A Quick and Easy Guide to U-Values

Updated January 2024

Understanding and measuring u-values is of huge importance as we aim to improve the sustainability and performance of our buildings.

This article is split into two parts. First we will look at the u-value meaning and some of the common terminology related to u-values and provide simple explanations to some of the key terms you need to know when understanding thermal performance. 

In the second part of this article, we will explore U-Value Calculations. We will look at how to calculate a u-value, starting with the u-value formula and then showing you a u-value calculation example. 

By the end of this article, you will have a clear understanding of the u-value meaning, along with knowing how to tackle u-value calculations. In addition we provide lots of useful resources to help you with u-value calculations and finding more u-value related guides and information.

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Scroll to the end to download this article as a handy PDF guide!

What is a U-Value?

“U-value is the measure of the overall rate of heat transfer through a particular section of construction. U-value is sometimes known as thermal transmittance.”

In other words, a u-value is used to measure how well or how badly a building component transmits heat from the inside to the outside. The slower or more difficult it is for heat to transfer through the component, the lower the u-value. So on a cold day, if it is more difficult for heat to transfer through an assembly, this means the assembly is performing better by keeping the inside of the house warm.This means that we are looking for a lower u-value.

The lower the u-value the better.

When we talk about a building element or component, we can mean a pane of glass, a timber door, or a complete building assembly such as a cavity wall. We can work out how much heat passes through each element of the building assembly and determine the u-value based on the amount of energy lost through a square metre of material. We are measuring the thermal performance of the construction. 

What are the units for U-Values?

The u-value (or thermal transmittance) is measured in W/m²K

This is broken down as: The rate of heat flow (in Watts) through 1m² of a structure when there is a temperature difference across the structure of 1 degree (K or C).

How do u-values work?

The following example shows two different cavity walls. External wall 1 has a layer of thick insulation in the middle of the building construction. External wall 2 has only half the amount of insulation in the wall cavity.

Wall 1 with U-Value of 0.1 W/m²K will lose heat at half the rate of Wall 2 which has a U-Value of 0.2 W/m²K.

01 Wall Types Example

Don’t forget: the LOWER the U-Value, the BETTER

The lower the u-value the more efficient the construction is at keeping heat flow through the structure to a minimum. This would suggest that Wall 1 will do a better job of keeping a more comfortable internal temperature.

We also need to take into account thermal bridges to ensure a thermally efficient structure, but for this article we will focus on u-values.

What is Thermal Resistance or r-value?

Thermal resistance or r-value is the measure of the ability of a building material to resist heat flow. It is measured in m²K/W. In the UK we tend to use the u-value more than the r-value to measure efficiency of a building assembly, however other parts of the world tend to focus on r-value.

A higher r-value figure will demonstrate a better performance of the material or assembly. This is the opposite to the u-value where a lower figure demonstrates a preferable thermal performance of the assembly.

What is Thermal Conductivity or k-value?

Thermal conductivity or k-value is the measure of a particular component’s ability to conduct heat. We use the thermal conductivity to calculate the u-value of a building construction. You will often see on insulation manufacturer websites the thermal conductivity of their products, which demonstrates the expected heat transfer across the material.

U-Values and the Building Regulations

What u-values do you need to comply with the Building Regulations? U-values are considered in the Building Regulations Approved Document Part L.

Achieving a specific u-value isn’t a tick box exercise. It is important that the building is considered as whole. With heating and powering buildings accounting for a large percentage of our total energy use in the UK, it is important that our buildings perform better.

Taking on a fabric first approach will result in higher quality, better insulated buildings that will require less power for heating, therefore being more energy efficient and requiring less energy resources. All new homes are expected to produce 31% less CO2 emissions than the previous regulations.

Specification of different insulation materials will play an important part in reducing heat loss of a construction element. 

What U-Values do you need for Building Regulations?

In June 2022 Approved Document Part L was updated, which included updates to specified u-values for both new dwellings and work to existing dwellings. The table below outlines the required u-values according to Approved Document Part L “Dwellings “ England:

02 Current U-Values

For a new build dwelling, the regulations specify the following u-values:

  • u-value requirement External Wall 0.18 W/m²K
  • u-value requirement Party Wall 0.0 W/m²K
  • u-value requirement Floor 0.13 W/m²K
  • u-value requirement Roof 0.11 W/m²K
  • u-value requirement Windows (whole window u-value) 1.2 W/m²K

How to calculate a u-value

Although online u-value calculators are really useful, and some more intelligent modelling software will calculate the u-value for you, it is worth learning the calculation methods yourself, just in case it pops up in an exam. After all, it is pretty simple when you know how.

To calculate the u-value of a particular part of the building construction you need to know a little about each building element.


Thermal Resistance (R)

U-values are calculated from the thermal resistances of the parts making up a particular part of the structure. Thermal Resistance is defined as a measure of a material’s ability to resist heat flow.

In order to calculate thermal resistance, you must know the thickness of the building materials and the Thermal Conductivity (K) value. These values can be found in the Metric Handbook, or the Architects Pocket Book among other sources.

The table below shows examples of thermal conductivity. These can vary according to density of material, manufacturer and so on.

03 Thermal Conductivity

Thermal Resistance is calculated as follows.



R= Thermal Resistance (m2K/W)

d= Thickness of material (in Metres – very important)

k= thermal conductivity of the material (W/m K)


You must know the thermal resistance (R) in order to calculate the u-value. If you are specifying standard products it is often easy to find the resistance values for these elements. Sometimes it is worth having a look at specific manufacturers’ websites for these details.


