The Natural Philosopher wrote:

K value for plasterboard is 0.16. So U value of 16 for 10mm thick.. or
16 watts per square meter per degree centigrade.

Nonsense - you've forgotten to include the surface boundary layer
resistances.

For calculation principles and up-to-date data see BRE publication 443:
http://www.bre.co.uk/filelibrary/BR_...6_Edition).pdf

--
Andy