Second skin for garage

[Mattk]

When converting a garage for domestic use I understand that a second skin has to be added for insulation but is there a quicker/cheaper solution e.g. similar to plasterboard with an insulating layer?

Thanks.

[TonyV]

you can stud it and add insulation to get the right u value - depends on what your local building Inspector is happy with. Do you need to floor as well?

http://www.insulation.kingspan.com/uk/k17.htm

[The Builder]

Always worth remembering Matt, that you can never have too much insulation. I agree with everything Tony has said. It's the way I usually do it.
Certainly no need for masonry. A single leaf wil let rainwater pass through easily, so it's a good idea to leave a 50mm cavity, then build a simple timber stud wall. Perhaps fill the studs with ordinary loft insulation, then cover with a mutifoil such as Thinsulex TLX.
http://www.just-insulation.com/tlx_silver.html
It's not cheap, but the results are excellent. Taping all joints and laps to prevent any draughts is the key to this kind of system.
You could simply batten the walls and fit Kingspan/Celotex between the battens. It really does depend on what you can agree with your Building Control guy. I guess, in theory, he could ask you to meet current (quite high) insulation values, as in a new build. In practice though, they're happy to see at least some attempt to get a good insulation value.