TOWARD THE HEATED SPACE
For the ceiling of a cool basement under a heated level: It’s always best to position the vapor barrier toward the heated space, that is, just under its floor. The opposite is true for an unheated attic, where the vapor barrier is usually placed against the ceiling of the space below. In both cases, the barrier holds back water vapor generated in the house, preventing it from condensing on cooler surfaces in basement or attic. Wet insulation is less effective and can lead to peeling paint and rot.