A rigid polymeric insulating foam board comprises an upper facing 2, a lower facing 1 and a foam layer 3 between the facings 1, 2. The foam sandwich is delivered into an oven 10 and the continuous length of foam board exiting the oven is delivered through a curing station. A stack 50 of the cut lengths of board 51 is formed by inserting spacers 55 between adjacent boards in the stack. The spacers are arranged longitudinally and are transversely spaced-apart to create channels 56 therebetween through which air can circulate during stack curing.