Do I need a vapor barrier?

Depends on which product you use. “Closed Cell” soy spray foam is the only insulation material that is a perfect vapor barrier both summer and winter. Soy closed cell foam prevents moisture laden air from entering the wall and ceiling systems  and condensing on the sheathing and rotting it out and/or growing mold.  When fiberglass and cellulose are properly installed with an external vapor barrier they can me effective insulations, but the very nature of these materials makes them prone to condensation and mold growth. Open cell foam requires a separate 4 mill poly sheet vapor barrier, as does fiberglass and cellulose (unless the cellulose is in “dense-pack” form).