Just had dorma extension built upstairs which will house extra bedroom and bathroom.
All finished outside – need to finish (more like start) inside now.
The wooden structure is still exposed inside, with thermal Recticel block insulation between the woodwork on the walls, and (deeper) Kingspan thermal block insulation between the flat roof joists, with vents on the soffits or airflow over that insulation. New building regs say I have to now have a thermal barrier on the inside of the wall woodwork before it’s plasterboarded as the cold can creep through the wood! Seems I can either a) put a thermal backed plasterboard so only one fix or b) line with something thermal then board over with dryliner board.

Anyone got any experience of this or got any recommendations?

Want to retain as much space as poss so the thinner the better – and cost-effective too!

You can get 12mm Celotex/kingspan, not sure of cost of that + plasterboard versus thermal
board. Check that BC wont want a vapour barrier too

12mm Celotex/Kingspan with foil taped joints (will also act as a vapour barrier), and then standard plasterboard.

Insulated plaster board at a guess

Insulated p/board as its a single fix operation – in either case the thickness required will depend on the u value you need to achieve. Don’ t just go for the thinnest product as it may not be enough – check with building control before you buy.

Kingspan in between joists, usually 75-100 mm then an insulated plaster board over that. We used 25 mm inso boards last year on dormas. Make sure you fix with 70 mm min screws.

Take a look at the Celotx website – they have a 96 page guide with all the various configurations (all compliant with building regs). I got my guide from Building Control who refer to it as ‘The Bible’.

Foil backed (well both sides actually) insulation foam, taped with Aluminium foil to create a vapour proof layer.

First insulation later complete by brf, on Flickr

On top of which you can put more insulation bonded to PB:

Fitting Celotex PB4050 onto GA4100 by brf, on Flickr