As a measure of the thermal efficiency of a material, it is important to get to grips with U-values when building a home and when choosing your windows and doors. We answer your most frequently asked questions and look in depth at why they matter.

WHAT ARE U-VALUES?

In a phrase, U-values are a measure of heat loss through a material. They’re used to measure how effective elements of a building's fabric are as insulators; that is, how effective they are at preventing heat from transmitting from the inside to the outside of a building. 

The more effectively insulated a structure is, the lower (and better) the U-value will be. They are measured in watts per square metre per Kelvin* (W/m²K).

U-values can be calculated for any building material. This allows effective, standardised comparison of all the component parts of your structure based on their thermal performance. This in turn allows you to catch the areas heat will be lost, eliminate poor insulators and achieve the best overall level of thermal efficiency for your project.

BUT WHAT ARE GOOD U-VALUES FOR WINDOWS? 

Current building regulations state that windows in a new-build house must have a U-value of 2.0W/m²K or better (1.4W/m²K or better recommended) . Modern double-glazed units use Low Emissivity Glass (Low-E) and argon or krypton gas in the cavity and the best double-glazed windows can achieve a U-value in the region of 1.2 W/m²K. By contrast, triple-glazed windows can have a U-value of less than 0.8 W/(m²K) – 50% better than the best double-glazed windows. 

AND WHAT DIFFERENCE DOES GLAZING TYPE MAKE? 

Our highest-performing triple-glazed windows have U-values as low as 0.64W/m²K. Two panes of heat reflective Low-E glass per window allow warming infra-red rays into your home but reflect heat back into the room that would otherwise be lost to the outside. A single glazed window has a U-value of around 5.0 – meaning it loses around 8 times more heat than our best triple glazed window; a difference that really does matter. 

In most houses, 20-25% of heat is lost though windows and doors – so 75% of heat loss originates from other elements of a structure. The best way to maximise the performance of your windows and doors is by adopting a holistic approach to the thermal envelope – by which we mean the walls, floor, roof, windows, doors, roof windows and roof lights - of your building. That said, investing in windows that improve on your previous u-values by as much as a possible 4.36W/m²K will still have a drastic impact on the thermal efficiency of your home – you are effectively targeting up to a quarter of all heat lost. Have a read of the Performance page on our website to learn about five of the key factors that make a thermally efficient window.




In most houses, 20-25% of heat is lost though windows and doors – so 75% of heat loss originates from other elements of a structure. The best way to maximise the performance of your windows and doors is by adopting a holistic approach to the thermal envelope – by which we mean the walls, floor, roof, windows, doors, roof windows and roof lights - of your building. 


That said, investing in windows that improve on your previous u-values by as much as a possible 4.36W/m²K will still have a drastic impact on the thermal efficiency of your home – you are effectively targeting up to a quarter of all heat lost. Have a read of the Performance page on our website to learn about five of the key factors that make a thermally efficient window. 


HOW DO YOU CALCULATE A U-VALUE? 

 In a sentence: u-values are calculated by finding the reciprocal of the sum of the thermal resistances of our target component. 

This might sound complex, but the calculation is quite straightforward. Thermal resistance (also known as the R-value) is simply a measure of the heat insulation capacity of a material. It’s how much heat is kept in across its thickness (as opposed to U value – which is how much it loses per degree C.)

R-values can be calculated as R = t/k , where t is the thickness, in metres, of the material in question, and k is the conductivity (sometimes referred to as the k-value). You can look up the k-values of common materials online. 

Finding your U-value, then, is a case of adding together the thermal resistances (the R-values) of each building material of the unit we want – say, perhaps, one of our beautifully insulated, high performance windows - then dividing 1 by this number. 

Your final U-value calculation might be dependent on the R-values of one or two component parts; it could comprise of a list of many different values. Sometimes, we look separately at the U-values for different parts of our windows for a more technical overview.

WHAT ABOUT PASSIVE HOUSE REQUIREMENTS?

