Many thanks to those who have been helping me with my heat loss calculations so far.

I felt this question also deserved a seperate post.

So I am now looking at the fabric heat loss through the roof which has a pitch of about 30deg at both ends, and a ridge along the top.

The ceiling will be insulated with 300mm of rockwool.

I want to do a before and after scenario, so fabric losses before were based on the entire area of the roof. Whereas now, the house is thermally insulated at the upper ceiling level, which not only is a smaller area, but no longer takes into account the roof material.

How can I do a fair comparison & calculate post fabric heat loss? Can I add the u-values of the insulated ceiling, the volume of air in the loft, and the uninsulated roof? (I know it's 1/(1/R + 1/R....) That's the only way I can see how to do it. As if it were one giant sandwich, with air, the filling. Although the air is not a flat area.

Cheers

Yep, disregard the roof entirely, it's just there to keep the rain off and has minimal thermal effect since the air circulating in the loft is pretty much the same temperature as the outside. Just model the heat loss through the flat ceiling.