Sub slab depressurization (SSD) – Most homes will be fixed with a “Sub slab depressurization” system, which uses a fan and PVC pipe to draw air from below the basement floor and exhausts it above the roof. The radon fan creates a vacuum under the basement floor. With the fan running, if you puff smoke by a crack or opening in the basement floor, you will see the smoke slowly pulled down through the crack or opening. Turn the fan off, and the smoke will blow back in your face. As long as the fan is running, there is literally no underground air entering the home. Typically, the pipe will run through the garage, and the fan will be in the garage attic. Other methods consist of the pipe in a closet and the fan in the attic, or both the pipe and fan outside.
(Images courtesy of Spruce Environmental Technologies)
SSD with a crawl space – In a home with exposed dirt in a crawl space, the dirt will normally be covered with a special plastic designed for crawl spaces. The plastic will be sealed to the perimeter walls. Then, a tee will be put in the pipe of the SSD system, and an arm of the pipe will be run into the crawl space and down through the plastic. Now, as the fan runs, it draws air from under the basement floor and from under the plastic.
Sealing openings – Years ago, the EPA experimented with various radon mitigation techniques. To assess the effect of sealing, they sealed basement walls, floors, and ceilings with epoxy, made the return air ductwork airtight, put an airtight door at the top of the stairs, installed sealed combustion furnaces and water heaters and more. After spending $4,000 to $5,000 per home, they got radon reductions ranging from 0 to 50%. For this reason, the EPA does not consider sealing to be a primary mitigation technique. Our experience has been similar. We have sealed 1,500 square foot crawl spaces and seen no change in the radon level. In 50% of the homes we have sealed, the radon level went up slightly, in 50% it went down slightly. A small number of homes will have a significant decrease or increase in the radon level after sealing. To maximize the performance of an SSD system, sump holes and large cracks near the suction point will generally be sealed.
Air exchangers – A small minority of homes have extensive additions, unusual conditions under the floor or large inaccessible crawl spaces. These homes can be hard to fix with an SSD system, and are sometimes fixed using an air exchanger (also known as an air to air heat exchanger or heat recovery ventilator). A typical air exchanger has two fans, one blowing air into the house and one blowing air out of the house. 60% to 80% of the heat in the exhaust air is recovered and returned to the house. The air exchanger doesn’t prevent radon from entering; it simply dilutes it with outside air. A typical home may change air naturally every two hours and have a radon level of 6 pCi/L. To cut the radon level in half, a contractor will install an air exchanger large enough to double the rate at which the house changes air. So now the house will change its entire volume of air every hour. This will add about 20% to the home’s heating and cooling cost. Often, the air conditioner can’t keep up with the extra humidity being brought in, so the air exchanger is shut off during the air conditioning season. An air exchanger will cost about twice as much as most SSD systems, and can actually make a radon problem worse if it’s not cleaned and professionally serviced regularly. An EPA study of radon systems found that 88% of air exchangers were not effectively reducing radon levels. For these reasons, air exchangers are generally seen as a “last resort” for radon mitigation.
Costs – Typical mitigation costs:
SSD: $1,300 to 1,600, including necessary sealing
SSD with crawl space: Add $200 to $400
Air exchanger: $1,700 to $2,200