Crack the code to the perfect crawl space by checking out local requirements
Always review local building codes before designing or altering your crawl space.
Crawl space code requirements regulate ventilation, size, and encapsulation.
Proper crawl space design protects the area from moisture, pests, and freezing temperatures.
Codes may vary depending on your local climate and flood risk.
A crawl space is more than an escape tunnel in a spine-tingling, suspense-filled horror flick. It’s an essential part of your home built to protect you and your family from what lies on the ground below—which can also send shivers up your spine—and it must be kept up to code.
We’ve rounded up the standard crawl space requirements to keep your home dry, safe, sanitary, and legal.
The IRC designed a set of residential code requirements to ensure the construction of buildings are sanitary and safe for people who live inside. While the IRC doesn’t mandate local governments to uphold the codes, cities and counties use them as a model to develop their own construction codes.
Read on to learn crawl space requirements homeowners and foundation contractors need to follow.
Today, building codes allow for both vented and unvented crawl spaces, as long as they comply with code requirements. Originally, crawl spaces all required vents, but after building commissions learned they became a breeding ground for mold, rodent nesting, and rotted wood building materials, the laws changed. Unvented crawl spaces, however, require proper crawl space encapsulation to meet building codes.
Additionally, determining whether the crawl space vents stay open or closed depends on the season and the level of moisture in your climate year-round.
When crawl spaces don't house a vent, the contractor needs to place a Class 1 vapor retarder, insulation material, and the right size dehumidifier for the space. Dehumidifiers come in a variety of sizes based on square footage and humidity levels. Below, you’ll find new dehumidifier sizes necessary for ventless crawl spaces (older models are around 30-pints larger).
| Crawl Space Size (Sq. Ft.) | Dehumidifier Size | Humidity Levels |
|---|---|---|
| 300 | 20 pints | 50% – 70% |
| 25 pints | 70% – 80% | |
| 30 pints | 80% – 100% | |
| 500 | 25 pints | 50% – 60% |
| 30 pints | 70% – 80% | |
| 40 pints | 80% – 100% | |
| 800 | 25 pints | 50% – 70% |
| 30 pints | 70% – 80% | |
| 40 pints | 80% – 100% | |
| 1,200 | 50 pints | 50% – 70% |
| 55 pints | 70% – 80% | |
| 60 pints | 80% – 100% |
If your crawl space area measures larger than 1,200 square feet, just do the math! Consider hiring a pro to encapsulate (or seal) the space. Encapsulation costs an average of $5,000, depending on the size of your crawl space and the materials used.
Building codes foundation require crawl spaces to maintain outdoor ventilation between the ground and floor joists. If your crawl space is vented, you’ll need to install vents on all the walls.
The size of vents must be 1 foot for every 500 square feet or 1 foot for every 1,500 square feet when they house Class 1 vapor barriers that contain a 6-mil thick sheet of polyethylene.
Crawl spaces require access doors that are easy to get to in case of an emergency or the need for repairs. According to the IRC, the size of access doors for crawl spaces must measure 18-by-24 inches.
Crawl space floors need drains or sump pumps in the crawl space. They must be separate from foundation perimeter drains and gutters, and drain away from foundations.
The IRC now requires all crawl space entrances to measure a minimum of 16 inches high and 24 inches wide.
When using a Class 1 vapor retarder, you’ll need to install thermal insulation on the walls. Foam insulation boards placed in crawl spaces and attics must contain a thermal barrier, which safeguards crawl spaces from igniting a fire. Never use insulation materials that are porous on the ground since they absorb water.
The terms vapor barrier and vapor retarder are often used interchangeably, so it's easy to get them confused when insulating a crawl space. However, on a basic level, vapor barriers typically have stronger ratings for protecting a space from moisture compared to retarders.
Vapor barriers and retarders are broken down by thicknesses and permeability. These factors are measured in mils, or millimeters of thickness, and a perm rating, or the level of permeability in the material. In either case, most regulations will require a Class 1 vapor retarder, which means it has a perm rating of less than 0.1 perms.
Local regulations, how susceptible your crawl space is to moisture, and the nature of the crawl space will all determine your ideal mils and perm rating. It will also determine whether you need a professional or if you can learn how to install the vapor barrier in the crawl space yourself. Specialized types of vapor retardants include:
String-reinforced and unreinforced polyethylene
Aluminum liners
Rubber membranes
Glass
Sheet metal
In many cases, the inside of your crawl space will fall below the grade of the ground outside your home. In order to pass a basic basement inspection after construction, contractors may need to fortify the outside of the wall with damp-proof materials such as concrete, asphalt, brick, or mortar.
Depending on the extent of your crawl space encapsulation, you may need to also add insulation to pipes and ductwork. Not only does this protect the inside of these areas from the buildup of moisture from the cold, but it can lower utility bills and protect pipes from the potential of freezing.
Homes located in flood zones deal with additional crawl space code requirements. For one, at least one wall of the crawl space must be above grade to the exterior. Additionally, doors that allow flood water to exit the crawl space must be installed to keep water from potentially rising into your home.
On a more regular basis, you'll be required as a homeowner to keep your crawl space free of debris. Lawn cuttings, leaves, large rocks, and garbage must be kept out of the space for proper access. It is also important to stay ahead of mold remediation should moisture issues creep up.
If you’re buying, selling, or building a home, the right pro to hire for the job is a foundation contractor near you. These contractors will get down and dirty and crawl under your house to check things out. If you’re hiring a local home inspector they can take a quick look, but when there’s any doubt, they’ll recommend you call a foundation contractor.
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.
A crawl space can be as little as 42 inches below the grade of your house, but this is not always recommended in flood-prone areas. At a minimum, there must be at least 16 inches of clearance between the ground and the floor joists to access the space in an emergency.
In most climates, contractors recommend closing your crawl space vents in winter. Keeping them shut protects your crawl space from freezing temperatures, pests, and cool breezes that can lead to chilly floors in your home. However, it's often best to speak with a local crawl space team about your specific design and its relationship to the winter season.
