I've got the opportunity to get my hands on a lot of 50mm(?) polystyrene board with a foil coating on one side and am wondering whether to use it as wall/roof insulation in the garage (which is really a workshop and never has a car in it). I tried burning a small sample and it melted, dripped and then burnt with a huge amount of soots. Is this stuff safe to use as intended? The intent would be to fix battens to the walls, fit the polystyrene between them and then plywood over the top; for the ceiling I'd just cut it to fit between the roof trusses and then hardboard over the top.
Dave S