Sealed Crawl Spaces and Radon: Why Encapsulation Helps Reduce Levels

Radon is a naturally occurring radioactive gas that forms when uranium in soil and rock breaks down. It seeps upward through the ground and enters buildings through any opening in the foundation. In homes with crawl spaces, the pathway is even more direct than in slab-on-grade homes.

A traditional vented crawl space has an exposed dirt floor. Radon gas rises from the soil into the crawl space, and from there it migrates into the living space above through gaps in the subfloor, around plumbing and ductwork penetrations, and through the natural air exchange between the crawl space and the house.

The stack effect makes this worse. As warm air rises inside your home and exits through the upper levels, it creates negative pressure in the lower areas. This pressure differential pulls air from the crawl space into the living space, bringing radon along with it. In winter, when the stack effect is strongest, this can significantly increase indoor radon levels.

Even crawl spaces with basic plastic ground cover can allow radon entry. Thin, unsealed plastic sheeting does not create an effective gas barrier. Radon can migrate around the edges, through tears, and at overlapping seams.

Crawl space encapsulation is the process of completely sealing the crawl space from the outside environment. It goes far beyond laying a thin sheet of plastic on the dirt floor. A proper encapsulation involves several key steps.

A properly installed crawl space encapsulation reduces radon entry in several ways. First, the heavy vapor barrier acts as a physical barrier between the soil and the crawl space air. While no plastic sheet is 100% impermeable to gas, a thick, properly sealed barrier significantly reduces the rate at which radon can migrate from the soil into the crawl space.

Second, sealing the vents and gaps reduces the air exchange between the crawl space and the outdoors. This means less unconditioned outdoor air is being drawn through the soil and into the crawl space, which reduces one of the mechanisms that pulls radon from the ground.

Third, by connecting the crawl space to the home's conditioned air system, you create positive pressure in the crawl space relative to the soil. This pressure differential works against radon entry rather than in favor of it.

The most effective approach for crawl space homes with elevated radon is to combine encapsulation with a sub-membrane depressurization (SMD) system. This is the crawl space equivalent of the active sub-slab suction used in slab-on-grade homes.

In an SMD system, a perforated pipe is placed beneath the vapor barrier in the crawl space. This pipe connects to a vertical vent pipe that runs to above the roofline, with an inline fan that creates active suction. The fan pulls radon from beneath the membrane and vents it safely above the roof before it can accumulate in the crawl space or living areas.

When both systems are installed together, the vapor barrier serves double duty. It acts as the seal for the radon depressurization system and also provides all the moisture and air quality benefits of encapsulation. This combined approach routinely reduces radon levels by 90% to 99%.

If you are already planning to encapsulate your crawl space for moisture control, adding the radon mitigation component during the same project is highly cost-effective. The additional cost for the radon pipe and fan is typically $1,000 to $1,500 on top of the encapsulation cost, compared to $1,200 to $2,500 if done as a separate project later. Regular radon system maintenance keeps the fan and seals performing properly. Contact our residential radon services team for a free crawl space assessment.

Radon reduction is one important benefit of crawl space encapsulation, but it is far from the only one. Homeowners who encapsulate their crawl spaces typically see improvements across several areas.

Georgia homes present some unique considerations when it comes to crawl space encapsulation and radon. The state's climate, geology, and building traditions all play a role.

Crawl space foundations are common throughout Georgia, particularly in older neighborhoods and areas with sloping terrain. Many of these crawl spaces were built with vents, which were once thought to be necessary for moisture control. Modern building science has shown that in humid climates like Georgia's, vented crawl spaces actually perform worse than sealed ones because the vents allow humid outdoor air to enter and condense on cooler surfaces.

The granite and gneiss bedrock in north Georgia produces higher levels of naturally occurring uranium, which means higher radon potential. Homes in counties like Forsyth, Cherokee, Gwinnett, and Fulton that have crawl space foundations should be tested for radon and should consider encapsulation as part of a comprehensive radon reduction strategy.

Georgia's building code now allows sealed crawl spaces under certain conditions, which means you will not run into code issues when encapsulating. However, proper installation is important. The work should meet current code requirements for vapor barriers, drainage, and conditioning.