On Dec 1, 8:49*pm, wrote:
U values... I thought brick/cavity/brick wall was 1.73?

I get something quite different...
Brick Commons - 102mm --- 0.132
Air Cavity - 50mm ------- 0.180
Brick - 102mm ----------- 0.132
Plaster - 6mm ----------- 0.025

Total R Value = 0.469 where U = 1/R = 2.132

Just noticed a cell in an old spreadsheet got corrupted and defaulted to 1.37 which I am sure is wrong, thought it was 1.73, but calculating from data I get 2.132 (6mm plaster, most are 13mm obviously).

Thanks.

The U values are only approximations anyway.
Cavity walls have a wide variance because it depends on the amount of
ventilation to the cavity.

The U value I have for an uninsulated cavity wall is 1.2
Ditto insulation filled 0.6

Calculating heat losses is a very inexact science.