Hello All,

I'm trying to answer the following question:

"Is there, by chance, a simple comparison between our 40 cm brick wall lined with Sempatap and painted with the Thermilate enhanced paint and a thinner modern wall with a filled cavity?"

Can anyone help me out here? Are U-Values cumulative or is there a specific formula for this? Can someone point me in the direction of any sites that compare these types of products?

Cheers,

Ben

Ok I'll attempt some calculations...

There are three components to your proposed wall..

1) The brick
Google says a 9" solid brick wall has a U-Value of about 2.0 so I would expect a 16" (40cm) wall to have a u-value approx.. 1/(1/2.0 * 16/9) = 1.1 W/m^2K

2) The Sempatap
Data here http://www.mgcltd.co.uk/Products/Thermal_and_Acoustic_Insulation/SEMPATAP_THERMAL/ suggests that 10mm of Sempatap isn't a great insulator... The thermal resistance is given as 0.158m² K/W
and that means a U-Value of about 6.3 W/m^2K (hardly worth bothering with?)

3) The Thermilate
Trying to find U-Value data on Thermilate paint isn't easy and some of the data out there is very missleading! This test was conducted on a sheet of painted plasterboard...
http://www.thermilate.com/pdfs/itdpdfs/Salford_Un_R_Value_test.pdf
R Value of untreated plasterboard = 0.136 m^2K/W
R Value of treated plasterboard = 0.199 m^2K/W
Improvement = 0.063 m^2K/W
So that a U-Value of 1/0.063 = 15.8 W/m^2K (hardly worth bothering with?)

Total U-Value
============

So the total for the wall would be approx:

1(1/1.1 + 1/6.3 + 1/15.8) = 0.88 W/m^2K


Modern Cavity Wall
================

The calculator on the Celotex Web site says that

4" brick leaf
25mm cavity
50mm celotex
Block leaf
wet plaster

U-Value approx 0.26 W/m^2K

So a modern wall would loose about three times less energy. 0.88/0.26 = 3.3

Fantastic. Many thanks for this.

this is a good easy to use calculator

http://www.xtratherm.com/software/software.php