with a continuous aluminium frame.

• Reducing Condensation

Apart from improving insulation, thermally broken profiles also reduce the risk of condensation forming on the interior side of the frame. This is particularly beneficial in colder climates and during the winter, preventing potential damp issues and damage to interior surfaces.

• Enhancing Overall Window Performance

When combined with double or triple glazing, a thermally broken aluminium frame can rival, or even surpass, the performance of alternative frames like uPVC or timber.

Selecting the right U-value

When selecting new windows and doors, aim for the lowest U-value within your budget to maximise energy efficiency. Consider factors like the direction the windows face (north-facing windows in the UK will generally lose more heat) and your local climate conditions.

In the UK, understanding U-values is essential to make an informed choice about windows and doors, whether you are building a new home or renovating. By prioritising lower U-values, you can benefit from energy savings, increased comfort and ensure that your home meets current Building Regulations.