The International Passive House Association describes the Passive House Standard as “the only internationally recognised, performance-based energy standard in construction”. To become Passive House certified, a new building must meet strict requirements in its thermal insulation, windows, ventilation heat recovery, airtightness and thermal bridging control specification. The end goal is to create a comfortable, affordable and impressively low-energy building. 

For a window to meet Passive House requirements, the entire unit, i.e. glazing and frame, should have a U-value of 0.80W/(m²K) or less – a specification easily met by most of our triple-glazed range. 

For more information on the Passive House standard and its history, visit our Passive House space.
That said, investing in windows that improve on your previous u-values by as much as a possible 4.36W/m²K will still have a drastic impact on the thermal efficiency of your home – you are effectively targeting up to a quarter of all heat lost. Have a read of the Performance page on our website to learn about five of the key factors that make a thermally efficient window. 
*Kelvin(K) is a temperature scale more commonly used in science, similar to Celsius but starting at absolute zero. A degree Celsius is the same as a single unit Kelvin. 

Frequently Asked Questions

what are u-values ?

U-values are a measure of heat transfer through a material. They are used to evaluate the effectiveness of building components as insulators. The more effectively insulated a structure is, the lower (and better) the U-value will be. They are measured in watts per square metre per degree Kelvin (W/m²K). 

how are u-values calculated ?

U-values are calculated by summing all the thermal resistances (the R-values) in the materials of the target component, and finding the reciprocal of that total. Thermal resistance is a measure of the material’s insulation capacity, calculated by dividing the thickness of the material by its thermal conductivity (k-value).

what's a good u-value for windows ?

UK Building Regulations require windows to achieve a U-value of 1.4W/m²K or lower. The best double glazing can achieve around 1.2 W/m²K. Passive House windows must achieve 0.80W/m²K or lower. Our highest performing triple-glazed windows achieve 0.64W/m²K for exceptional insulation, warmth and comfort.

what is the u-value of double glazing ?

Double glazed windows generally achieve U-values ranging from 1.2 - 3.0W/m²K. Our double glazing ranges from 1.24 -1.29 W/m²K approximately, sitting on the highest end of the double glazing performance spectrum and well within UK Building Regulation requirements for windows.

what is the u-value of triple glazing ?

Most triple-glazed windows have a U-value less than 1 W/m²K. Our triple-glazed windows range from 0.64-0.94 W/m²K, and several models are suitable for use in Passivhaus and ultra low-energy projects with exceptional U-value performance of under 0.80 W/m²K.

u-value vs r-value 

Thermal resistance (denoted by an’ R-value’) is a measure of the heat insulation capacity of a material. R-values measure how much heat is preserved across its thickness, whereas the U-value is how much it loses per square metre across a 1 degree temperature difference between the inside and outside surfaces. 

what is the u-value of glass ?

Sometimes called the centre pane U-value or Ug-value, the U-value of a glass package depends on several factors: whether it is double or triple-glazed, the spaces between glass panes, the insulating gas used and the thickness of the glass panes. Because it is measured away from the edges of the window where more heat is lost, it is usually lower than the whole window U-value.

What U-value is a Passive House ? What are Passive House standards for U-values ?

Thermal insulation is usually considered the most important requirement in the design and building process of a Passive House. All opaque surfaces of a building must have u-values of 0.15 W/(m²K) or less. Passive House windows must achieve 0.80 W/m²K or less to meet the requirements.

Are U-values the same as thermal conductivity ?

No. U-value = 1 / (R0 + R1 + …. + Rn), the reciprocal of the sum of all the requisite R-values in a component. The R-value of each material = thickness of the material / conductivity of the material. So the U-value = thermal conductivity / thickness, averaged out across all the materials in a component.

What U-values are required for the Building Regulations?

U-values can be calculated for any building material. This allows effective, standardised comparison of all the component parts of your structure based on their thermal performance. This in turn allows you to catch the areas heat will be lost, eliminate poor insulators, and achieve the best overall level of thermal efficiency for your project.