Crawl space basics: moisture, vents and vapor barriers

A crawl space is the short open area between the ground and the first floor of many homes. It holds the floor framing, and often the plumbing, ductwork and sometimes the furnace or water heater. Because it sits right on the soil, it is the part of the house most likely to collect moisture.

Over time, crawl space moisture can lead to biological growth, rot in the floor framing and conditions that attract termites. This guide covers what a healthy crawl space looks like, the two basic designs, and the tools used to keep one dry. For what fixes cost, see crawl space moisture.

What a healthy crawl space looks like

• No standing water. The ground may be cool, but it should not be wet or muddy.
• Ground covered. A plastic vapor barrier covers the soil. NC State Extension calls for at least a 6 to 8 mil plastic vapor barrier over the soil. [6]
• Dry, solid wood. Joists, beams and the subfloor are firm, without dark stains, white fuzz or soft spots.
• No wood touching soil, and no scrap wood or cardboard on the ground. North Carolina's termite report rules treat wood-to-soil contact and cellulose debris under a house as conditions that invite termites. [8]
• Insulation in place. Floor insulation is not sagging or hanging down, and ducts are connected.

Signs of trouble include a musty smell in the house, standing water, white powdery deposits on the walls (efflorescence), water stains on the framing, condensation dripping from ducts or pipes in summer, and wood that feels damp. North Carolina's termite report rules also flag wood with a moisture content of 20 percent or more as a condition that favors termites. [8] See the high crawl space moisture and standing water findings for how these show up on a report.

Fix the water first

Most crawl space water starts outside. Before anyone changes how the crawl space is built, the drainage around the house should work. NC State Extension's checklist says gutters and downspouts should be clear and carry water away, downspouts should extend 6 to 10 feet from the foundation, and the ground should slope away from the house. [6] It also says to keep sprinklers off the house and not to overwater plantings next to the foundation. [6]

The U.S. Department of Energy's crawl space checklist for contractors says the same: confirm foundation drainage and waterproofing before sealing anything, and add perimeter drainage if the crawl space floor sits below the ground outside. [2] Plumbing leaks under the house need to be fixed too.

Vented crawl spaces

Many older homes have a vented crawl space, with small openings in the foundation walls. The idea is that outside air flows through and carries moisture away. Building codes based on the International Residential Code set how much vent area is needed: 1 square foot of vent for every 150 square feet of crawl space, or 1 for every 1,500 square feet when the ground is covered with a vapor barrier, with a vent near each corner. [4]

The problem is summer air in humid climates. Warm, humid outside air cools when it enters a shaded crawl space, and cooler air can't hold as much moisture. A Penn State housing research center example shows outside air at 86°F and 77 percent relative humidity reaching 100 percent humidity once it cools to 68°F, which means condensation. [4] The Department of Energy notes that in humid regions, vents can bring moisture in rather than remove it. [1]

A vented crawl space can still work well when the ground is covered, the outside drainage is good and the vents are open. Blocked or missing vents show up on reports as poor ventilation.

Sealed crawl spaces and encapsulation

A sealed crawl space, also called a closed, unvented or encapsulated crawl space, takes the opposite approach. Instead of letting outside air in, it closes the crawl space off and dries the air inside. Research by Advanced Energy in North Carolina, sponsored by the Department of Energy, compared vented and sealed crawl spaces in matched homes in the Southeast and found the sealed ones were substantially drier and used less energy for heating and cooling. [3] The Department of Energy reports heating and cooling savings of 15 to 18 percent and humidity reduced by more than 20 percent in Building America research, and that research helped change the model code to allow this design. [1]

Encapsulation typically involves these steps [2, 4]:

1. Fixing drainage and leaks, and repairing rotted wood.
2. Removing old insulation and debris, and clearing the ground.
3. Covering the whole floor with a heavy plastic liner, overlapping the seams by at least 6 inches and taping them, and running the liner up the foundation walls (at least 6 inches, per the Department of Energy checklist) and sealing it there.
4. Closing and sealing the foundation vents.
5. Insulating and air sealing the foundation walls and the rim joist (the board around the edge of the floor framing).
6. Adding a sealed, insulated access door.
7. Drying the air, usually with a dehumidifier or a small supply of conditioned air from the house.

Codes require one of a short list of drying methods. North Carolina's code lists five: a dehumidifier, supply air from the HVAC system, house air, an exhaust fan, or treating the crawl space as conditioned space. [5] In North Carolina, the supply-air and conditioned-space methods need a heating contractor with the right state license. [5] Rules differ in Georgia, Texas and between local jurisdictions.

Dehumidifiers

A crawl space dehumidifier is a heavy-duty unit sized for the space. It needs an electrical outlet and a place to drain the water it collects. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, to limit mold growth. [7] A dehumidifier can help a vented crawl space through humid months, and it is the most common drying method in a sealed one.

Safety checks for a sealed crawl space

Closing a crawl space changes how air moves in the house. The Department of Energy checklist calls for [2]:

• Combustion safety testing if a furnace or water heater that vents by natural draft sits in the crawl space, so exhaust gases do not spill back in.
• A radon test after the work, since the crawl space air now connects more closely with the house. See the safety guides for radon.
• Ongoing checks for water after the work is done.

Vented or sealed: a quick comparison

	Vented crawl space	Sealed crawl space
How it handles moisture	Outside air moves through vents	Vents closed; air dried by a dehumidifier or conditioned air
Ground cover	Vapor barrier recommended	Sealed liner on the ground and up the walls
Insulation	Usually in the floor above	On the foundation walls and rim joist
Best fit	Drier climates, or homes where outside drainage keeps it dry	Humid climates, homes with ducts or equipment in the crawl space
Ongoing care	Keep vents clear, check the vapor barrier	Keep the dehumidifier running and drained, watch for water

Three levels of crawl space work

HomeCloud looks at crawl space moisture in three levels, from basic to premium:

1. Vapor barrier. Plastic sheeting over the dirt floor, after any water coming in is fixed. This is where most homes start.
2. Seal and dehumidify. For a crawl space that stays damp: close the vents and other openings to outside air and add a dehumidifier.
3. Full encapsulation. The premium level: the vapor barrier runs over the floor and up the foundation walls with overlapped, taped seams, every foundation vent is closed, the walls are insulated, and a dehumidifier keeps the air dry.

Our article on vapor barrier vs. encapsulation explains how to choose, and crawl space vapor barrier cost covers the basic level.

What to do next

If your report mentions crawl space moisture, start with the water source, then look at the vapor barrier and how the crawl space is vented. Damp framing that has grown mold or rotted is its own repair: see biological growth removal. Insulation and air sealing are covered in the insulation and ventilation guides. If you are buying a home, our article on termites when you're buying a home explains why crawl space moisture also shows up on termite reports.

HomeCloud is a licensed general contractor that evaluates homes and prices and does the repairs. We can send you a priced estimate for your crawl space.

Sources

1. U.S. Department of Energy, Unvented, Conditioned Crawlspaces: Building America Top Innovation
2. Building America Solution Center, Vented to Unvented Crawl Space checklist
3. Advanced Energy (sponsored by U.S. DOE), A Field Study Comparison of the Energy and Moisture Performance Characteristics of Ventilated Versus Sealed Crawl Spaces in the South
4. Pennsylvania Housing Research Center, Crawlspaces
5. NC Office of State Fire Marshal, Closed Crawl Space Moisture Control (2019)
6. NC State Extension, Mold and Moisture Checklist
7. U.S. EPA, A Brief Guide to Mold, Moisture and Your Home
8. NCDA&CS Structural Pest Control, Homeowners Guide to the Wood Destroying Insect Report