In the UK, a U-value is the critical metric for a building’s insulation. It measures how much heat can pass through a material.

This is known as ‘thermal transmittance’. At its simplest level, a U-value is a number, and lower is more insulated. A double-glazed window might be 1.4, whereas an external wall might be 0.18 (I’ve listed more examples below).

Understanding U-values is vital if you want to make a home energy-efficient. Hitting U-value targets is also a legal requirement through building regulations.

Warning: this article contains maths.

How is a U-value calculated?

A U-value works as a total score for a building surface. E.g. an entire wall, or window. But of course not all walls are created equal. They’re made of lots of different materials: plaster, insulation, blockwork, etc. Walls are complicated.

This is where the Lambda value comes in. A Lambda value, or K-value, is the score for an individual material. A U-value is the combination of all these Lambda values.

Sort of.

Let’s start with Lambda values. These are expressed in W/mK. It’s how much energy (in Watts or ‘W’) is transferred through a metre (M) of material, when the outside is 1 degree colder (in Kelvin, or ‘K’) than the inside.

• Mineral wool has a Lambda value of 0.035.
• Bricks have a Lambda value of 0.77.

However, you might have spotted a problem. Thickness. More wool means more insulation. And how many bricks? This is where we calculate thermal resistance.

Thermal resistance is found by dividing the width of the material by the Lambda value. Let’s stick with our example, constructing a cavity wall with two layers of brick:

• 115mm of brick = 0.115/0.77 = 0.149
• 170mm of mineral wool = 0.17/0.035 = 4.857
• 115mm of brick = 0.115/0.77 = 0.149

You then need to account for things like air gaps, finishes (e.g. plaster or render), plus the outer and inner resistance of the thin layers of air that cling to the surfaces. If we add everything up, we get the following:

This 5.348 is called an R-value. The R-value measures the total thermal resistance, i.e. how much heat the material stops. It is measured in m²K/W. It is the inverse of a U-value. Some countries, such as Australia, leave it like that. However, in the UK we go one step further, and divide the R-value by 1 (finding the reciprocal). 

1/5.348 = 0.187 W/m²K

This 0.187 is the U-value. Or, at least, it is nearly. You then need to account for things like mortar and wall ties, which can add between 0 and 0.01 to the U-value. This would give a true U-value of up to 0.197.

In the UK you need to achieve a U-value of max 0.18 for external walls on new builds, and 0.13 for roofs, as of July 2024. So our hypothetical wall would have just failed building regulations and would need to be rebuilt at great cost. Well done.

How are U-values measured?

In most cases, like this. By using known ratings and calculating the total. Often people use online calculators, such as our one below. Because it is a bit complicated.

Of course, this U-value is just theoretical. Materials all differ slightly, and tradespeople differ a lot. If you want a really accurate idea of your U-values, you need to measure it.

You measure U-values using heat flux sensors. This measures the external and external temperatures, along with the heat passing through the surface you’re testing. Generally you’d test over a number of days to improve accuracy.

If you want to measure your own U-values, and have £3500 to spare, here’s an example.

What are the U-values of typical surfaces?

• Solid brick walls have a U-value of 1.3* to 2.1 W/m²K
• Unfilled cavity walls have a U-value of around 1.5 W/m²K
• Insulated cavity walls have a U-value of around 0.18 W/m²K
• Uninsulated roofs have a U-value of around 2.5 W/m²K
• Roofs with 100mm of loft insulation have a U-value of around 0.3 W/m²K
• Roofs with 270mm of loft insulation have a U-value of around 0.13 W/m²K
• Single glazing has a U-value of around 5.2 W/m²K
• Older double-glazing has a U-value of around 3.1 W/m²K
• New double-glazing has a U-value of around 1.4 W/m²K
• Triple glazing has a U-value of around 0.8 W/m²K

These are, of course, oversimplified. A cavity wall can be any number of dimensions, and hold different amounts of insulation. But the numbers are useful to give you an idea of the scale of the improvements you can make, and to highlight the most obvious opportunities – namely, moving to double-glazing from single, and insulating solid brick walls.

*Note: most sources will say that solid brick walls have a U-value of ~2, however there is evidence to suggest this is too pessimistic. The essence of the article is that we assume a depth of brick nowadays which may not be true in older buildings, making them better insulated than we thought (although still very badly insulated).