Many older homes in rural areas have pier foundations. The piers may be made of wood (for example, creosoted posts or pressure-treated lumber), poured concrete, CMUs, or bricks. The space between the dirt and the underside of the floor framing may be enclosed or may be entirely open to the wind.
This type of home is more common down South than up North, because cold-climate builders have to find a way to keep the plumbing pipes from freezing. (It’s easier to keep pipes from freezing if a house has a crawl space or a basement than if the house is on piers.) That said, even up north, many rural homes (including mobile homes) have pier foundations.
Owners of homes on piers face several questions:
Should the area under the house be enclosed?
As long as there is no evidence that the insulation in the floor assembly is being disturbed, or that rodents are chewing holes to gain access to the house, there is no pressing need to enclose the space under a house on piers.
Leaving this space wide-open to the weather works better when piers are relatively high than when piers are relatively low, and works better when the floor assembly is airtight and well insulated than when the floor assembly is leaky and poorly insulated.
Problems occur when the spaces between the floor joists are insulated with poorly protected fiberglass batts. If these batts are held in place by chicken wire, it won’t take long for the mice, squirrels, and raccoons to move in. On the other hand, if the underside of the joist bays are sealed with a layer of carefully installed plywood or OSB, there are far fewer reasons for concern.
What’s better: a conventional crawl space foundation or a skirt?