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, read our article here or refer to the Passive House Institute’s website. 



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.