After reading your post, I'm not quite exactly sure what you intend to create with your design, in regard to where the styrofoam is used in your build. Is there any way you could post a diagram or sketch of your idea? With that said, I'm not sure where styrofoam would produce any benefit in a bass trap build. People are right in that it won't "stop sound" but it isn't acoustically transparent either. It's going to reflect, which is kind of the opposite of a bass trap's goal. Again, a sketch or diagram would definitely help as I may be completely misinterpreting your idea, but styrofoam is going to be reflective in any scenario.
EDIT: Oh, where are you located? Reason being, materials vary in availability and price region to region and somebody might be able to help you by pointing out something that's both attainable and affordable in your area.
Last edited by CarloBadalamenti; 15th October 2012 at 04:33 PM..
Reason: Location post-script