Questions on insulating fish house?

Hi all
I have just built a 10ftx10ft breeze block fish house , now the question is i want to insulate the fish house the roof is a flat roof i have read that u have to leave a 2inch gap between the insulation and the roof is this correct because i was planning to fill the roof with 8 inch insulation this would take it to the underside of the roof basically filling the whole void and then put the foilbacked plasterboard over this and then seal the plasterboard with gloss paint would this cause condensation not having the 2 inch gap as my builder friend said that i should be ok to do this but i would like a second opinion on this matter as i dont want my timber to rot. Also i was going to insulate the walls now could i use the 8 inch rolls and squeeze it back to 3inch's or would i be better of using 3 inch celotex type insulation for the walls, the reason i want to squeeze the insulation to that size is i need as much room as possible inside the fish house by squeezing it would it lose any preformance the reason for the 8 inch rolls is my friend has some going cheap he also has some 4 inch as well would i be better of using that instead any help would be welcomed regards .

You must ensure a 50mm airspace is maintained between the insulation and the roof. this is for ventilation of the enclosed area to reduce condensation. I would use celotex in the roof void.

Then a waterproof membrane is fixed over all, make sure you have a good overlap on joints.

The plasterboard is then screwed to the wall/roof joists using 50mm drywall screws . the reason P/board is screwed up is, speed, less damage to board surface and personal pref.

You will need a lot of force to keep insulation compressed, 3" would be cheaper, why use 8"?

OILMAN the reason for 8' was i could get it at half price although i could also get 4 ' at half price too.