I've read a number of replies a naval architect gave to people who wanted to do his designs in glass/foam. His stadard reply was for double the money the hull will be as heavy or heavier than wood core. The reason is foam has a low resistance to penetration so more layers of glass and epoxy are required. On bigger boats the interior furniture could be made from glass/foam and considerable weight savings can be had.
For kayaks I have a theory why we don't see glass/foam being used but I'm not going to build a hull just to find out if I'm right.

: Unless you've got alot of experience using advanced composites, it
: will be very tricky using foam and it'll likely be quite a bit
: heavier rather than lighter. Strip built can be very light. You
: need much more fabric if you use divinycell since it doesn't
: have the compression strength that the wood does and that
: fabric/glass is the heavy part, the wood is actually very light.

: As you mentioned it would also be very expensive.

: Foam sandwich construction can be very light on a bigger boat, but
: as you get smaller the skin weight gets to be a higher
: percentage of the weight.

: Fairly easy btw to have a sub 30 lb. 17' kayak using wood strip,
: very difficult to match that using other materials other than
: plywood with S&G.

: Bill H.