What is a U value?

April 4, 2015

A U value is the measurement of heat loss (or heat flow) per m2 of surface area per 1 degrees C (Kelvin) temperature difference.

The calculation measures the heat flow through a series of elements and includes internal and external resistance. An example is displayed below:

In order to calculate a U value, you need the following information for each layer:

Thermal conductivity (W/mK)
Material thickness (mm or metres)
To get the ‘R’ value, you divide the thickness (metres) by the thermal conductivity (W/mK). Example. 200mm Loft Roll with 0.044 W/mK = 0.2m/0.044W/mK = 4.545 m2K/W
The U value of the element is then 1/R. Example = 1/4.545 = 0.22 W/m2.K. So we know that just adding 200mm insulation with 0.044 W/mK thermal conductivity will achieve 0.22 W/m2.K on it’s own (assuming no thermal bridging)
The total U value for the construction is this adding up the ‘R’ value of each element PLUS the internal and external resistance ‘R’ values. Divide the total ‘R’ value into 1 (1/R=U W/m2.K)

The equation is generally displayed as follows:

U Value = W/m2.K

W = Watts – measurement of heat flow (or heat loss)
m2 = 1 metre squared of surface area (of the wall, roof, floor or door/window)
K = Kelvin or Centigrade. in this case just 1 degrees C

To calculate the heat loss of a wall then measures 5 metres long by 2.5 metres high, and assume the internal temperature of the room of 20 degrees and the external temperature is -1 degrees. The U value of the wall is 0.30 W/m2.K

Wall area = 5 x 2.5 = 12.5m2
Temperature Difference = 21 degrees
U value = 0.30 W/m2.K

The heat loss at that given state is therefore:

0.30W/m2.K x 12.5m2 x 21 degrees = 78.75W heat loss.

If we improve the insulation so the U value is now 0.20 W/m2.K, then we get the following revised result:

0.20W/m2.K x 12.5m2 x 21 degrees = 52.50W heat loss. A 33 % reduction in heat loss.

When a SAP Calculation, thermal model or SBEM calculates the heat loss, it will use weather data to determine the external temperature. It will use the calculated U value for the construction. it will use the geometry of the surface to calculate the area.

The lower the U value, the less heat loss occurs. This will reduce heating demand. Reduce the size of the heating plant (ie less overall heat loss at the highest temperature difference) and reduce carbon emissions.

We also have an overview of Thermal Bridge Details and their impact on SAP Calculations and SBEM. Just click HERE to see out article with links to standard details and heat loss values (PSi values).

Manufacturer U Value Calculations

Need U values? The I recommend the following manufacturers websites:

Knauf Insulation – Select, the element type (i.e. wall) and the type of wall (i.e. timber frame) and it will take you to a brochure and give an overview of the construction, their insulation production and the U value result. You can then contact Knauf for the full U value calculation.
Celotex Insulation – You need to register, but they have an excellent online U value calculator to make life easy. You can also get their mobile app.
Kingspan Insulation – Recently added. Simple to use and you can use a slider to target the U value and the website app will tell you the thickness of insulation to use.