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Evgenii Konkin
Evgenii Konkin

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Radon Mitigation Sizing: Airflow Is Not a Radon Level Prediction

#hvac #engineering #ventilation #buildings

Radon mitigation sizing is easy to misunderstand.

A designer, contractor, or building owner may look at a basement area and ask:

“How much fan airflow do I need to remove radon?”

But that question can be misleading.

A radon mitigation airflow estimate does not directly predict the final indoor radon concentration. It is a preliminary sizing step for the mitigation system — usually active soil depressurization or sub-slab depressurization.

The better question is:

“How much airflow may be needed to create a practical depressurization strategy under this slab condition?”

That difference matters.

A radon mitigation system is not just a ventilation fan. It is a soil-gas control system. The required airflow depends on the treatment area, slab leakage, sub-slab communication, and design uncertainty.

The basic sizing model

The calculator uses a fixed preliminary area-based sizing model.

For Imperial units:

CFM_required = A × L × C × S
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For Metric units:

Q_required = A × L × C × S
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Where:

A = treatment area, ft² or m²
L = base leakage airflow intensity
C = sub-slab communication multiplier
S = safety factor multiplier
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The safety factor multiplier is:

S = 1 + (SF / 100)
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Where:

SF = design safety factor, %
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The formula is simple, but the inputs are not trivial.

A larger treated area increases airflow.

A leakier foundation increases airflow.

Poorer sub-slab communication increases airflow.

A larger safety factor increases airflow.

That is why two buildings with the same floor area may need very different mitigation approaches.

Leakage class changes the result directly

The calculator uses different base leakage airflow intensities depending on the foundation condition.

For Imperial units:

Tight slab:   0.05 CFM/ft²
Typical slab: 0.10 CFM/ft²
Leaky slab:   0.20 CFM/ft²
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A leaky slab is not a small adjustment.

It can double the base airflow compared with a typical slab, and quadruple it compared with a tight slab.

That is why visible cracks, joints, penetrations, open sumps, and poor sealing should not be ignored during early sizing.

If the foundation is treated as “tight” when it is actually leaky, the airflow estimate may look artificially low.

Sub-slab communication matters

The calculator also applies a communication multiplier:

Good communication: 1.00
Fair communication: 1.25
Poor communication: 1.50
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This represents how easily suction is expected to extend beneath the slab.

Good communication means one suction point may influence a larger area more effectively.

Poor communication means the system may need more effort, more airflow, more suction points, or a different layout strategy.

This is one of the most important practical points in radon mitigation:

Airflow alone does not guarantee pressure field extension.
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A fan can move air, but if the suction field does not reach the right areas beneath the slab, mitigation performance may still be poor.

Example: typical basement with fair communication

Suppose a preliminary radon mitigation estimate uses these inputs:

Treatment Area = 1,200 ft²
Foundation Leakage Class = Typical
Sub-Slab Communication = Fair
Design Safety Factor = 15%
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Step 1: Select the leakage airflow intensity.

For a typical slab:

L = 0.10 CFM/ft²
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Step 2: Select the communication multiplier.

For fair sub-slab communication:

C = 1.25
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Step 3: Calculate the safety factor multiplier.

S = 1 + (15 / 100)
S = 1.15
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Step 4: Apply the sizing formula.

CFM_required = A × L × C × S
CFM_required = 1,200 × 0.10 × 1.25 × 1.15
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Calculate step by step:

1,200 × 0.10 = 120 CFM
120 × 1.25 = 150 CFM
150 × 1.15 = 172.5 CFM
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So the preliminary result is:

Required Mitigation Airflow ≈ 173 CFM
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This would fall into a high airflow range for a preliminary residential-style mitigation screening.

That does not automatically mean the system is impossible. It means the project deserves closer review: fan curve, pipe routing, suction point strategy, slab communication, and actual field diagnostics all become important.

What happens if the slab is leaky?

Now keep the same area, communication condition, and safety factor, but change the leakage class from Typical to Leaky.

Inputs:

Treatment Area = 1,200 ft²
Foundation Leakage Class = Leaky
Sub-Slab Communication = Fair
Design Safety Factor = 15%
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For a leaky slab:

L = 0.20 CFM/ft²
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Apply the formula:

CFM_required = 1,200 × 0.20 × 1.25 × 1.15
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Calculate:

1,200 × 0.20 = 240 CFM
240 × 1.25 = 300 CFM
300 × 1.15 = 345 CFM
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So the estimated airflow becomes:

Required Mitigation Airflow ≈ 345 CFM
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The airflow doubled because the leakage intensity doubled.

