Things You'll Need
Ventilating the crawl space under a structure's foundation prevents mildew and rot from attacking the subfloor. Water vapor in the soil escapes into the atmosphere. Without proper ventilation, that water vapor can collect under your foundation and invade the subfloor wood. Eventually this moisture will cause damage. There are several methods used to prevent excess moisture from gathering under your foundation. Installing foundation vents is the best and most practical solution.
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Determine how many foundation vents local building codes suggest per square foot of crawl space. Most building codes specify one square foot of ventilation per 150 square feet under the foundation. For example, if your home is 1000 square feet, you will install 7 square feet of ventilation. Purchase the required number of vents. Measure one of the vents. The standard size is 4" X 10", but may vary.
Select locations on the 2-by-12 rim joist between two subfloor joists. Measure and mark openings to fit the vents you purchased.
Drill a quarter-inch hole in one corner of the first marked opening. Slip the blade of the jigsaw into the hole and cut along the marked lines.
Cut out the remaining vent spaces. You will probably have one every 3rd or 4th bay. The bay is the 16 inches between each floor joist.
Slide a foundation vent into each space. Attach the vents to the rim joists with a small nail at each corner and each side.
Foundation vents are only necessary on raised foundations. Foundation vents are non-structural so they require only a minimum number of nails. In addition to installing foundation vents, laying out a vapor retarder over the soil under the foundation helps prevent moisture problems.
If you live in an extremely cold region, unless you use automatic vents, they must be opened and closed with seasonal changes.