Insulate Bristol - Internal Wall Insulation

Insulate Bristol - Internal Wall Insulation



U-values - The lower the better!

U-value calculation service

If you would like to know how much energy you could save having internal wall insulation installed we can provide U-value calculations for your walls.

Calculating the U-value of a building element

U-values are a measure of thermal transmittance (heat transfer) and they express the rate of heat transfer through any element of a building, such as a wall, roof or window.

U-values are measured in watts per square meter per kelvin (W/(m²K)

For example, a solid wall with a U-value of 2.0 W/m²K, means that for every degree difference in temperature between the inside and outside of the wall, 2.0 watts will be transmitted every square meter.

The lower the U-value, the better insulated the building element.

Understanding U-values is important when considering insulation that will be the most effective at reducing heat loss. 

Part L of the Building Regulations stipulates minimum standards for building energy efficiency and it includes target U-values for new build and refurbishment works.

U-value calculators can be used to determine the U-value of a building element before and after insulating, alternatively some Insulation manufacturers offer a U-value calculation service for their products. 

To calculate the final U-value (after insulation) we first need the base U-value of the building element. For example a solid brick wall (225mm thick) may have a base U-value of 2.16 W/m²K. It is important to include all layers of the building element in the base U-value calculation. For example a solid brick wall may be plastered internally and rendered externally and in this example including these 'layers' will lower the final U-value. 

Once the base U-value of the building element is determined the insulation layer can be added to the calculation to get the final U-value.

Below is a worked example of adding different thickness of wood fibre board insulation to a solid brick wall with a base U-value before insulation of 2.16W/m²K.

For context PART L of the building regulations requires existing walls have U-values no higher than 0.70 W/m²K to gain compliance.

Un-insulated 9” solid brick wall: 2.16 W/m²K

9” solid brick wall with 20 mm wood fibre insulation: 1.01 W/m²K.

This example shows how 20 mm of wood fibre insulation doubles the thermal efficiency of a solid brick wall.

Increasing the thickness of the Wood fibre insulation lowers the U-value further reducing heat loss.

30mm = 0.80 W/m²K

40mm = 0.66 W/m²K (may comply with building regulations)

60mm = 0.49 W/m²K (may comply with building regulation)

Comparing thermal conductivity of insulating materials


The insulating performance of different types of insulating materials e.g. PIR rigid foam, wood fibre boards, Aerogel can be compared by their thermal conductivity also known as the Lambda value.

The Lambda value measures how easily heat flows through a specific type of material, independent of the thickness of the material in question. Materials with a low lambda value are good insulators and will reduce heat loss.

For example:

Celotex PL4000 (PIR) insulation board - Lambda value: 0.022 W/mK

STEICO Therm wood fibre boards - Lambda value: 0.038 W/mK 


The R-value is a measure of resistance to heat flow through a given thickness of material. So the higher the R-value, the more thermal resistance the material has and therefore the better its insulating properties.

With the R-value the thermal resistance of insulating materials can be compared and the impact at different thicknesses. 

For example:

Celotex PL4000 (PIR) insulation board - 60mm PIR insulation + 12.5mm plasterboard - R value: 2.75 m2K/W

Steico Therm wood fibre board - 60mm - R value: 1.50 m2K/W