"Andy Hall" wrote in message
...
The U value for 100mm of mineral fibre in a pitched roof construction
is 0.36 according to BS 5449.

So in fact Neil's heat loss via this route is less than suggested and
more like

70 x 0.36 x 21 = 529W.

This is about 4% of his total 12.3kW

Increasing the insulation to your suggested level might achieve a U
value of 0.15 taking the heatloss down to 220W.

This represents a difference of 2.4%

In fact I deliberately increased the U-value of the loft because I don't
know exactly how much insulation there is throughout the whole thing -
part of it is inaccessible at the moment - and some of it appears to be
somewhat low quality.