Peter,
you can add as much FOAM, etc, as you like, you will NOT REDUCE the
displacement of the HULL by ONE gram !! All you have done is ADD the
weight of the foam.
Like many, you are confusing WEIGHT with VOLUME (displacemnet) of a hull.
All you can do is try to remove actual weight from your hull or superstructure.
( Like most of us, adding food, even so called 'lite' foods, adds weight !!! :-)