JohnB is right . . . foam provides NO significant structural benefit, other than to provide a male mold for the stringer. It's only benefit is flotation (and additional weight when it gets wet!
) Stringers function like a beam, and most of the stress in any beam occurs in the outer fibers so that's where you want the most glass. I-beams put most of the material at top and bottom for that reason, but a box section section stringer is used because it's light and also has more torsional stiffness than an I-beam.