On Thursday, March 6, 2014 10:19:56 AM UTC, Mathew Newton wrote:
On Thursday, March 6, 2014 2:08:45 AM UTC, wrote:



I'd lay the cables on top of the plasterboard.



That does seem to make sense. Indeed now that I've asked the question in public I can't help feel it's blindingly obvious!



Presumably recessed downlighters would be out of the question

Hopefully you'll come up with a better idea. Not all fashions are good.



He he. Quite. Bizarrely, I've often felt in other people's houses that that whilst I like the look of them in a kitchen environment they don't seem to always function particularly well, especially given I always end up sitting right underneath one of them feeling like I'm on Mastermind. There's no reason to believe my implementation would be any better so I'll scrap that idea.



On a related note, what are the benefits of adopting an insulation

between AND under the rafter approach?



Better insulation. When only between rafters, the rafters still
conduct thermally. OTOH you can stick cheap squishy stuff between
them.

I am still getting to grips with calcuating U-values so I might give
that alternative a thought. I'd always assumed PIR boards were
generally always better performers than mineral wool etc for a given
thickness though?

PIR is better than rock-wool batts per mm, not as good per UKP.

*Personally*, I would stick the "over the rafters" insulation *above*
the rafters (so the rafters are in the warm, and hence dry). I'd
probably leave the rafters exposed too (but that's just me).

Else thread you also said:
On the subject of rafter depth this will be a new extension and so
I am assuming I could specify 175mm rafters in order to accomodate
the necessary insulation between them (say 150mm) and air gap (25mm)
beneath the breathable felt.

Why? Rafters are expensive. Don't make them bigger than you need
(unless you want them big for visual appearance). Much better to have
a solid layer of insulation above them.