Updated: Mar 31, 2020
Even after you have eliminated leaks, you may still have moisture in the basement or the crawl space. If its unventilated and unheated and especially if it has a dirt floor moisture condenses on joist and the underside of floorboards, and spreads upward inside the house walls. Covering a dirt floor with a vapor barrier 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 or 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 rare instances when more than two are need, 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 the summer, so you can reduce air circulation and still get rid of unwanted moisture by partially closing ventilators but leave some opening.
How to monitor the ventilator? With chisel and hammer make an opening in the foundation wall slightly larger than the ventilator. If you expose block core, 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 25 hours. Half way back and on the bottom of the opening, lay a bead of mortar the width of the ventilator and 3/4 in 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.
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