Q.: For sidewalks placed directly on the subgrade soil, is a sheet-plastic vapor retarder useful or harmful?
A.: In temperate climates, we see no advantage to placing a vapor retarder beneath a sidewalk. If you're in a cold climate and the subgrade soil is poorly drained, a vapor barrier might be marginally useful. It could protect the concrete from freeze-thaw damage by reducing the amount of water it absorbs from the wet subgrade. But it might be harmful to seal this concrete with a nonbreathing sealer, thus trapping moisture within the concrete pores