Quick Question!

I just got my Ultratherm UTH from RBI today. Instructions say to put polystyrene underneath it to reflect and insulate. I went to our hardware store and all they had was the blue polystyrene (it's blue... sometimes it's pink... that stuff).

Will this work? On the blue polystyrene it says "Combustible - protect from from high heat sources". Arrrgh, does this mean I can't use it?

Thanks in advance!