Written by: Kate Holder

Category: Radon Mitigation

Published: January 4, 2025

Energy Recovery Ventilation (ERV) and Radon Removal: A Technical Overview for Homes Built on Clay Slab Foundations with French Drains

Radon, a naturally occurring radioactive gas, poses a significant health risk, especially in homes with inadequate ventilation. Traditional radon mitigation methods include active soil depressurization (ASD) and sub-slab depressurization systems. However, Energy Recovery Ventilation (ERV) is an innovative solution that can be employed to reduce radon concentrations while enhancing indoor air quality and energy efficiency. This post delves into the technical aspects of how ERV systems work in radon removal, using a 1940s house with a clay slab foundation and an open French drain system as an example.

What is Energy Recovery Ventilation (ERV)?

An Energy Recovery Ventilation (ERV) system works by exchanging heat and moisture between incoming and outgoing air streams. This process helps maintain energy efficiency while ventilating a building, which is crucial for balancing indoor air quality and maintaining comfortable temperatures. ERVs are particularly beneficial in climates with extreme seasonal temperatures, where heating and cooling needs can strain energy resources. The system captures the heat or coolness of exhausted air and transfers it to incoming fresh air, minimizing the energy required for conditioning the incoming air.

Radon Mitigation Using ERV

The core physics behind radon removal is based on pressure differentials and airflow patterns. Radon gas typically enters a building through the soil beneath the foundation, migrating through cracks, gaps, and porous building materials. The level of radon in the home depends on factors like soil permeability, foundation type, and ventilation.

In homes built on a clay slab foundation, the permeability of the clay is typically low, which can increase radon accumulation as the gas has fewer escape routes. The open French drain system, a key feature in many older homes, serves to direct water away from the foundation but may inadvertently contribute to radon intrusion. French drains often allow air to enter the sub-slab area from the surrounding environment, which could bring radon into the house if pressure is not carefully controlled.

Here’s how ERVs work in this context:

1. Ventilation Effectiveness: ERVs rely on the principle of air exchange. By creating a continuous flow of fresh outdoor air into the home while expelling indoor air, they create a slightly negative pressure inside the building. This negative pressure helps prevent radon from accumulating within the home by drawing radon-laden air out of the sub-slab area.
2. Pressure Differential: The physics of radon entry into the home are governed by the pressure differential between the indoor and outdoor environments. In the case of a clay slab foundation with an open French drain, an ERV system can be used to carefully control the pressure differential by introducing fresh, filtered air into the home while exhausting stale air. This balanced airflow can help reduce the entry of radon gas into the living spaces.
3. Humidity and Temperature Control: The interaction between temperature and humidity levels in the building affects radon concentrations. A high-humidity environment can increase radon solubility in water, causing radon to cling to surfaces and thereby reducing its potential for air exposure. The ERV system regulates both temperature and humidity by transferring moisture between the incoming and outgoing air streams, which ensures that the building maintains optimal humidity for radon management while minimizing excessive moisture buildup that could exacerbate radon concentrations.
4. Heat Recovery and Radon Airflow: On a clay slab foundation, heat transfer dynamics are critical. Radon typically enters through cracks or porous areas in the foundation, which are more prevalent in older homes. The ERV system’s ability to capture heat from outgoing air and use it to temper incoming air can help maintain a stable interior environment, reducing the risk of radon influx by maintaining consistent air pressure and minimizing the influence of temperature fluctuations that might create pressure imbalances between the indoors and the outside.

Physics Behind Radon Removal with ERVs

The underlying physics of radon mitigation in this scenario can be broken down into several key principles:

• Convective Heat Transfer: Radon enters the home through convection currents driven by temperature differences between the indoor and outdoor air. When an ERV system moderates the indoor temperature by transferring heat from exhaust to incoming air, it dampens extreme temperature fluctuations, thus reducing convection currents that might pull radon into the house.
• Advection: The process of moving air through a system driven by a pressure differential is called advection. ERVs enhance advective airflow by promoting the movement of air in and out of the house. In homes with clay slab foundations and open French drains, advection can be used to mitigate radon by ensuring a constant outflow of air from the foundation area, thereby limiting the radon concentration inside the home.
• Diffusion: Radon naturally diffuses from areas of high concentration (soil) to areas of low concentration (the interior of a building). While this process is passive, it can be mitigated by controlled ventilation strategies such as those employed by ERVs. The system reduces the amount of radon-laden air that diffuses into the building by keeping the interior environment under slightly negative pressure, which discourages radon entry.

Example: Radon Mitigation in a 1940s Home with a Clay Slab Foundation and Open French Drain

Let’s consider a 1940s home with a clay slab foundation and an open French drain system. Over time, the structure’s foundation and drainage system have allowed radon gas to accumulate due to limited ventilation pathways. Despite the French drain’s purpose of diverting water, it may also unintentionally draw in radon-laden air from the surrounding soil, especially if the system is not properly sealed or ventilated.

By installing an ERV system, the home can achieve the following:

• Pressure Balancing: The ERV system creates a controlled airflow that balances the pressure inside and outside the home. This ensures that the radon gas is expelled from the house, rather than being drawn into it through the open French drain and foundation cracks.
• Moisture Control: The ERV system also ensures that humidity levels remain at optimal levels, reducing the moisture content that can interact with radon gas. This keeps radon in its gaseous state, where it can be more effectively ventilated out of the home.
• Energy Efficiency: While mitigating radon, the ERV system helps maintain energy efficiency by conditioning the incoming air without excessive reliance on heating or cooling, which is particularly beneficial for older homes with less efficient insulation and ventilation systems.

Energy Recovery Ventilation (ERV) systems offer a powerful solution for radon mitigation in homes with challenging foundation types, such as a clay slab with an open French drain. By leveraging the physics of air pressure, temperature, and moisture exchange, ERVs can create an optimal indoor environment that limits radon intrusion while improving overall energy efficiency. This makes ERV an ideal choice for homeowners looking to manage radon levels in older homes while enhancing comfort and sustainability.