How Radon Mitigation Works: A Complete Guide

The Core Principle: Reversing the Pressure

To understand radon mitigation, you need to understand why radon enters your home in the first place. Your house acts like a gentle vacuum. Warm air rises and escapes through the upper floors and roof, creating a slight negative pressure at the lower levels. This pressure difference — even just a few Pascals — is enough to draw soil gases, including radon, up through every tiny crack, gap, and opening in your foundation.

Radon mitigation reverses this pressure. By creating a stronger negative pressure beneath your foundation than inside your home, the system intercepts radon before it enters your living space and vents it safely outdoors, where it disperses harmlessly into the atmosphere.

This principle — called depressurization — is the basis of over 90% of residential radon mitigation systems. The specifics vary by foundation type, but the core concept is the same: create suction beneath the foundation, capture the radon, and vent it above the roofline.

Sub-Slab Depressurization (SSD) — The Gold Standard

Sub-Slab Depressurization is the most common and effective radon mitigation method, used in approximately 90% of residential installations. It's the EPA's recommended approach for homes with basement or slab-on-grade foundations.

How it works, step by step:

1. Suction point creation: A 4-5 inch diameter hole is cored through the concrete slab, typically in a utility area, closet, or along an exterior wall. A small amount of aggregate or soil beneath the slab is excavated to create a pit about the size of a basketball — this becomes the collection point for sub-slab gases.
2. Piping installation: A 3" or 4" diameter PVC pipe is inserted into the suction point and routed either through the interior of the home (up through closets, walls, or utility chases) or along the exterior of the house, extending to above the roofline.
3. Fan installation: An inline radon fan is installed in the piping — typically in the attic or on the exterior wall above the first floor. This fan runs continuously, creating consistent negative pressure beneath the slab.
4. Sealing: Visible cracks, gaps, construction joints, and pipe penetrations in the slab are sealed with specialized caulk to maximize the system's effectiveness and prevent radon bypass.
5. Monitoring: A U-tube manometer (a simple liquid gauge) is installed on the piping, providing a visual indicator that the system is creating suction. When the liquid levels are unequal, the system is working.

Sump Pit Depressurization

If your home has an existing sump pump pit, it can often serve as a ready-made suction point for radon mitigation — potentially saving the cost of coring a new hole in your slab.

The sump pit is sealed with a custom-fit airtight cover (with penetrations for the sump pump discharge pipe, power cord, and radon vent pipe). The pit's connection to the home's perimeter drain tile system means it often has excellent communication with the sub-slab area, making it an effective collection point for radon.

Your sump pump continues to function normally beneath the sealed cover. This method is typically the most cost-effective option at $800-$1,800, with installation taking just 2-4 hours.

Crawl Space: Sub-Membrane Depressurization

Homes with dirt-floor crawl spaces require a different approach since there's no concrete slab to work with. Sub-Membrane Depressurization (SMD) creates an artificial barrier and then depressurizes beneath it.

A heavy-duty polyethylene vapor barrier (typically 6-20 mil) is laid across the entire crawl space floor and sealed to the foundation walls, piers, and any penetrations. A suction point with PVC piping is placed beneath the membrane, connected to a fan that creates negative pressure under the barrier — pulling radon out before it can rise through the membrane into the home.

This system has a double benefit: it mitigates radon AND acts as a moisture barrier for the crawl space. Many homeowners notice reduced humidity, less musty odors, and improved air quality throughout the home. Cost: $1,200-$3,000.

Water Radon Treatment

Homes with private wells may have radon dissolved in their water supply. When this water is used for showering, cooking, or laundry, radon is released into the air. Two main treatment options exist:

Passive vs Active Systems

Passive systems rely on natural air pressure differentials and thermal convection to move radon — no fan is used. These are most common in new construction, where radon-resistant features (aggregate, vapor barrier, vent pipe) are built in during construction. Passive systems are less effective, typically achieving only 30-50% radon reduction.

Active systems add a fan to the piping, which dramatically increases effectiveness to 90-99% radon reduction. If you have a passive system and your radon levels are still above 4.0 pCi/L, "activating" it by adding a fan is a simple, affordable upgrade ($300-$800).

What to Expect During Installation

A typical radon mitigation installation follows this timeline:

Maintenance & Long-Term Monitoring

Radon mitigation systems are designed to be low-maintenance, but they do require some attention:

• Check your manometer regularly — A quick glance at the U-tube manometer confirms the fan is creating suction. If the liquid levels are equal, the fan may need attention.
• Listen for the fan — A properly working fan produces a gentle hum. Changes in sound (grinding, rattling, silence) indicate a potential issue.
• Retest every 2 years — The EPA recommends retesting your home's radon levels every 2 years to ensure the system remains effective.
• Fan replacement — Radon fans typically last 5-10+ years. Replacement costs $150-$300 for the fan plus labor.
• Annual electricity cost — A radon fan uses about $50-$100/year in electricity — comparable to leaving a lightbulb on.