Radon Mitigation Design for a Slab-on-Grade Home With a Sump
A slab-on-grade home tested at 5.8 pCi/L on a short-term test in a basement bedroom that is used as a guest room. The home has a sump pit in the mechanical room with a loose-fitting lid, a floor drain, and visible shrinkage cracks in the slab. There is no existing radon system. The client wants the cheapest fix and proposes sealing the cracks and putting a tight lid on the sump. The house is on a well and has a pressure tank in the same room. The client also asks whether they can just keep the guest room door closed. A second short-term test in the living room read 3.9 pCi/L.
1. Evaluate the client's proposed approach and explain what the two test results mean.
Read the worked answer
Evaluating the proposals. Sealing cracks and installing a tight sump lid are useful components but are not a mitigation strategy on their own. Research and EPA guidance treat sealing alone as ineffective at reliably reducing radon, because soil gas will find other entry points; sealing is a supplementary measure that improves the performance of a sub-slab depressurization system. A tight sump lid is valuable because an open sump is a large direct entry point, but it must be part of a system that creates a pressure field under the slab.
Keeping the door closed does not reduce radon entry. Radon enters from the soil beneath the slab and accumulates wherever air exchange is lowest. A closed bedroom door reduces exchange with the rest of the house and can raise concentrations in that room. It may also increase the pressure difference between the room and the soil, drawing more soil gas in.
What the two results mean. The basement bedroom at 5.8 pCi/L and the living room at 3.9 pCi/L are both short-term tests. Neither is a long-term average. The difference between floors is expected because radon is denser than air and soil gas enters at the lowest level, but the living room result shows the whole house is affected. The action level is 4 pCi/L, and the basement result exceeds it. A confirmatory long-term test is advisable, but a result this far above the action level with an obvious entry point (open sump) warrants proceeding with mitigation rather than waiting.
Test quality. Confirm the devices were placed per protocol: closed-house conditions, lowest livable level, away from drafts and exterior walls, and for the required duration. If the short-term test was done under abnormal conditions, a long-term test clarifies.
2. Design the mitigation system and specify the commissioning and verification steps.
Read the worked answer
System design. Active sub-slab depressurization (ASD) is the appropriate technology for a slab-on-grade home with a sump.
Suction point. Use the sump pit as the primary suction point if it is properly constructed, with a sealed, gasketed lid and a dedicated pipe penetration. Add a second suction point under the slab if communication testing shows poor pressure field extension, particularly toward the bedroom.
Piping. Route schedule 40 PVC from the suction point up through the conditioned space and out the roof, sized to the fan. Slope horizontal runs back toward the suction point to drain condensate. Insulate the portion in unconditioned space. Do not terminate at grade or in the attic.
Fan. Select a radon fan rated for continuous duty and sized to the pressure field; install it outside the conditioned envelope or in an accessible location with a disconnect.
Sealing. Seal slab cracks, the floor drain with a trap primer or a proper trap seal, and all penetrations. Install the tight sump lid. These improve pressure field extension and system efficiency.
Monitoring. Install a U-tube manometer visible to the occupant and a system label with the installer's contact information. Add an audible or visual alarm for fan failure if the client wants assurance.
Commissioning. Measure pressure field extension at multiple points under the slab to confirm the sub-slab is being depressurized. Verify the manometer reading is within the design range and that no backdrafting of combustion appliances occurs; the water heater and any vented appliance in the mechanical room must be tested for spillage after the fan is running, since the fan can depressurize the space.
Verification. Post-mitigation radon test after at least 24 hours of operation, then a long-term confirmatory test. Target below 2 pCi/L as a design goal, and certainly below 4 pCi/L. Retest every two years and after any major renovation. Advise the client to retest the guest room specifically, since that is where the highest reading occurred.