I used homasote on my 40x50 layout, works well. It is much more dense than ceiling tiles (I use that a lot for scenery purposes) it also is much better for sound abatement than foam.
Foam is A hard smooth surface, and while good for scenery, not so good to absorb sound. Soft uneven foam - like the shape of egg containers, is good for sound, but that is much different than foam insulation.
while homasote is not cheap, it works well.
