Garage Roof Insulation

Hi all,
I've got a concrete sectional garage with a corrugated (asbestos?) roof. Due to the place always being cold, I am thinking of putting insulation of some sort under the asbestos, and relaying that on top of it. My question is, other that twin wall polycarb or similar, what would be the best bet? I don't want it to be thicker than 1/2 in. max.
TIA
--
--dave

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