Marcus,

As Tapio pointed out, most balsa is used in the end grain orientation. Low shear strength parallel to the grain is usually cited as the reason not to use balsa planking or strips (the shear strength across the grain is good). However, the stresses in a kayak (when not banging into things) may be low enough that you could use balsa strips. You might like to use thinner strips to reduce the effect of shear stresses, but the money you save in the cost of wood might well be offset by the additional epoxy and glass necessary. Let us know what you decide to do.

Sam