Total Resistance (Rt)

Rt can be calculated as:

Rt= Rsi + R1 + R2 +Ra + Rso


04 Rt
05 Calculation example

Ra is airspace cavity and values for this can also be found in Architects Pocket Book.


How do you know the Rso and Rsi Value?

Rso is the outside surface resistance (sometimes referred to as Rse), and Rsi is the inside surface resistance. The values according to BREEAM are:

06 Rso Rsi


07 U Value calc

U-Value Example Calculation

We will work out the u-value of a standard masonry cavity wall. The wall is a partial fill cavity with PIR insulation. We have kept this example simple, and avoided the technical aspects of calculations that include adjustments for mortar, or adjustments for timber frames. This is more advanced and for the purpose of this example we are keeping things as straightforward as possible. 

08 Partial Fill Cavity wall

The table below shows the thickness of each element, the thermal conductivity, and the thermal resistance. From here we will go ahead and make our calculation.

09 Working Example

To calculate the U-value we have to first calculation the Rt.

Rt = Rsi + R1 + R2 +Ra + Rso

Rt = 0.04 + 0.1331 + 0.6651 + 4.5455 + 0.2500 + 0.0591 + 0.1300

Rt = 5.8228

U-Value = 1/Rt

U-Value = 1/ 5.8228

U-Value = 0.171 W/m²K


Based on this calculation we can see that the partial fill cavity wall would comply with the requirements of the building regulations that stipulate a u-value of 0.18 W/m²K for walls in new build dwellings.

Online U-Value Calculator

If you don’t want to do it the old fashioned way, you can use various different online u-value calculator options.

There are some really useful u-value calculators online that are worth checking out. Some are for calculating any build up, whereas others have been developed by insulation manufacturers that specifically calculate the u-values of a building assembly using their own products. See some of our favourites below.

11 DL249 Brick Slip detail masonry substrate mechanical fix 3D

Useful Resources


BRE Guide to U-Values

One of our readers, Brian, has very kindly provided access to the U-Value calculations of a live project so that you can get a feel for what is involved. Click on the link below to view. (this is from a pre update project so the u-values are not current).

Calculations of area weighted u-values



McMullan, R. 2007. Environmental Science in Building


Written by Emma Walshaw, Architectural Technologist and founder of First In Architecture and Detail Library. Emma has written a number of books about construction and architectural detailing.


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  1. Easy to understand even by a lay man who doesn’t have the subject knowledge. Explanation simple & precise

  2. What is the difference between m2 KW and W/m2K if we have an thermal value of 0.090m2KW what does this equate to re W/m2K

  3. to have a maximum ‘U’ value of 0.28 W/m2K and to be taken to the underside of the first floor boarding. B3. L1B

  4. how would you transpose this equation to make the density of a given material the subject so you could obtain a specific U value

  5. This article was very intuitive and helpful about U-Value. Thank you for sharing, very well explanation.

  6. hi, you mention above ‘U-value requirement Party Walls 0.0 W/m²K’ i have been informed by building control that if the adjoining units are provided with heating and in different tenancies the wall needs to achieve 0.55 W/m²K, i can’t see where it says this but as they sign it off, i’m going with it. BTW this is for Wales mind. cheers

  7. over what time period is the U value calculated?

  8. I’ve read through Brian’s calculation and I don’t understand the area weighted comparisons.
    W/m2K multiplied by m2 should result in W/K not W/m2.

  9. Are windows part of the wall, please? When you calculate the average for the wall, do you have to include the windows? So if they are 1.4, say, then does the rest of the wall have to be below 0.18 to compensate. Or are these separate? Thanks

  10. The U values you are specifying are out of date since June 2022 when the 2021 AD part L1 came out

    • Thank you Antony, we are in the process of updating the article, it should be online very soon.

  11. Note the research done by Historic England performed on various, soft red brick has shown the standard figures of U-value are over-estimated by up to 3 times the in-situ, measured values on historic properties.

  12. How does the U value affect the performance of a building component. For instance if out side temperature is 5 degree Celsius, and I have a wall with a U value of 0.27 W/m2K, what will be the internal temperature?

    • @MichaelOsafo it’s not that simple. The internal temperature is defined by the amount of heat energy you put into the inside. With no heating, the inside will be 5 degrees too. What U value tells you is the walls lose 0.27 watts per square meter per degree of temperature difference between inside and outside. Work out the area of the walls (m2), the temp difference (K) and do area x tempdiff x U to know how much energy is lost through the wall – this is how much energy you must replace to maintain the temperature difference

  13. Energy comes as mass or radiation according to Mr. Einstein and the “U” value calculations recognise this as the surface transmission of inside and outside surfaces. What happens in between is essentially ‘conductance’ through the material which is measured by the reciprocal of the ‘R’ value. The ‘R’ value is the speed of sound through the material and is considerably slower than the speed of light . Air having little mass does not conduct well but will account for considerable amounts of lost energy if it moves from inside to outside. The inside air will have little affect on the mass of the internal surfaces so these should have a low “U” value whereas the rest of the construction should RESIST the passage of radiation. DISCUSS

    • @Barrie Moore, this is true IF there are no airchanges, however airchanges would also need to be factored in. I really like the way you have explained the conductance element of the u values. thank you.


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