What is a U-value?

What is a U-value and how do you explain its importance in a way that connects? Enter the world of When Alan met Sharon (A U-value Romance), a story which leads us in just 2 minutes 44 seconds to a deeper understanding of both U-values and the art of seduction.

We challenged the students the London-Loughborough (LoLo) Centre for Energy Demand to answer the question ‘What is a U-value?’. This short animation is the award-winning video response.

For readers still wondering what a U-value is, you are under 3 minutes away from a much better understanding!

Whilst the U-value of materials is a crucially important measurement when it comes to making older homes more energy efficient, it is not yet widely understood by the public.

Please share this video. U-values aren’t usually this much fun!

Understanding U-values

Improving our insulation is a great way to save on energy for heating the home.

To determine the extent to which a material may be responsible for heat loss we need to understand heat transfer coefficients, more commonly known as U-values.

U-values describe how easily a material transmits heat (1) so the lower the U-value the better it is as an insulator. They have the units W/m²K. If we break these units down we can see precisely what a U-value tells us about the heat flow from our building.

Watts (W) are the units of power. We’re used to seeing this in the context of how much electricity an appliance uses, but essentially it just means the amount of energy being transferred every second. A Watt is one joule per second. In this case this is the amount of energy you are losing in a second.

Square meters (m2) refers to how much of the surface area of our building will be made of this material. For some materials, such as brick or glass, this might be quite large, while others may only cover a very small area. By taking the area into account you can compare like with like, and know that the U-values will be in the same range, regardless of how large or small an element of your building you are looking at.

Kelvins (K) refers to the temperature difference between the inside and outside of your building. A Kelvin of temperature is the same as a degree Celsius, the only difference between the two scales is that Celsius puts zero at the freezing point of water, while Kelvin puts it at absolute zero (a chilly -273.15 oC) For all intents and purposes you can treat a kelvin as a degree, just expect to see far larger numbers when using this scale.

So when you look at the U-value of a material it tells you the amount of energy lost every second, for each square meter of material, for every degree of temperature difference between the inside and outside of your house. The lower the U-value, the less energy is being lost, and so the better that material is as an insulator.

UK Building regulations (2) define the maximum U-Values that certain building components need to have. Walls should have a U-value below 0.28 W/m2K, while roofs must be around 0.18 W/m2K, depending on the type or roof and how it is to be insulated. Floors must be below 0.2 W/m2K. Glazed elements like windows and patio doors typically have much higher U-values in the range of 1.4-1.6 W/m2K. Typical U-values for various building materials are summarised online (3). Choosing materials that give you the lowest possible U-value is a simple step that can have a dramatic effect on how much energy it takes to heat your house.

More about the video

This video, produced by students Faye Wade, Kate Simpson, Louis Fifield and Mike Fell, first picked up a competition prize from the Sustainable Energy Academy then went on to win second place in the Durham Energy Futures Film Festival.

Mike Fell, spokesmen for the winning team, said “We’ve been really pleased with people’s reactions to it, and it’s great to be recognized by SuperHomes. We hope that with their help we’ll be able to get the basic message of the video – the importance of insulating and lowering U-values – to a wider audience. We probably spent about six hours on the video in total so time was pretty tight. The advantage of this is that you have to go with your first impulse, rather than over-thinking things.”

The Music used is “Our House” (c) 1970 Crosby, Stills, Nash and Young.
With thanks to the UK Confectionary in Building Association


  1. Designing Buildings Wiki
  2. Planning Portal (pdf file)
  3. The Green Age