Veranda ceiling - insulating + airtightness + panneling

[JFDIY]

I've got three bays above my veranda to panel in, they are 1.6m in both directions, above is a built in cupboard/wardrobe into the master bedroom.

I want to fit UFH to minimise heat loss in the room, I've already got spreader plates so that is my plan. 

 

What would the standard detail be regarding air tightness layer, breathable membrane etc. Joists are 220mm deep.

 

My plan is some thin battens under the top chord of joist giving 45mm on the top side, this having 25mm PIR to support the spreader plates and pipes.

 

This leaves 175mm for mineral wool insulation underneath, I'm feeling there should be I a breathable membrane, should it have air space or can it be in contact with anything?

 

Finally the boarding to the underside presumably needs to be moisture proof? Not sure fire proof is needed, building inspector wasn't over fussed, but I would like the finished surface to be rendered I think.

 

Anyone got any standard details of an overhanging floor like thid by any chance? I'm guessing a built over carport would be a similar situation.

 

This was missed off the plans and i'm no longer on talking terms with those who drew up the building regs.

 

Many thanks