Passive vs. Active Radon Control in New Construction

In new construction, a passive radon control system moves soil gas out of the building on natural stack effect alone, while an active system does the same job with an inline fan running under power. The standard sequence for a new building is to design and build the passive system first, then convert it to active only if a post-construction test shows indoor radon is not low enough. That order, passive by default and a fan added on evidence, is what ANSI/AARST CC-1000-2018 is written to make possible, and it is the cheapest path to a result that a plan reviewer and a lender will accept.

This page explains how the two modes differ, why a new building almost always starts passive, the single test that decides whether it stays that way, and what your design package has to contain so the switch is a field change instead of a redesign.

Both systems are built from the same parts: a gas-permeable layer under the slab, one or more suction points, a soil gas barrier, and a vent stack that carries collected gas above the roofline. The difference is what drives the air.

• A passive system relies on stack effect. Warm air rising inside the vent stack creates a slight upward draft that pulls soil gas from beneath the slab and discharges it above the roof. It adds no fan, no electrical load, and no noise. When soil permeability is favorable and the draft is strong, a passive stack can cut sub-slab gas entry meaningfully, but the draft rises and falls with outdoor temperature and wind, so its reduction is not guaranteed.
• An active system takes the identical pipe network and adds an inline fan that continuously depressurizes the soil under the slab. The EPA's radon-resistant new construction guidance notes that builders install complete fan-powered systems especially in areas of high radon potential, where a passive rough-in on its own is less likely to hold indoor levels down.

In short, the pipe is the same, the fan is the variable, and the fan is the reason an active system performs predictably while a passive one performs conditionally.

A new building is the one moment when the collection layer, barrier, and stack can be built into the slab at almost no incremental cost. That is why the design starts passive rather than switching a fan on from day one:

• The full system is integrated during the pour, when a sub-slab layer and continuous membrane are cheap to install and impossible to add later without demolition.
• CC-1000-2018 sets minimum requirements for soil gas control in new construction of buildings intended for human occupancy other than one- and two-family dwellings, and it requires the design to provide for a future fan even when no fan is installed at construction.
• Running a fan that the building does not need wastes energy and adds noise. Starting passive means the building only pays for active operation if a measurement proves it necessary.

The result is a system that is fully built, code-referenced, and ready, but not yet powered.

Passive versus active is not a judgment call made from the plans. It is settled by a measurement taken after the building is closed in. The EPA action level for radon is 4 pCi/L; at or above that level the building needs to be fixed.

The sequence is straightforward:

• The building is completed and the passive system is left to run on stack effect.
• Indoor radon is tested under occupied conditions, because winter stack drafts and a closed building envelope are when passive performance is weakest.
• If the result is below 4 pCi/L, the passive system stands. If it is at or above 4 pCi/L, a fan is installed to convert the stack to an active system, and the building is retested to confirm the fix.

Because the deciding test happens late, the design has to assume conversion might be required and be built for it in advance.

The whole economic case for passive-first collapses if converting to active means cutting a finished slab or rerouting a buried stack. A CC-1000 design package earns its value by making the conversion a matter of hours. That means the drawings and specifications carry, up front:

• A fan location designated on each vent stack, in an accessible run of conditioned or exterior space rather than buried behind finishes.
• An electrical rough-in beside the fan location, so wiring a fan does not require opening walls.
• A system failure indicator or monitor so an activated fan can be confirmed to be working.
• Discharge placement that already meets the clearances an active fan needs, with the exhaust point above the roofline and clear of operable windows and outdoor air intakes.

When those provisions are in the permit set, converting a passive system to active is mounting a fan, making an electrical connection, and retesting. When they are missing, the same conversion turns into demolition and a change order, which is exactly the cost the passive-first approach was meant to avoid.

For any building other than a one- or two-family dwelling, the passive-versus-active decision is not left to the contractor in the field, it is documented in the stamped design. A complete package specifies the passive layout the building is constructed with and the active-conversion provisions that let it be upgraded, both drawn to the standard a plan reviewer keys their checklist to. The CC-1000 reference page explains how that standard governs the new-construction design, and building-type detail such as unit stacking is covered on the design pages linked from it.

What to do next: if you are scoping radon control for a new building, send your architectural set and we return a fixed price for a stamped design package that is built passive and ready to activate. Pricing is flat by building type and published on the pricing page, so the number is known before any work starts. When you are ready, request a quote with your plans and you get back a delivery date for a permit-ready, standard-referenced design.