Radon Fan Sizing Helper

A labeled planning helper: get a suggested radon fan suction class (low, standard or high) from your foundation type and how well air moves under the slab.

Radon is a serious health risk (EPA action level 4 pCi/L). Test first, and use an NRPP/NRSB-certified mitigator. This is a cost estimate, not a diagnostic or health opinion.
Planning typicals: equipment counts are planning typicals from IICRC/AHAM rules of thumb. Actual counts depend on the class of water, materials and airflow on site — a certified technician must confirm the drying/containment plan.

Calculator

How freely air moves under the slab — a certified diagnostic measures this.
Recommended fan classStandard-suction fan
FoundationFull basement
Sub-slab communicationModerate

Standard-suction fan. A mid-range fan covers most homes with average sub-slab conditions. A crawlspace usually uses a sub-membrane system over a sealed vapor barrier rather than a slab suction point. This is a labeled planning helper — a certified mitigator sizes the fan from a real diagnostic (a post-mitigation test must confirm the level is below 4 pCi/L).

The fan is the heart of an active sub-slab depressurization system, and picking one is a balance: it must pull enough air to hold the space under the slab at negative pressure, but an over-powered fan wastes energy and pulls conditioned air out of the house. The right choice depends on sub-slab communication — how freely air moves through the soil or gravel beneath the floor — and on the foundation type. This helper gives a labeled starting point (a low-, standard- or high-suction class); a certified mitigator sizes the actual fan from a real diagnostic.

Formula

This is a labeled decision helper, not an arithmetic formula. It follows the standard rule of thumb:

  • Poor communication (tight or clay soil) → a high-suction fan, and often extra suction points, to overcome the resistance.
  • Good communication under a slab-on-grade → a quiet, efficient low-suction fan is usually enough.
  • Everything in between → a mid-range standard-suction fan.

A crawlspace normally uses a sealed sub-membrane over the soil rather than a slab suction point, which changes the airflow picture entirely.

Worked example

Take a full basement with moderate sub-slab communication (mixed fill). The helper suggests a standard-suction fan — the common mid-range choice for an average home. Change the soil to poor (clay), and it steps up to a high-suction fan because tight soil resists airflow and the fan has to work harder to hold the slab under negative pressure. A slab-on-grade with good gravel underneath drops to a low-suction fan, the quietest and most energy-efficient option.

These are planning categories, not a model number. The mitigator measures the actual sub-slab pressure field and picks a fan curve to match — then proves it with a post-install test under 4 pCi/L.

Reading the suction class & confirming on site

“Suction class” is shorthand for where a fan sits on its pressure-vs-airflow curve. A low-suction fan moves a good volume of air at low static pressure — ideal when the soil or gravel lets air flow freely. A high-suction fan sustains a strong vacuum through resistant, low-permeability soil, at the cost of more noise and energy. Choosing wrongly is a real problem: an under-powered fan leaves radon above the action level, while an over-powered fan on porous soil short-cycles, drones and pulls heated or cooled air out of the home.

Because the answer hinges on a physical measurement, treat this helper as a labeled planning guide only. A certified NRPP/NRSB mitigator performs a diagnostic — drilling a test hole, measuring the pressure field extension and confirming how far a single point can depressurize — before committing to a fan and a number of suction points. The cost side of that decision lives in the radon mitigation cost calculator, and the labeled bands by foundation are in the radon, asbestos & lead cost table. Whatever fan is fitted, the system is only proven by a follow-up test showing the indoor level is below the EPA action level of 4 pCi/L.

Frequently asked questions

What is sub-slab communication?
It is how freely air travels through the material under your floor slab. Loose gravel gives “good” communication — a single suction point can depressurize a wide area. Tight clay or fine fill gives “poor” communication, so the fan needs more suction and the system may need extra draw points. A mitigator measures it with a diagnostic test.
Can I pick a fan from this tool alone?
No. This is a labeled planning helper that points you at a suction class so you can talk sensibly with a contractor. The actual fan is sized from an on-site pressure-field diagnostic by a certified mitigator, and confirmed by a post-mitigation radon test below 4 pCi/L.
Why does a crawlspace behave differently?
A crawlspace usually has exposed soil rather than a poured slab, so the standard fix is a sealed sub-membrane (heavy poly over the soil) with a suction point drawing from beneath it. That is a different airflow problem than depressurizing a slab, which is why crawlspaces are called out separately.
Does a bigger fan always lower radon more?
Not necessarily. On porous soil an over-sized, high-suction fan can short-cycle, get noisy and waste energy by pulling conditioned air from the house, without improving results. Matching the fan to the soil is the goal — more suction only helps when the soil resists airflow.
Where does the fan get installed?
For safety and to keep any radon-laden air out of living space, the fan is mounted outside the conditioned envelope — in an attic, garage or on an exterior wall — with the discharge routed above the roofline and away from windows. Placement is part of the mitigator’s design and affects noise, efficiency and how the fan class performs; treat this helper as a planning starting point only.

Related calculators

Radon Mitigation CostEstimate radon mitigation cost: base sub-slab system by foundation plus extra suction points and a contingency. EPA 4 pCi/L action level; enter your own quote.