Nothing changed about the basement area.

Nothing changed about the safety factor.

Only the assumed foundation leakage changed.

That is the practical lesson:

Radon mitigation airflow is highly sensitive to slab leakage assumptions.
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What happens if communication is poor?

Now return to a typical slab, but change communication from Fair to Poor.

Inputs:

Treatment Area = 1,200 ft²
Foundation Leakage Class = Typical
Sub-Slab Communication = Poor
Design Safety Factor = 15%
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For poor communication:

C = 1.50
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Apply the formula:

CFM_required = 1,200 × 0.10 × 1.50 × 1.15
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Calculate:

1,200 × 0.10 = 120 CFM
120 × 1.50 = 180 CFM
180 × 1.15 = 207 CFM
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So the estimated airflow becomes:

Required Mitigation Airflow ≈ 207 CFM
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The result is higher than the fair-communication case.

But the bigger warning is not only the airflow number. Poor communication may also mean that one suction point cannot influence the whole treated area. The solution may require additional suction points, better sealing, different pipe layout, or more detailed diagnostic testing.

Common engineering mistake: treating airflow as the final radon answer

The most important mistake is interpreting mitigation airflow as a direct prediction of indoor radon level.

For example:

“The calculator says 173 CFM, so the building will be safe.”
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That is not what the calculation means.

The formula estimates preliminary mitigation airflow demand.

It does not calculate:

Final indoor radon concentration
Pressure field extension
Sub-slab vacuum distribution
Fan curve performance
Pipe friction losses
Fitting losses
Actual suction point effectiveness
Post-installation test result
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Radon mitigation must be verified after installation.

A sizing estimate is only the starting point.

Another mistake: using total building area instead of treatment area

The input should be the foundation zone that the mitigation system is designed to treat.

That may not always equal the total building area.

For example, a building may have:

Basement area
Slab-on-grade area
Crawlspace area
Garage slab
Separate foundation zones
Additions with different slab conditions
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If the wrong area is entered, the airflow result can be misleading.

Overstating the area may oversize the preliminary system.

Understating the area may hide a real mitigation challenge.

Another mistake: assuming one suction point is always enough

A single suction point can work well in some conditions.

But it is not guaranteed.

Weak sub-slab communication, large treatment areas, compartmentalized slabs, footings, grade beams, or different fill materials can limit how far suction extends.

A calculated airflow number may look acceptable while the pressure field still fails to reach part of the slab.

That is why diagnostic communication testing and post-installation verification matter.

Practical design responses

If the calculated airflow is high or very high, the next step is not only “choose a bigger fan.”

Better engineering questions include:

Can slab leakage be reduced by sealing cracks and penetrations?
Is the treatment area defined correctly?
Is sub-slab communication actually poor, or just unknown?
Would multiple suction points reduce the burden on one point?
Are pipe losses and fitting losses included in fan selection?
Does the fan curve support the required airflow at the needed pressure?
Will the system be tested after installation?
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A larger fan may help in some cases.

But if the real issue is poor communication or excessive leakage, system layout and sealing may matter more than fan airflow alone.

Practical engineering takeaway

Radon mitigation sizing starts with a simple formula:

Required Airflow = Area × Leakage Intensity × Communication Multiplier × Safety Factor
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But the result should be interpreted carefully.

Before accepting the number, ask:

1. Is the treatment area correct?
2. Is the foundation leakage class realistic?
3. Is sub-slab communication known or only assumed?
4. Is the safety factor appropriate for the uncertainty?
5. Will fan pressure capability and pipe losses be checked?
6. Could multiple suction points be needed?
7. Will post-mitigation radon testing verify the result?
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The airflow estimate is useful because it gives the project a starting point.

But it is not the same thing as a final mitigation design or a radon concentration guarantee.

For a quick first-pass estimate, you can use the Radon Mitigation System Sizing Calculator

It estimates required mitigation airflow from treatment area, foundation leakage class, sub-slab communication condition, and design safety factor for preliminary ASD or sub-slab depressurization review.

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