OK, we have an affordable housing development where I have been asked to look at the u-value for the ground floor slab, approximately 40% of which is existing and the rest new build. The former is going to have at most 50mm some polystyrene beads, (not my specification) and the latter can have something more substantial thermally.

The balance between new and old varies from flat to flat...

However how would you go about calculating the U-values in compliant (if not realistic) way?

Assess each flat with the perimeter as the outside edges only ie not adjacent to other flats and hearted circulation spaces

The whole slab - average out the R-value of the insulation, use whole area vs whole perimeter...

Obviously if the former then I'll end up with either a range of U-values or a range of slab/insulation levels (which aint gonna happen...!) If a range of U-values then this will result in a range of Code scores as pretty much everything else in Code (in this case) is common to all flats, ie the lowest flat would effectively set the pass (Code3) score...

Any ideas...?



J

PS we are the contractors architects and Code assessors and have taken over the Code for construction stage from someone else, who helpfully set a U-value of 0.13...

Along edges where the slabs adjoin, you'll be double-counting losses where there are none at all.

You could calc ea slab individually, as Tony suggests, on the perimeter:area ratio rule; deduct from ea slab's loss a %age equal to the %age of its perimeter that abuts another one (i.e. that isn't a true external perimeter); then add them.

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