Budget garage roof insulation

[mrmarcopolo]

I've inherited a problem garage with the problem house I bought. I now mm n need to replace the roof on my garage and want to add some insulation. I'm going to put it between the ply and the rafters for simplicity. I know I can use foil conferred insulation boards, but what about using floor insulation (http://www.selcobw.com/products/insulation/floor-insulation/jabfloor-70-floor-insulation-2400-x-1200-x-25mm-bba) to save some cost? It's a detached garage that is built of aerocrete bricks and I've lined the inside with 12mm marine ply and flexible tile adhesive to keep the walls from cracking apart. After the roof, I'm going to clad the outside to hide the disaster.

 

[noseall]

Many people will be wondering why you are insulating a garage?

If you insulate from inside you will be effectively creating a cold deck. How will you ventilate this space?

 

[mrmarcopolo]

I'd like to use the space as a workshop and for my wine/beer making. Actually I wanted to create a warm deck by putting it between the ply and the rafters (i.e. on top of the rafters rather than underneath). I am more concerned about the compressive strength of the polystyrene, but imagine on reflection that if you have 50mm thick sheets it should be fine on reasonably spaced joists.