Airing a Dank Basement

Even after you have eliminated leaks and see page, you may still have moisture in the basement or the crawl space. If it's unventilated and unheated — and especially if it has a dirt floor — moisture condenses on joists and the undersides of floorboards, and spreads upward inside the house walls. Covering a dirt floor with a vapor barrier is the essential first step. But if your basement or crawl space lacks ventilation, also install ventilators. With a vapor barrier on the floor, most experts recommend one ventilator for every 300 or so square feet of floor area.

Install the ventilators near the top of the foundation walls and close to the corner, but not right next to it. At a minimum there should be two ventilators in opposite walls. In the rare instances when more than two are needed, distribute them equally in all the walls.

In a cold climate you will need to insulate the underside of the first floor if it has not already been done. Outside air in winter is as much as five times drier than in summer, so you can reduce air circulation and still get rid of unwanted moisture by partially closing ventilators—but leave some opening.

Mortaring the ventilator. With chisel and hammer make an opening in the foundation wall slightly larger than the ventilator. If you expose block cores, pack them with paper to within an inch of the surface and fill them the rest of the way with mortar. Let it cure for 24 hours. Half way back and on the bottom of the opening, lay a bead of mortar the width of the ventilator and ¾ inch high. Place the ventilator on the mortar and then pack mortar around the other edges of the ventilator with a mason’s tool, called a joint filler, or with a blunt kitchen knife.

Making a coping. So that rain water does not ac cumulate on the sill outside or inside the ventilator, build up a coping, or sloping surface, on both sides of the ventilator. Trowel mortar up to the bottom cit the ventilator and then slant it toward the edge of the foundation wall. Place damp rags on the mortar to cure it for 24 hours.