Dilemma re Insulation in conversion

[Glyn1959]

Help! I have a long integral garage which at some time in the past has been split into a garage and workshop buy a 9" cavity wall. The two remaining walls of the workshop are single brick. I am converting the workshop into a cloakroom. A builder has screeded the floor having fitted a shower tray and associated waste under the screed.
Re the single brick walls, his advice was to screw 2x2 wooden battens, fit kinspan insulation board between and then 12.5mm plasterboard on top. He has set the shower tray with these measurements in mind. When I contacted kinspan technical help line for advice on what thickness to use, they recommended leaving a void between the battens and screwing the kingspan in full sheets onto the front of the battens and screwing the plasterboard on top and lag any pipes that are going in the void.
The shower tray in now set too close to the wall for me to do this. I do not want the upheaval of digging up the floor to reset the tray and waste. Any ideas? Advice? Similar experience?

 

[Deluks]

Tricky one. You're gonna have to use thinner insulation and that's that. You might have to keep quiet about it if building inspector comes round though. Or you might be able to use the tri-iso stuff. Which is a foil sandwich insulation. In any case you definitely don't want insulation up against that external wall, the airgap is essential.

 

[Glyn1959]

Thanks for your points. 2 x 2 is roughly 50mm and the kingspan I had bought before all this blew up is 25mm. This would leave a gap, but how would I fix to ensure no contact with the brick? I am assuming it needs to be away from the brick but can be against the back of the plasterboard.

 

[Glyn1959]

I have noticed on the follwing site: http://www.walesatheart.com/Features/DIY/brighten up your home.htm extract as follows:
"To line a single brick wall, tack a 1000 gauge polythene sheet to the walls and then hold it in position using 50mm square vertical battens at 400mm spacing. Fill the gaps with 50mm thick expanded polystyrene foam and then line the walls with 12mm plasterboard".
What is your view on this?