A. Henri de Marne responds: Extruded polystyrene (such as Styrofoam, Foamular, and Amofoam) and aluminum-faced polyisocyanurate rigid insulation (such as Thermax) make an effective vapor retarder if installed on the bottom chord of any style truss as long as the joints are sealed with a compatible foil-faced tape. If the foil tape is applied carefully at the joints, there should be no need for an additional poly vapor barrier.
By contrast, expanded polystyrene or "beadboard" would not perform as well as a vapor retarder, since this type of rigid insulation can absorb moisture. Assuming that you have R-19 or greater fibrous insulation between the trusses, a 6-mil poly vapor barrier should be installed on the bottom chord of the trusses prior to installing the beadboard.
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