A barrier assembly comprising a horizontally deployable vapor barrier assembly with a frame with opposing side guides extending between a header and footer. A guide rod contained in each side guide extends between the header and the footer. A vapor barrier is deployable horizontally with side edge sleeve portions that slide axially over the guide rods. At least one roller is connected to each of the side guides adjacent to the guide rod, and each roller supports a portion of the vapor barrier's side edge portions. The rollers rolls on the edge portions as the sleeve portion slides axially over the guide rods as the vapor barrier moves between the deployed and stowed positions. Lifter plates are attached to each of the side guides and positioned to engage support carriages and temporarily lift the support member out of engagement with the support surface over segment joints as the vapor barrier moves between the stowed and deployed positions. A drive system moves the vapor barrier between the stowed and deployed positions.