: You probably don't need the 1/8"--the foam should work well by itself.
: Pete Rudie put expanding foam in the bow and stern of his Guillemot 17.

Yeah, it seemed like a good idea at the time. Trouble is, the foam does not adhere very well to the epoxy and glass, allowing water to wick into the dissimilar-material joint. But there is insufficient ventilation in those spaces to ever completely dry them out. Result: rot. All my other boats will have removable float bags instead.