Wall Audit

The house comprises of the original, main part plus two later additions; an entrance area and a kitchen extension. And in order to make the design of the original part more attractive, three different wall construction types were used by the designer, as follows

1The front has decorative wooden cladding on a wall section with no cavity. *
2The second is pebble dashed with a slightly reduced cavity.
3The third has a 100mm cavity.

Information contained in this section

The heat loss from each of the different wall types has been calculated and these are shown in the Calculations section of the site. The combined heat loss has then been estimated for the house in order to evaluate the effectiveness of insulation and the heating system.

Important Note: The unit of temperature Kelvin (K) can be considered to be the same as Celcius (C) for temperature differences. Kelvin has been used because these are the accepted units used in heat calculations.  

Insulate Walls

The total heat loss from the house has been estimated by calculation to be 69.39W/K. That is, 69.39 Watts of heat escape for each degree K (C) that the inside of the house is warmer than the outside.

So what does this represent in real terms.

Using the same conditions as used in the Windows section of the site, the scenario is a cold winters day with the outside temperature at -1C and the inside temperature at the recommended 18C. In such conditions the heat loss is approximately 1.057kW.

Thermal loss = 69.39W/K
Temperature inside = 18C
Temperature outside = -1C
Temperature difference = 19C
Total heat lost through walls = 19x69.39 = 1318W (1.318kW)

In terms of the boilers maximum heat output it is 8%.

Repeating the above without the wall insulation gives a heat loss of 176.66W/K. In the above scenario the heat loss would then be

Total heat lost through walls = 19x176.66 = 3356W (3.356kW)

In terms of the boilers maximum heat output it is 21%. In other words the insulation makes a massive difference to the heat loss!

Calculations on the thermal performance of the above three wall types reveal the relative effectiveness of the insulation.

Construction Type

U Value without insulation


U Value with insulation




Front wall, cladding, no cavity.




Side wall, 100mm, air cavity.




Rear wall, Pebble dashed, slightly reduced cavity.




 Problem Impact Rectified Remedial Work
In summer the inside of the front wall can get warm from the heat of the sun. The warming effect suggests that the insulation is not high. Unfortunately, the front wall construction does not allow for cavity wall. Calculations show that this wall amounts for 46% of the total loss through the walls which is approximately £109 per annum. A 56% saving in heat loss could be made by installing insulation under the wooden cladding. TBC
The picture shows the bottom of the kitchen wall, taken from underneath the units. It shows a gap between the plaster board and floor. The plaster board is fixed on wooden battens and so there is a small cavity behind the plasterboard.The cavity was allowing air to travel from the roof void down the back of the plasterboard and into the room. DoneNow filled with expanding foam. (An example of which can be seen on the right)