Combining u values

legs akimbo.

Would appreciate any advice combining u values of building elements.
For example : calculating together the value of a 325mm solid masonary wall with 52.5 mm overall Kingspan Therma board drylining.
Default for a circa 1940 construction is 1.6 u value but from the table of contents on standard building materials thermal conductivity using stated TC's does not equate to a 1.64 default value.
The method I employed was to work in reverse to determine a T.C.
0.325 / 0.52 = T.R of 0.625 1 / 0.625 = 1.6 u value (of 325mm masonary wall)

Taking the thermal resistance of the above with for example, the T.R of 52.5mm Therma board @ 1.80 equates to an overall u value of 0.41

0.625 (tr) + 1.80 (tr) = 2.425..... 1 / 2.425 = 0.41 overall u value.

Anyone have a better way ? or indeed is my equasion even correct before I employ theese calculations.

Regards
Legs.

No6

Use a U Value Calculator legs. Use the resistance for stone (not the DEAP default u Value)with all the other relevant restistances for an accurate U value of the construction. (assuming of course that you have proof of the drylining etc!!)

sydthebeat

if you are calculating u values you MUST refer to this document.

http://www.bre.co.uk/filelibrary/rpts/uvalue/BR_443_(2006_Edition).pdf

always be aware of air space resistance and thermal bridges.

Its extremely difficult to assess existing construction without a large invasive survey.