"Mathew Newton" wrote in message
...
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

I have seen a few new buildings/roofs done that way.
I was insulating an existing roof.

Mind, there is an advantage with multiple layers and that is you can plaster
directly on to the finish of the insulation board rather than having to add
plaster board.
ie it will be big sheets on the inside if you see what I mean, few joints.

BTW, important there are absolutely no gaps. Even a tiny gap defeats the
whole project.
I cut my insulation undersize (by about 20mm) and filled the gaps with foam.
Trim excess off when set.
Slabs supported in place with temporary nails (removed when foam had set)