Thermal Conductivity, Thermal Resistance, R & U-Values

Thermal Conductivity, Thermal Resistance, R & U-Values
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Thermal Conductivity, Thermal Resistance, R & U-Values

When you decide to insulate your home and begin to look at insulation materials, you will come across the terms, ‘Thermal Conductivity’, ‘Thermal Resistance’, ‘R-Value’ and ‘U-Value’.

Read on to discover what these terms mean.


What is Thermal Conductivity?

Thermal Conductivity is the property of a material to conduct heat. It is evaluated in terms of Fourier’s Law for heat conduction. It can be assessed in a lab test measurement involving a set thickness and environment for the material. This can then be used to find the Thermal Resistance or R-Value. From here you can calculate a U-value which will vary depending on the application.

What is Thermal Resistance?

Thermal Resistance measures the material's resistance to heat flow and is sometimes referred to as R-Value. The higher the value of this, the more efficient the material is.

For example, if an insulation material has an R-Value of 2 per inch thickness and the required R-Value to be reached for the insulation of the required space is 6, then 3 inches of the product would need to be installed.

If you choose an insulation product with a higher R-Value, then the required thickness of the product used is reduced. However, other factors come into play, such as the suitability of the insulation material for the space to be insulated.

The R-Value varies with thickness. It is calculated by dividing the thickness of the board (expressed in metres) by its thermal conductivity.


Thermal Resistance/R-Value Recommendations

Thermal Resistance is worked out by dividing the thickness of a material with the conductivity. When you buy insulation material from a high street store or online, you’ll see that the R-Value or Thermal Resistance has already been worked out for you.

There are different recommendations for different insulating locations. For example, if you are carrying out a loft insulation, the recommendation is between 6.1 and 7.

When you buy loose fill fibreglass, you’ll find it has an R-Value between 2.2 and 2.7 per inch of thickness. Therefore, to reach the R-Value of between 6.1 and 7, you are going to have to install a minimum thickness of around 3 inches. Insulating materials have various Thermal Resistance levels.

Here is an example of how thermal resistance can be improved by insulation:


Insulant Thickness (mm) Thermal Resistance (m2-K/W)
20 0.85
25 1.05
30 1.30
40 1.70
50 2.15
55 2.35
60 2.60
65 2.80
70 3.00
75 3.25
80 3.45
90 3.90
95 4.10
100 4.30
105 4.55
110 4.75
115 5.00
120 5.20
125 5.40
130 5.65
135 5.85
140 6.05
145 6.30
150 6.50
155 6.700


What is U-Value?

The U-Value is the flow of heat through the building materials used to make up the building itself. This includes bricks, flooring, roofing and windows. The value is measured in watts at the rate of heat transfer. The lower the U-Value of a material the better insulator it will be.

For example, if you have a home with cavity walls and your home was built at the end of the seventies and into the eighties, it is likely that your U-Value uninsulated is 1.0 W/m2k. If you insulate the cavity walls you’ll reach a U rating of between 0.2 and 0/5 W/m2k depending on the materials you use. The significant reduction in the U-Value of the cavity wall after insulation will relate to a reduction in the required energy consumption to keep your house warm.


Possible Reductions in U-Values by Housing Material

  • Walls – Solid walls can be insulated from the inside or outside and this can lower your U Rating from around 2.70 W/m2k to as low as 0.29 W/m2k.
  • Roof – Uninsulated roof space will have a U-Value of about 2.5 W/m2k and this can be brought down to around 0.15 W/m2k. This can lead to a major saving for roof and wall spaces.
  • Windows – If you are building a new home, the government recommends U-Values for windows are a maximum 1.6 W/m2k. If you want to insulate windows in an older property, then having them double glazed will lower the U Rating. Windows are now sold with a Window Energy Rating which includes the U-Value and the solar benefits. It goes from A to G with A being the most energy efficient.

This guide is intended to get you started with understanding insulation. Refer back to this information to help find the right material and amount of insulation required to keep your home well insulated.

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