What is a U-Value and why it is important

What is a U value, and why is it important?

The quick answer:

The U-Value is a way of measuring the thermal efficiency of a building element such as a wall, floor, window etc. The thickness of a wall, what it’s made of, and the form of insulation used (if any) all have a bearing on its U-Value.

A wall with a U-Value of 2 loses twice as much heat than a wall with a U-Value of 1
(if the area is the same).

A solid brick wall typically has a U-Value of around 2 W/m2K. When we have installed solid wall insulation to the same wall, we would expect a final U-Value of 0.35 W/m2K which is in line with government regulations for walls in new build domestic properties.

When we talk about U-Values, lower = better = warmer = lower fuel bills.

The longer answer:

In considering U-Values you may also encounter K-Values and R-Values. Whereas the latter two are scientific measurements of the thermal efficiency of a particular substance in the laboratory e.g. phenolic insulation board, plasterboard, concrete block etc. U-Values measure the efficiency of different materials when combined together in the real world.

So a wall made of plasterboard, concrete “breeze” block, a 50mm cavity, and an outer layer of brick will have a particular U-value.

The same wall with a wider cavity will have a higher U-Value. The same wall with internal insulation will have a substantially lower U Value.

So what is a U-Value ?  It is the measure of the amount of energy required to keep a room at a certain temperature and is measured in W/m2 K, where:

W = Watts
m2 = square metres
K = Temperature in Kelvin (the same as Centigrade or Celsius).


Take a room at 20OC, where the outside temperature is 15OC and consider just three square metres of wall which has a U value of 2 W/m2K.

How much energy do we need to put into the room to maintain it at the same temperature ?

U = 2                The U value.
m2 = 3              The wall area we are considering.
K = 5                The difference in temperature between inside and outside.

Rearrange the formula so Watts = U x m2 x K

The energy input would need to be = 2 x 3 x 5 = 30W

- not much, enough to power an energy efficient light bulb. But of course we cannot consider just part of the wall in a room, we have to look at the entire wall area.

Now consider a house with 100 square metres of wall, where the internal temperature is 21OC and the outside temperature is 0OC, and the U value of the wall is 2.0 W/ m2 K.

Now, we would need:

   2 x 100 x 21 = 4,200 Watts

of constant heat input just to make up for the heat lost through the walls. That’s in addition to the heat lost through the roof, the floor, windows, and doors. To say nothing of the draughts whisking away more energy.

As a nation we have a dilemma. By 2050 it’s estimated that 70% of the houses standing have already been built. In a world with rising fuel prices, uncertainty over supply, and concerns over climate change we will need to consume much less energy.

Keeping Yorkshire warmer
cold walls, no ventilation cause damp, condensation and mould