On Thursday, March 6, 2014 8:17:00 AM UTC, harry wrote:

On a related note, what are the benefits of adopting an insulation
between AND under the rafter approach?

I have that arrangement in part of my house. Most rafters are only 100mm deep.
So you get more insulation. You can get the rigid foam with a crinkly plastic finish that can be plastered direct. (Joints need scrim) I fixed mine with big (galvanised) nails, washers and canned foam. Seems OK after three years.

Thanks Harry. I must admit to being rather bewildered by the range of options available. Even within a manufacturer's range I'm finding it difficult to always discern the differences between some of their offerings or, if it's obvious, it doesn't always seem significant to justify both their existances!

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.

Mathew