cavity wall insulation

hello, i was wondering can polystyrene board be used in the cavity of a kitchen extension?
i have 2 sheets of 70 mm kooltherm which i am going to use and was wondering about polystyrene?
i know the kooltherm is foiled both sides and the polystyrene is not, if i foil both sides of the poly can this be used? at least then it will have a vapour barrier on both side just like the kooltherm.
if not why not?

thanks in advance

 

thx phil, will invest in kooltherm