How Does an Under-Slab Radon Mitigation System Work?

On this page (9 sections)

1. How does an under-slab radon mitigation system create suction?
2. What are the main parts of a below-slab radon system?
3. Why does a slab foundation need a different approach?
4. How do you test before and after installation?
5. What should you check around the system?
6. Can an under-slab radon system affect other safety devices?
7. What if one suction point does not lower radon enough?
8. Is sealing enough without an under-slab fan?
9. How should you choose an under-slab radon mitigation system?

An under-slab radon mitigation system draws radon from the soil beneath a concrete slab and vents it outdoors through a sealed pipe. A fan creates the pressure difference that keeps soil gas from entering the home.

This approach is often used in homes with slab foundations, where there is no basement or crawl space to use as the main collection area. The exact design depends on the slab, soil, joints, utility openings, and available routes for the vent pipe.

For background, see how does radon mitigation system work before comparing the parts of an under-slab setup.

How does an under-slab radon mitigation system create suction?

The system creates lower pressure beneath the slab than inside the home. Soil gas then moves toward a collection point instead of escaping through cracks, joints, drains, or openings into the living space.

A typical system has a suction point, a vertical vent pipe, an in-line fan, and sealed connections. The suction point may be made through the slab, or it may use an existing opening or access point when the foundation layout allows it.

The fan pulls soil gas into the pipe. The pipe carries that gas to an outdoor discharge location, where it can disperse away from doors, windows, and other openings.

What are the main parts of a below-slab radon system?

A below-slab radon system needs a collection point, an airflow path, and a fan that maintains suction. Sealing visible openings supports the system, but sealing alone does not replace active soil-gas removal.

Where is the suction point installed?

The suction point is usually placed through the concrete slab into the material below it. A professional may select a location near a foundation wall, in a utility area, or where the sub-slab material allows air to move toward the pipe.

The best location is not always the center of the room. The goal is to influence the largest practical area beneath the slab while avoiding utilities, drainage components, and structural problems.

Some homes need more than one suction point. That decision depends on the slab layout, soil permeability, measured radon levels, and how well one point affects the rest of the foundation.

How does the pipe move radon outdoors?

The vent pipe connects the suction point to the fan and then continues to an outdoor discharge point. Joints should be sealed so the pipe carries soil gas instead of leaking it into the building.

The pipe route may run through an interior space before passing outdoors. It should be planned so it does not block access, conflict with utilities, or discharge near openings where the gas could return indoors.

For design choices, compare how to choose a radon mitigation system design. The pipe route, fan location, discharge point, and suction coverage all affect the finished system.

What does the fan do?

The fan keeps air moving from beneath the slab to the outdoors. It is usually installed in a location where the system can draw soil gas without placing the fan inside an occupied area.

A fan that stops working can remove the system's active pressure control. Check any warning indicator regularly and pay attention to changes in sound, airflow, or visible system condition.

Do not assume a quiet system is working or a loud system is failing. Use the installed indicator and follow-up radon testing to judge performance.

Why does a slab foundation need a different approach?

A slab foundation places the occupied space directly above the soil and concrete assembly. Since there is no open basement or crawl space to ventilate, the mitigation system usually collects soil gas through the slab itself.

Radon can move through cracks, construction joints, gaps around pipes, floor drains, sump openings, and other paths. The concrete slab may slow that movement, but it does not make the home airtight.

A professional should inspect the slab and foundation before drilling or routing a pipe. Drilling without checking for utilities or drainage can create an avoidable safety and repair problem.

Read how to choose a radon mitigation system for a house on a slab for the foundation-specific choices that affect installation.

How do you test before and after installation?

Test the home before mitigation to establish a baseline, then test again after the system has operated. Testing helps show whether the system lowered the indoor radon level enough or whether the design needs adjustment.

The First Alert Radon Gas Test Kit is suited to an initial screening because it includes test materials and lab fees, excluding New Jersey state fees. It is listed under the EPA radon gas measurement proficiency program, and results are emailed within 72 hours of lab receipt.

Follow the kit instructions for placement, test duration, packaging, and return steps. The kit is recommended for use only within the United States, so check local requirements if you live elsewhere.

The EPA radon guide explains why testing matters and what steps homeowners can consider when results are raised.

What should you check around the system?

Check the fan indicator, pipe connections, discharge route, and visible slab openings on a regular schedule. A system can lose effectiveness if the fan stops, a connection separates, or a new opening creates an easier path for soil gas.

