Insulating garage/room above floor divide from cold side

23 Sep 2012
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United Kingdom
Good morning everyone,

Hopefully a relatively easy question - we have a garage with a bedroom above it. Pre-pre-owners did the conversion to make garage liveable space plus room above, pre-owners (who we purchased from) returned the garage to well, a garage, leaving the room above.

This has now left the garage with a plasterboard ceiling > joists > caber floor > finished room. We have since discovered that despite having a building control certificate for converting the liveable space back to a garage that there is no insulation in the ceiling; how annoying.

Anyway, keen to get it done properly before next winter and before any condensation problems arise so would like to pull the plasterboard ceiling down, insulate and refinish.

I have previously DIY'd the insulation in a 1930s cavity floor at our last house - pull the original boards up, PIR between joists, expanding foam any gaps and then foil tape as the vapour barrier, all inspected and signed off by building control 'as a really good job' (very pleased with myself).

What I'm really not sure about here however, and this is the real crux of the question, is how the vapour barrier should be done in this instance? I thought, from past research into how to do the floor that the vapour barrier always goes on the 'warm' side of the room? So would need to be between the insulation and caber floor above? Obviously I'm really keen to avoid pulling the floor up to do this as the room above is carpeted, finished and in use, and equally would like to try and avoid the loss of any ceiling height in the garage by insulating under the joists (making it a 'warm' floor/ceiling I guess if using the same terminology as a flat roof?). Is there a way to insulate between joists from the 'cold side'?

Any thoughts and/or advice gratefully received.
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