Insulating garage ceiling (bedroom above)

[Diybeginner5]

I have a bedroom extension above my garage (built before I purchased house) which always feels cold. I have insulated the loft space well above it but can feel the cold in the carpet which leads me to want to insulate the garage ceiling - I have a decent roller door so drafts minimal into garage.

Been reading threads on here and think that celotex or similar is best bet on garage ceiling. But looking at plastboard options like http://www.planetinsulation.co.uk/ZPL4025.php but wondered if I can then tape between the boards and then expanding foam around the edges or if it would be better to do without plasterboard so can tape joints better? Advice much appreciated!

Also would like some advice as to which of these or an alternative is better?

http://www.planetinsulation.co.uk/ZGRTLP27.php or http://www.planetinsulation.co.uk/ZPL4025.php

 

[screamer]

if you're going to re-plasterboard it, just take down the plasterboard, glasswool between the joists and re-plasterboard, but would be better double sheeted (staggering the joints) or use of fire retardant plasterboard due to being over a garage, also a upstand with fire retardant mastic butted up to the PB and fixed to the wall around the edges.