Keep the area around the fan and pipe accessible. Do not cover the system with stored items or make changes to the pipe route without understanding how the alteration affects suction.

Water management also matters in slab homes. A drain, sump, or low point can be close to a suction area, so early warning for unwanted water can protect the surrounding space.

The Basement Watchdog 110 dB Water Alarm has a detachable sensor with a 6 ft wire. Its sensor detects as little as 1/32 in. of water, and the alarm can be heard throughout the house.

The sensor can be mounted on a wall or extended into a sump pit. The alarm accommodates more than 100 ft of additional 24-Gauge wire, but that extra wire is not included.

This alarm does not measure radon or confirm fan performance. It is a separate water warning tool for a location where moisture could create another home-maintenance concern.

Can an under-slab radon system affect other safety devices?

An under-slab radon system does not replace smoke or carbon monoxide alarms. Radon is a radioactive soil gas, while carbon monoxide comes from incomplete combustion, so each hazard needs its own detection method.

Keep carbon monoxide alarms on each level and in each bedroom as appropriate for your home's layout. Follow the alarm instructions for placement, testing, battery backup, and replacement.

The First Alert Plug in Carbon Monoxide Alarm with Digital Display uses an electrochemical carbon monoxide sensor, an 85 decibel alarm, and a battery backup. Its digital display shows the highest measured carbon monoxide concentration, while the plug-in power provides normal operation.

The CO615 works with a standard outlet and includes 2 AA batteries for backup. The battery backup is not intended to power the alarm for an extended period without AC power.

A carbon monoxide alarm cannot tell you whether the radon fan is running. Use the radon system's indicator and repeat radon testing for that purpose.

What if one suction point does not lower radon enough?

A single suction point may not control every slab foundation. If follow-up testing remains high, the system may need a different fan setting, improved sealing, another suction point, or a revised pipe route.

The result can depend on soil conditions beneath different parts of the slab. Thick footings, separated slab areas, additions, and low-permeability materials may limit how far suction spreads.

Do not block a suction point or seal a crack without considering the whole design. Some openings may need sealing, while others may be part of the drainage or pressure strategy.

See which type of radon mitigation system should you choose when comparing under-slab suction with other approaches.

Is sealing enough without an under-slab fan?

Sealing cracks alone is usually not a complete radon strategy for a slab home. It can reduce some entry paths, but soil gas may still enter through hidden gaps, joints, porous materials, and openings that are difficult to seal permanently.

Active sub-slab suction addresses the pressure source by drawing soil gas away from the building. Sealing can support that work when it is suitable for the slab and foundation.

A test result should guide the decision. If testing shows raised radon, discuss the foundation conditions and mitigation options with a qualified radon professional.

How should you choose an under-slab radon mitigation system?

Choose the system design after testing the home and assessing how air can move beneath the slab. The important decisions include suction-point placement, fan capacity, pipe routing, discharge location, sealing, and post-installation testing.

Pick an under-slab approach when the slab and sub-slab area can support effective suction. Plan for monitoring after installation, because the final test is what shows whether the indoor radon level changed.

Use the First Alert kit for a practical screening and follow-up check. Add the Basement Watchdog alarm where water could threaten a low or hard-to-reach area, and use the First Alert carbon monoxide alarm for a separate combustion hazard.

An under-slab radon mitigation system works by controlling soil gas before it enters the home. Test first, match the suction design to the foundation, confirm the fan operates, and test again after installation.

Frequently asked questions

Can an under-slab radon mitigation system work under every concrete slab?

No. The system needs a suitable route for drawing soil gas from beneath the slab, such as a sub-slab layer or accessible suction point.

A professional should assess the slab, foundation layout, soil, openings, and drainage before choosing the suction method.

Where does an under-slab radon mitigation pipe end?

The pipe usually carries soil gas from beneath the slab to an outdoor discharge point above or away from the occupied areas. The final location and height should follow local building rules and the system design so radon does not reenter the home.

Does an under-slab radon system remove radon from indoor air?

Its main job is to stop soil gas before it enters the home. By lowering pressure beneath the slab and venting that gas outdoors, the system can reduce the source of indoor radon.

Test after installation to confirm the result.

How do you know if an under-slab radon system is working?

Check the system indicator if one is installed, listen for unusual fan changes, and test indoor radon after the system operates. A follow-up test shows whether the home's radon level has fallen and whether more work may be needed.