Insulating a vaulted ceiling

[MeldrewsMate]

I'm fitting out my new extension, the next job is to insulate the vaulted ceilings in the porch and new kitchen. The rafters are 150mm x 50mm on 400mm centres.
I've been advised by one builder that I have to use 100mm celotex between the rafters, then use a fancy and expensive insulating blanket over the whole construction, and finish off with silver backed plasterboard. Another says he never uses the blanket and doesn't have any problems with Building Control.

I'd like to discuss it with the Building Control people, but not sure what to call these components, nor how to lead into the subject. Any suggestions?

 

[tony1851]

You're referring to the multi-foil insulation products and they're a waste of time (and space).
Just put 100 board between the rafters, and finish off underneath
with 25 thick board (to reduce the pattern staining) and then normal plasterboard.

 

[freddiemercurystwin]

If you are doing this yourself why are you asking builders?

 

[noseall]

The most recent vaulted fella we built, featured 100mm celotex betwixt, with the option of multi-foil stapled beneath or 40mm celotex.

We opted for 40mm celotex then used 90mm drywall screws to fix the duplex boards.

Pattern staining or shadowing or cold bridging is when cold travels through a joist revealing itself as a discolouration on the ceiling where condensation has been occurring in line with the rafters or joists.