A Quick and Easy Guide to U-Values

Updated September 2022

Understanding and measuring U-values has become increasingly important as we aim to improve the sustainability and performance of our buildings.

What is a U-Value?

U-Value – is the measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction.

In other words, a U-value is used to measure how well or how badly a 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. This means that we are looking for a lower U-value.

The lower the U-value the better.

When we talk about a 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.

Don’t forget you can download our handy guide by clicking on the button below:


What are the units for U-Values?


The U-Value is measured in W/m² K

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)


Wall 1 with U-Value of 0.3 W/m2 K will lose heat at half the rate of Wall 2 which has a U-Value of 0.6 W/m2 K


So, 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.


Are U-values important in the Building Regulations Approved Document Part L?


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 the building is considered as whole. With heating and powering buildings accounting for 40% 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.


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:
  • U-value requirement External Walls 0.18 W/m²K
  • U-value requirement Party Walls 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


U-Value Calculator:


There are a few good u-value calculators online, that if you are short on time are worth checking out. Some are for calculating any build up, where as others have been developed by insulation manufacturers that specifically deal with their own products.

Scroll down to view our recommended Online U-value calculators.


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 how to do it 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 element of the construction.

Thermal Resistance (R)

U-values are calculated from the thermal resistances of the parts making up a particular part of the structure. Transmission of heat is opposed in varying amounts dependent on material and surface. Thermal Resistance is defined as a measure of the opposition to heat transfer offered by a particular component in a building element.

In order to calculate thermal resistance, you must know the thickness of the material and the Thermal Conductivity (K) value. These values can be found in the Metric Handbook, or the Architects Pocket Book (the Architects Pocket Book is extremely useful, I would recommend every architecture student has a copy of this book).


Thermal Conductivity of materials

Thermal conductivity of materials (w/mK)



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)


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, and Rsi is the inside surface resistance. These values are specified in the Architects Pocket Book as:



Now you have your Rt value, the calculation is simple, one divided by the Rt. There you have your U-Value.

Other helpful bits:


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


Online U-value calculators:

Rockwool U-Value Calculator – This is for Rockwool products only



McMullan, R. 2007. Environmental Science in Building


Don’t forget you can download our handy guide by clicking on the button below:


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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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