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The primary reason homes have crawl spaces is so the foundation can reach down below the frost line to avoid dangerous soil movement and structural damage when the ground freezes. They’re more popular in colder climates where the ground freezes down to a few feet, and slabs are most popular in warmer areas where there is no frost line. Crawl spaces also provide a space for utility lines to run where they’re at less of a risk of freezing.

A vapor barrier keeps moisture out of the crawl space, while insulation reduces the amount of unconditioned air—and sometimes moisture—that enters the space. Coupling a vapor barrier with insulation can help prevent humidity and related moisture damage in a crawl space while also making your home more energy-efficient. Just be sure to use insulation that won’t get damaged from moisture, like rigid foam board or expanding spray foam insulation.

Yes, the air from a damp basement can travel up into your living area through a process called the stack effect, so moisture, mold spores, and other debris in your basement air can all decrease your indoor air quality on your main floors. Additionally, damp basements can promote the rotting of building materials, which can damage flooring, insulation, and framing on your first floor in addition to materials in the basement.

In nearly all cases, you are required to install a vapor barrier in your crawl space. However, local regulations will change based on the local climate and the crawl space's susceptibility to moisture. Even if a vapor barrier is not legally required, it is often a good idea to keep your crawl space dry.