What's the kayak shell construction?

What's the kayak shell construction?

Single layer
Solid LDPE shell, which is widely applied to most short boats
2 layers
LDPE shell with 1 inner layer of PE foam, better rigidity and lighter weight than solid LDPE.
3 layers
2 layers of LDPE with 1 layer of PE foam in the middle, the 3rd layer of solid LDPE makes inner side smooth.