The Slab-on-Grade Dilemma
A home has a slab-on-grade foundation with a visible crack along the perimeter. The radon concentration in the living room is 5.8 pCi/L, measured over 48 hours with a continuous monitor. The homeowner reports that the radon level is highest in the morning and lowest in the afternoon. The house has a forced-air heating system with returns in each room. The outdoor temperature is 30°F, and the indoor temperature is 70°F. The soil is sandy with high permeability. The professional must determine the likely entry routes and recommend mitigation strategies.
1. Based on the given information, what are the most probable radon entry routes and driving forces? Explain how the diurnal pattern and HVAC system influence radon levels.
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The most probable entry route is the perimeter crack, as it provides a direct pathway from soil to indoor air. Other potential routes include gaps around plumbing penetrations and sump pits, but the crack is the primary suspect. The driving force is likely stack effect (winter condition: indoor 70°F, outdoor 30°F, so indoor air is warmer and buoyant, creating negative pressure in the lower level) combined with depressurization from the forced-air system. The HVAC system with returns in each room can create negative pressure in the living space if the return ductwork is leaky or if supply registers are closed, increasing soil gas infiltration. The diurnal pattern (higher in morning, lower in afternoon) is consistent with stack effect being strongest when outdoor temperature is lowest (morning) and weakest when outdoor temperature rises (afternoon), and also with variable HVAC operation. The high-permeability sandy soil allows easy soil gas movement, so the crack is an effective entry point. To confirm, the professional could perform a sub-slab communication test or use a smoke pencil to detect airflow at the crack.
2. What mitigation strategy would you recommend and why? Include specific design considerations for the sandy soil and the existing HVAC system.
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Recommend active soil depressurization (ASD) with a sub-slab depressurization system. Because the soil is sandy and highly permeable, a single suction point may suffice, but a communication test should be done to determine the radius of influence. If the slab is large, multiple suction points may be needed. The system should include a sealed sump lid if a sump exists, and sealing of the perimeter crack and other openings. The ASD fan should be sized to maintain a negative pressure under the slab of at least 5 Pa (measured with a micro-manometer). For the HVAC system, ensure that return air is balanced and that ductwork is sealed to prevent depressurization. Consider adding a fresh air intake or balancing the HVAC to maintain slight positive pressure in the living space. Because the soil is sandy, the system should be designed to handle high airflow with low resistance; a radon fan with a performance curve suitable for high flow at low pressure may be appropriate. Post-mitigation testing should be conducted after 24 hours of operation and again after 30 days to confirm reduction below 4 pCi/L (or 2 pCi/L if using EPA recommendations).