A vapor barrier is a waterproof material used to prevent or retard moisture from traveling between an interior and an exterior surface. Vapor barriers are installed on interior walls to protect them from condensation or, in new construction, beneath a concrete slab or underneath siding as a wind and moisture break.
A major source of ambient moisture in historic houses is dirt cellar floors: installing a vapor barrier on a dirt cellar floor will reduce moisture throughout the building. Polyethylene sheeting is recommended. Seams should overlap six inches and outside edges should extend six inches up the wall. Placing bricks along the floor at the foundation will secure the vapor barrier. Opening cellar windows, using screens to prevent small animal and insect intrusion, will provide further ventilation during good weather.