: Installing foam bulkheads seems easier, lighter, & quicker than plywood,
: however I am concerned over their watertightness. Are the foam bulkheads
: watertight if well caulked?

Everything but lighter. I doubt a 2" or 3" thick piece of minicel is lighter than 1/4" of 'glassed cedar. That minicel is pretty dense.

I wouldn't worry about watertightness, though--5200 is good stuff.

Another benefit of the minicel bulkheads is the lessening of stress risers that can form near rigid bulkheads.

Shawn