sfag, say you have two dwellings
one is a detached 1350 sq ft bungalow
the other is a 1350 sq ft 2 storey semi-d
under the new regs (2007 part L) the two dwellings could require different insulation thicknesses in the walls (of the same material) to comply with regulations... why??
because the bungalow has a lot more exposed wall surface area for heat to escape from than the semi-d..... if you picture the actual shapes it becomes obvious.
then say you consider the air tightness of each.. .say the bungalow is 25% less air tight than the semi d.. it may require 25% more insulation to meet the same energy efficiency criteria.
The days of simply specifying a thickness for floor / wall / roof are over. the whole house has to be looked at as one entity, with all factors such as orientation, solar gains, air tighntess, heating system and controls etc