Radon in crawl space homes: submembrane suction, ventilation and what works best

Many people assume radon is mainly a basement problem. In fact, homes built over crawl spaces can have high radon too, because the soil under the house is exposed and air from the crawl space is drawn up into the living area. The good news is that crawl space homes have a well-proven fix. This guide explains how radon behaves in crawl space homes and how the main reduction methods compare, based on EPA's Consumer's Guide to Radon Reduction.

How radon gets from a crawl space into your home

EPA explains that air pressure inside a home is usually lower than the pressure in the soil around and beneath the foundation. The house acts like a gentle vacuum, drawing soil gas in through any available opening. In a crawl space home, those openings include:

• the exposed earth floor of the crawl space;
• gaps around plumbing, wiring, and ductwork that pass through the floor above;
• ducts and air handlers located in the crawl space, which can pull crawl space air into the home if they leak;
• open joist bays and rim joists.

In cold weather, warm air rising and escaping from the upper parts of the house increases this suction, which is one reason radon levels often run higher in winter.

The most effective method: submembrane suction

EPA describes the main method for crawl space homes this way. The earth floor is covered with a high-density plastic sheet, and a vent pipe and fan draw radon from under the sheet and vent it outdoors. EPA calls this submembrane suction and says that, when properly applied, it is the most effective way to reduce radon levels in crawl space homes.

A typical installation includes:

1. Preparing the crawl space: removing debris and sharp objects that could puncture the membrane.
2. Laying a durable plastic membrane over the entire soil floor, overlapping seams and sealing them, and often sealing the membrane to foundation walls and piers.
3. Installing one or more suction points under the membrane, often a perforated pipe or a collection mat.
4. Running a vent pipe to a fan located outside the living space, such as in the attic or on the exterior, and discharging above the roof.
5. Adding a warning device, such as a manometer or alarm, so you can see the system is running.

EPA's comparison table lists submembrane depressurization in a crawl space as typically reducing radon by 50 to 99 percent, with less heat loss than natural ventilation in cold winter climates.

Why the discharge goes above the roof

EPA explains that radon concentrations at the discharge point can be very high, but they fall to near background levels within 3 to 4 feet. EPA disallowed ground-level discharge because radon could re-enter the house and children could be exposed. Ask your contractor where the discharge will be and confirm it meets the national standard and any state requirements.

Alternative: active crawl space depressurization

Another technique draws air directly out of the crawl space with a fan, without a membrane. EPA says this method generally doesn't work as well as submembrane suction. It requires special attention to combustion appliance backdrafting, meaning that a furnace or water heater in the crawl space could spill exhaust, and to sealing the crawl space from the rest of the house. It may also raise energy costs because conditioned air is lost from the home.

Alternative: crawl space ventilation

EPA notes that radon levels can sometimes be lowered by ventilating the crawl space, either passively by opening or adding vents, or actively with a fan blowing air through the crawl space. Ventilation lowers indoor radon both by reducing the house's suction on the soil and by diluting radon beneath the home.

But there are trade-offs:

• EPA's table lists natural ventilation in a crawl space at 0 to 50 percent reduction, with variable costs.
• In colder climates, EPA notes that water pipes, sewer lines, and appliances in the crawl space may need to be insulated against the cold.
• Ventilation can increase energy costs and may add moisture in humid climates.

Ventilation can be a reasonable choice for modest radon levels in mild climates. For higher levels, submembrane suction is usually the better long-term answer.

Combination foundations

EPA notes it's common for a home to have a basement under one part and a crawl space or slab under another. In those cases, a combination of techniques may be needed, such as sub-slab suction for the basement plus submembrane suction for the crawl space, often tied into a single fan.

Crawl space encapsulation vs. radon mitigation

Some contractors sell "crawl space encapsulation," which typically means a heavy vapor barrier, sealed vents, and sometimes a dehumidifier, mainly for moisture control. Encapsulation can help with radon, but only if it includes active suction beneath the membrane and a proper vent. A sealed crawl space without suction may still let radon into the home. If radon is your concern, ask specifically for submembrane depressurization designed to radon mitigation standards, and require a post-mitigation radon test.

Testing and follow-up

EPA's guide says a post-mitigation test should be done within 30 days of installation, but no sooner than 24 hours after the system is running with the fan on. After that, EPA suggests retesting at least every two years to make sure levels stay low. The fan must run continuously. EPA notes fans may last five years or more, and manufacturer warranties tend not to exceed five years.

Questions for your contractor

• Will you install submembrane suction, and how will seams and edges be sealed?
• How many suction points will you use?
• Where will the fan be, and where will it discharge?
• What warning device will you install?
• How will you protect the membrane from damage when people need to access the crawl space?
• What post-mitigation result do you guarantee, and what happens if it isn't met?

See hiring a radon mitigation contractor for credentials and contract tips.