Crawl Space Planning

Instructor: Sunday Moulton

Sunday recently earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

This lesson discusses crawl spaces in home planning and construction. Specific topics include the decision to build a crawl space instead of a basement, types of crawl spaces, and environmental concerns with crawl space design.

What is a Crawl Space?

You may have heard of a crawl space as part of a house before and, like many, the image that probably comes to mind is a dirty, musty area full of spider webs and unspeakable horrors. Well, some crawl spaces may live up to that reputation, but those are rare exceptions. Properly constructed and well-maintained crawl spaces provide homeowners with an affordable alternative to a basement when local conditions present challenges to basement construction.

Creepy Crawl Space
Creepy Crawl Space

So what exactly is a crawl space? It is an area between a home's floor and the ground the house is built on. This space often provides access to a home's plumbing as well as the heating and cooling ductwork.

Why a Crawl Space instead of a Basement?

The decision to build a crawl space instead of a basement depends on the local environment and financial concerns. Living in an area with a high water table, the depth below ground where water is present, makes basement construction difficult and expensive. No one wants a leaky basement, and the more moisture in the soil, the more likely leaks will develop.

Another reason to include a crawl space in a building design involves the consistency of the ground below the house. Rocky soil poses a challenge to building and requires more time, effort, and money to remove stones. If the stones are too large to remove with heavy equipment, owners may have to use explosives at the site.

Design Concerns

There are two major concerns when designing a crawl space, the most important of which is moisture control. Moisture beneath the floor can cause extensive damage to a home, such as mold, decay, and attraction of termites looking to snack on wet wood. Developers and homeowners must ensure proper ground drainage so rain flows away from the home. Next, the ground beneath the home should be covered with a sturdy moisture barrier and the floor, plumbing, and ductwork should be insulated to prevent moisture penetration and condensation.

Ground cover keeping back excessive moisture during a flood
Moisture Barrier

The second concern is radon. Radon is an odorless, colorless, radioactive gas that causes lung cancer. The protective ground cover should be a minimum of six millimeters thick and have a label for soil-gas retardant. Installing a ground vent that directs this gas to the outside of the structure will help if building in an area with significant radon gas risk.

EPA Radon Threat Map
Radon Threat Map

Types of Crawl Spaces

While the measures listed above will help with moisture and radon, selecting the best measures to use depend on the type of crawl space selected. The two types built today are the unconditioned and vented crawl space and the conditioned and unvented crawl space. Their main difference is that the vented crawl space is open to the elements in some way while the unvented crawl space is sealed from the outside. Each offers benefits and drawbacks depending on the home's needs and the local environment.

Unconditioned and Vented

Traditionally, the unconditioned and vented crawl space was the only style allowed by most building codes. These styles installed mandatory vent openings at specified intervals along the foundation wall on all sides of the building. These openings allow for cross ventilation, airflow through openings on either side, so air has entry and exit points. The most extreme example, though not technically a crawl space design, would be a house on stilts or small piers.

This design prevents moisture from becoming trapped beneath the floor while small amounts of radon gas can dissipate. Remember, higher levels of radon will still require venting and ground cover barriers.

Unconditioned and Vented Crawl Space
Crawl space vent

Still, vented crawl spaces possess a few problems. First, the vents provide openings for pests like mice and insects to enter the home. While the vents should be screened, the screens must be regularly maintained and the mesh should be small enough to prevent infestation while still wide enough to allow ventilation. The effectiveness of vented crawl spaces for moisture reduction significantly decreases in humid environments like the American Southeast or the Pacific Northwest.

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