A.David Bowyer responds: Our company has built many additions on wooden posts and concrete piers, and we’ve developed a simple technique to close off the open area between the floor system and the ground. We install a pressure-treated plywood skirt that matches the thickness of the wall sheathing above (see illustration). The top edge of the plywood skirt is nailed to the rim joist; the lower portion extends about 1 foot below grade and is fastened to a treated 2x4 nailer that is held a few inches above the ground. The plywood can be sided to match the walls, or parged (over wire lath) to simulate the look of a masonry foundation.
We’ve never had a problem using this approach in our area of Indiana, where the frost line is 3 feet and local soils drain well. In colder regions with heavy clay soils, it would be wise to surround the buried portion of the plywood skirt with gravel, and place a drain to daylight in the bottom of the trench.
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