U-Values

When using Terraco EIFS, in order to determine the thickness of the insulation material required, you need to determine the Thermal Resistance (R) value required to achieve the required U-Value.

Remember: The lower the U-Value – the better insulation and performance you get from the system.

What is a U-Value?

“A U-Value is a measure of heat loss in a building element such as a wall, floor or roof. This means that the higher the U-Value the worse the thermal performance of the building envelope. A low U-Value usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.” - The Royal Society of Architects (RIBA)

Why use U-Values?

“U-Values are important because they form the basis of any energy or carbon reduction standard. In practice, nearly every external building element has to comply with thermal standards that are expressed as a maximum U-Value. Knowledge of how to simply calculate U-Values at an early stage in the design process avoids expensive re-working later on in a project. It allows the designer to test the feasibility of their project at an early stage to ensure it is fit for purpose and will comply with regulatory frameworks.” - The Royal Society of Architects (RIBA)

When to use U-Values?

A critical milestone in any building project is obtaining building regulation approval where energy or carbon reduction standards are required as part of the approval process. To calculate the U-Value of a building element such as a wall, the architect or consultant needs to know the build up of that element or wall, where each building material is positioned properly in sequence, where the thickness of each building material has been determined, as the U-Value is determined as the sum of all the thermal resistances (R) of the materials found in the wall.

U-Value calculation

To determine the U-Value of a building element, i.e. a wall, the architect needs to know the different elements and thicknesses that make up a wall. Each element in that wall has a certain thermal resistance (R) which is derived from that materials thermal conductivity (K) and its thickness (d).

This data is then entered into the following formula: R=(1/K) x d

These Resistance (R) values then need to be added up to give you the combined U-Value of that wall.

Thermal transmittance (U-Value) = sum{1/R}

There is a simple rule: The higher the K-Value, the higher the U-Value, and vice versa. Therefore an insulation board with a lower K-Value can be applied in a lower thickness to achieve the same U-Value as a board with a slightly higher K-Value.

Terraco EIFS uses insulation materials (EPS, G-EPS, XPS, MW) with low thermal conductivity (λ) to get low U-Values and reduce thermal bridging as shown in the table below.

Insulation Board  Thickness (mm) Thermal Conductivity*
K-Value
[W/(m.K)] 
Thermal Resistance
R-Value [(m².K/W] 
Thermal Transmittance
U-Value
[W/(m².K)] 
 EPS  50
100
150
200
0.033  1.52
3.03
4.55
6.06
0.66
0.33
0.22
0.17
 G-EPS
(Graphite enhanced)
 50
100
150
 200 
 0.030 1.67
3.33
5.00
6.67
0.60
0.30
0.20
0.15
 XPS 50
100
150
 200 
 0.030 1.67
3.33
5.00
 6.67 
 0.60
0.30
0.20
0.15
 MW  50
100
150
 200
 0.040 1.25
2.50
3.75
5.00
0.80
0.40
0.27
0.20

* K-Values are indicative and will vary depending on insulation board density and specification.

Calculations:

  • K-Value: Constant
  • R-Value: Board Thickness (m)/K-Value
  • U-Value: 1/R-Value