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"Calculation of the heat loss through twin skin or composite panel metal construction brings
particular difficulties. Simplified methods are possible in some cases, but 2- or 3-dimensional
heat flow calculations must be carried out for some U-value and for all thermal bridge calculations. A number of software packages are available but many important decisions, which have a very significant effect on the results, are left to the user."

"Some standards, including BS EN 1252414, and the BRE U-values Conventions7 quote a conventional value of 50 W/mK for the conductivity of steel. The available information suggests that 60 W/mK is a more appropriate value for metal cladding systems. That value was used in the preparation of MCRMA Technical Note 143 and has been assumed in this guide."

The following steps are necessary to find the U-value,  (Omega)-value or f-value:

Details to be included in the model
a) Spacers – Section 5.1
b) Cladding rails - Section 5.3
c) Air cavities – Section 5.4
d) Profiles – Section 5.5
e) Fasteners - Section 5.6
f) Size of the model - Section 5.10

http://www.mcrma.co.uk/pdf/mcrma_t18.pdf

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