Our double garage was converted before we moved in many years ago, the converted half is now used as an office and occasional crashing room (sofa bed). The room is damn cold though and there appears no real insulation between the wall and the garage.

Its a block and brick wall so would I be best off using insulated plasterboard inside the garage? If so, do I screw the board straight into the bricks/blocks or batten first then screw into the battens? If the latter, should I also squeeze some rockwool type stuff into the gap or could that lead to damp?

Thanks

Batten, insulate and plasterboard.

You can buy Plaster Board backed closed cell foam. You can fix that to battens or just fix to the wall direct.

Hammy, insulate with rockwool or a board type like cellutex?

Either tbh – whichever you feel is best priced!
However all modern houses tend to use Cellotex, etc to attain the U values as its cleaner, easier to fit, etc.
If you really want to go full on then use the 20mm backed plasterboard over the top of the battens/insulation to complete the thermal break.

This is how I did my workshop:

Celotex PB4050 fitted over GA4100 by brf, on Flickr

Using hammer in fittings

Insulation fixings for 170mm of insulation by brf, on Flickr

You then need to put a 5mm bonding coat under the skim if you’re plastering it, to cover up the heads of the fixings.

Alan skimming the workshop by brf, on Flickr

For ease and the least disruption I’d insulate on the garage side of the wall with a rigid PUR type insulation (Kingspan, Recticel, Celotex). 50 – 75mm on that side, either screwed or bonded/glued to the wall will make it a lot warmer. Could cover with ply or something sturdier rather than plasterboard if you use the garage properly.