Home Radon Mitigation Runtime Planner

Why radon mitigation timelines matter

Radon is an odorless, radioactive gas generated by the decay of uranium in soil and rock. When it accumulates indoors, especially in basements and crawlspaces, long-term exposure increases the risk of lung cancer. Many homeowners test for radon with a two-day charcoal canister or a digital monitor, receive a result above the U.S. Environmental Protection Agency action level of 4 pCi/L, and then install a mitigation system. The process often stops there: once the fan is running, people assume the problem is solved. Yet radon behaves like any other contaminant subject to ventilation. The concentration declines exponentially over time and approaches a steady-state level determined by the balance between the removal rate (air changes per hour) and the entry rate from the soil. Knowing how long the system must run before retesting and what residual level to expect is essential for compliance, peace of mind, and scheduling follow-up measurements.

This planner treats a basement or slab-on-grade space as a single well-mixed zone. While real buildings have dead zones and pressure gradients, the one-zone model is widely used for quick assessments because it matches how short-term radon tests integrate air samples. By entering the conditioned volume, the initial measurement, the target level, and mitigation parameters, you receive an estimate of both the time to reach that target and the ultimate equilibrium. The timeline can guide decisions about when to perform a post-mitigation test, when occupants can return to the space, and how aggressively to ventilate during the first few days. It can also highlight if the selected fan is oversized or undersized for the space volume.

Professional mitigators often size fans by rules of thumb and field experience. However, homeowners adding a radon fan to an existing passive system or builders designing a new home sometimes lack that intuition. The exponential decay model used here clarifies how air changes per hour relate to radon removal. With every air change, a fixed fraction of radon is removed, so the time constant—the time required for the concentration to drop to about 37% of its starting value—depends on the combined air change rate from the fan plus natural infiltration. If the fan provides 1.1 ACH and the natural leakage adds 0.3 ACH, the total removal rate is 1.4 ACH, implying a time constant of roughly 0.71 hours. In three time constants (just over two hours), the concentration falls to about 5% of its starting value. That dynamic explains why radon readings drop quickly after mitigation but can climb back to a steady level determined by the rate of radon entering the space.

Equations driving the mitigation forecast

The planner relies on the classic well-mixed room equation for a contaminant with a constant source. Let $C\left(t\right)$ represent radon concentration at time $t$, $C$₀ the initial concentration, $C$_s the source term expressed as an equivalent concentration increase per hour, and $\mathrm{\backslash lambda}$ the total removal rate in air changes per hour. The differential equation is

The solution combines an exponential decay and the steady-state concentration $C$_\infty:

Here $C$_\infty equals the source term divided by the removal rate: $C$_\infty=C_s\lambda. The fan airflow rate expressed in cubic feet per minute converts to air changes per hour by multiplying by 60 and dividing by the zone volume. The infiltration rate, usually estimated between 0.2 and 0.6 ACH for basements, adds directly to the mechanical rate. To find the time required to reach a target concentration $C$_target, the calculator rearranges the solution:

If the target concentration is below the steady-state value, the logarithm becomes undefined—meaning the system cannot achieve that target with the current airflow and source rate. The planner detects that situation and alerts you to increase the fan size, seal entry pathways, or add active sub-slab depressurization.

Worked example: sealing a basement and adding a 220 CFM fan

Consider a 1,500-square-foot basement with an eight-foot ceiling. The volume is 12,000 cubic feet. Pre-mitigation testing shows a radon level of 12 pCi/L. The homeowner wants to bring the space below 3 pCi/L. After sealing sump pits and major cracks, a 220 CFM fan is installed. The house experiences about 0.3 ACH of natural infiltration, and post-sealing smoke tests suggest the radon entry rate now contributes the equivalent of 1.2 pCi/L per hour. The calculator converts the fan airflow into 1.10 ACH (220 CFM × 60 / 12,000). Adding infiltration yields a total removal rate of 1.40 ACH.

The steady-state concentration becomes 1.2 / 1.4 = 0.86 pCi/L, well below the 3 pCi/L target. The time constant is 1 / 1.4 = 0.714 hours. Plugging into the logarithmic formula shows that reaching 3 pCi/L takes about 1.61 hours. Extending the timeline to eight hours reveals that the concentration approaches the steady-state value asymptotically. Within three hours the level drops below 1.5 pCi/L, and after twelve hours it is effectively at equilibrium.

The downloadable timeline generated by the calculator lists the concentration every 30 minutes. Homeowners can use that information to decide when it is safe to occupy the basement again, schedule a follow-up continuous radon monitor, or determine how long to leave windows closed to avoid biasing the test. If an immediate post-mitigation test is required, running the system for at least two hours ensures the target is reached with the given assumptions.

Comparison of fan configurations

The table below compares three fan sizes for the same basement and radon entry rate. Scenario A uses the 220 CFM fan, Scenario B a 150 CFM fan, and Scenario C a 300 CFM upgrade. In each case the infiltration is 0.3 ACH and the source is 1.2 pCi/L per hour.

The comparison shows diminishing returns from oversized fans. While the 300 CFM fan shaves 22 minutes off the time to reach 3 pCi/L, it also uses more electricity and can increase noise. The 150 CFM fan still meets the target but takes longer and leaves a higher steady-state level. Seeing the trade-off quantified helps homeowners balance operating cost, noise, and compliance.

Limitations and assumptions

This tool simplifies complex building physics into a single-zone model. Stratification, closed doors, sump pits, and HVAC ducting can create pockets with higher radon that take longer to clear. Continuous radon monitors may show a sawtooth pattern as HVAC cycles change pressure. The calculator assumes the radon entry rate remains constant, but seasonal soil moisture and stack effect can cause large variations. It also assumes the fan flow remains steady; in reality, suction fans may pull less air as filters clog or moisture accumulates in pipes. Finally, the planner does not account for occupant behavior such as opening windows, which would increase ACH but might also draw more radon into the home. Use the results as a planning baseline, pair them with long-term testing, and consult certified mitigators for site-specific adjustments.

To refine the model, consider measuring actual airflow with a pitot tube or flow hood, sealing major foundation cracks to reduce the source term, and tracking radon over several weeks with a continuous monitor. Those data can feed back into the calculator to validate the predicted steady-state. If the steady-state remains above your target despite adequate airflow, the next step is to address the radon entry rate through sub-slab suction points, drain tile depressurization, or active soil pressurization. The planner encourages this iterative approach by making the relationship between source, removal, and concentration explicit.