Garage Insulating

Verb

I'm going to insulate my garage and would like a little advise if anyone has it.

Its currently red hot in the summer and freezing in the Winter

Its a flat roof with nothing above with 200mm rafters. Whats the best insulation to use?

The furthest wall from the house is only a single wall of brick (no inner breeze block or cavity) I'm planning to stood the wall to the depth of 100mm and insulate, What is the best insulation for this?

The wall and ceiling I'm to cover with OSB board. Do I need to put any vents in the wall or ceiling?

I'm looking at Kingspan or Insulation Wool or Jablite which looks to be a cheaper alternative or a combination

DOH_a

Have you checked if you need Planning or Building Regulation approvals as the specification would normally come as part of the latter?

Doozergirl

Solid insulation in ceilings and on the walls for me.   There isn't enough depth to meet current regs with rockwool, so 125mm or more of solid insulation is the simple choice. 

Again, solid insulation doesn't need to be as thick as rockwool equivalents, so I'd choose insulated plasterboard with the insulation depth to the improve / meet specification. 

You want OSB, so your not painting' properly?

Petriix

If you want it to actually be properly insulated then do some research about how to avoid thermal bridging. Consider creating a proper warm roof by adding insulation on top of the roof; otherwise you will have to add insulation below the joists, reducing your ceiling height.

Verb

I think my main question is as its only a garage.
Jablite or Celotex Insulation Board?
Or 50mm of each. Jablite on the back and Celotex on the front directly behind the OSB board