Two grids per bay in each purlin space drop in from the topside and require no fasteners or special tools . When used with white insulation facing it gives a bright reflective surface for lighting. The erectors place the support grids from the main frames without the necessity to walk on purlins. The support grids easily slide out from main frames and drop into position. No fasteners or tools required, all work is done from the topside.
Vapor barriers are important in controlling moisture in insulation.
Each building condition should be discussed with your architect, engineer, or insulation supplier.