What Is Sub Slab Depressurization? Arizona Guide

Quick answer: Sub slab depressurization is the method used in nearly every radon mitigation system built today. A pipe is sealed into a hole cored through the concrete slab, a fan pulls a slight vacuum under the foundation, and radon laden soil gas is carried up and released above the roofline before it can enter the house. It works because the fan makes the pressure under your slab lower than the pressure inside your home, which reverses the direction the gas wants to travel.

How does sub slab depressurization actually work?

Under any Arizona home there is a thin layer of air trapped between the soil and the bottom of the concrete. Radon forms as uranium in the soil breaks down, seeps into that space, and then gets pulled indoors through cracks, control joints, plumbing penetrations, and the gap where the slab meets the stem wall. The house does the pulling. Air conditioning, exhaust fans, and duct leakage in a hot attic can all leave the interior at slightly lower pressure than the ground below it, so soil gas takes the path of least resistance and comes up through the floor.

Sub slab depressurization flips that pressure relationship. The fan creates a low pressure field in the material under the slab, so instead of gas moving from soil into house, it moves from soil into pipe. Nothing gets filtered, scrubbed, or neutralized. The gas is simply intercepted before it reaches you and released outdoors where it dilutes into open air almost immediately.

The important word is field. A single suction point can influence a surprisingly large area under a slab if the material down there lets air move sideways. How far that field extends is the whole design question, and it is the part that separates a system that works from one that just makes noise.

What are the parts of a sub slab depressurization system?

• A suction point, meaning a cored hole through the slab with a small pit excavated underneath
• Sealed PVC pipe, typically three or four inches in diameter, routed from that pit to the exhaust point
• A radon fan rated for continuous duty, mounted outdoors, in the garage, or in the attic, never inside living space
• A U tube or digital manometer on the pipe so you can see at a glance that the system is pulling
• Sealing of major slab cracks, expansion joints, and penetrations to keep the fan from wasting suction on indoor air
• An exhaust point above the roofline, positioned away from windows, doors, and other openings

Standards are specific about that last item. The discharge should sit at least ten feet above grade, at least a foot above the roof at the highest point of the penetration, and at least ten feet from any window, door, or air intake, or two feet vertically above one. That distance exists so the gas cannot loop back into the house.

Why is sub slab depressurization different in Arizona?

This is where a lot of generic radon advice falls apart. Most national guidance assumes a basement with a clean gravel bed under the slab. More than 95 percent of Arizona homes are slab on grade with no basement, no sump, and no French drain, so a mitigator here is working with a very different set of conditions.

• What is under the slab varies wildly. Some Valley builders placed a clean aggregate base, which lets a suction field spread easily. Others poured over native soil, sand, or compacted fines, which resists airflow and may require more than one suction point.
• Caliche. That cemented layer of calcium carbonate is common in central Arizona and it does not behave like ordinary soil. It can block a suction field in one direction and channel it in another.
• Expansive clay. Tight clay is the hardest material to pull a field through, and it is common in the East Valley and up through Prescott and the Verde Valley.
• Post tension slabs. These dominate newer Maricopa and Pinal County construction. The cables inside cannot be cut, so any coring has to be located carefully first. This is not a job for guesswork or a rented hammer drill.
• Attic temperatures. A Phoenix attic can hit well over 150 degrees in July, which is hard on a fan and on PVC. Exterior routing is common here for good reason.

There is also a seasonal wrinkle. In cold climates the stack effect drives radon indoors in winter. Here, homes are sealed up tight against 110 degree heat with the air conditioning running for months, which is its own version of a closed house condition. That is part of why a summer reading in a Phoenix home can look very different from a spring one.

How does a mitigator know where to put the suction point?

Before installing anything, a certified professional performs a sub slab communication test, sometimes called a diagnostic. A small test hole is drilled, a vacuum is applied, and pressure is measured at other points around the slab to see how far the influence reaches. If the field spreads well, one suction point may cover the home. If it dies out after a few feet, the design needs more points or a different location.

Skipping this step is one of the most common reasons a system underperforms. A fan mounted on a pipe that only communicates with a small pocket of soil will move air and lower nothing meaningful. Sizing, placement, and fan selection all follow from that diagnostic, which is why proper radon mitigation starts with measurement rather than with a drill.

Is sub slab depressurization the same as an active system?

Sub slab depressurization can be built either way, and the difference matters.

Many newer homes have a capped pipe running from the slab to the roof with no fan attached. That is a passive stub, and it is a gift, because activating it is far simpler than starting from scratch. It is not proof the home is protected. Passive systems in a hot dry climate lean on a temperature difference that mostly is not there, and the only way to know whether yours is doing anything is to test.

Is a soil moisture system the same thing as a radon system?

No, and this trips people up because both start with venting the space under a slab. They are two separate systems, installed in different locations, sized for two different jobs.

A radon system is built to capture and exhaust a gas. It uses a relatively small in line fan and it is judged by the pCi/L number it produces. A soil moisture system, used to address foundation heave in expansive clay, is a larger unit designed to move a much greater volume of air continuously, because water is heavy and clay gives it up slowly. Ambient intake ports along the perimeter stem wall let dry Arizona air sweep across the whole sub slab space and carry humidity out with it, and a moisture sensor buried under the slab regulates the drying so the soil never goes too dry near the footings. That approach was developed in Arizona in 2004 specifically for our clay and our climate.

Because both pull from beneath the slab, a running moisture system often lowers radon as a secondary effect. That is a bonus, not a plan, and it does not replace a properly designed radon system or the testing that verifies one. If foundation heave is your actual problem, expansive soil mitigation is the right conversation to have.

How do you know if it worked?

By testing again. A radon system is judged by the number it produces, not by how it looks on the wall. Radon is measured in pCi/L, the EPA action level is 4 pCi/L, and the EPA also suggests considering a fix between 2 and 4. Arizona data from roughly 7,000 homes since 1993 shows an average of 2.9 pCi/L, with 26 percent of homes above the action level and a recorded high of 55.8 pCi/L. Phoenix homes usually fall between 1 and 4, but some in hot spots like Scottsdale and Cave Creek have come back far higher.

A post mitigation radon test confirms the reduction, and ongoing radon monitoring tells you whether it holds across seasons. Worth knowing honestly: no system guarantees a specific number forever, and levels can shift if the fan fails, the house changes, or soil conditions change. That is what the manometer and periodic retesting are for, and it is also why radon can come back if a system stops working.

Thinking about a system for your home?

If your test came back above 4 pCi/L, or you have a capped passive pipe and no idea whether it does anything, we are happy to take a look and walk you through what your particular slab would need. Every job starts with a free estimate after we assess the home, with transparent upfront pricing and no surprise add ons. If you would rather ask a few questions before anyone comes out, get in touch and we will give you a straight answer.

Frequently Asked Questions

Is sub slab depressurization the same thing as radon mitigation?

Sub slab depressurization is the most common technique used to accomplish radon mitigation, so in most homes the two mean the same thing in practice. Other approaches exist, including crawl space membranes and ventilation changes, but for a slab on grade Arizona home this is almost always the method.

Does a radon fan have to run all the time?

Yes. The fan maintains the pressure difference that keeps soil gas moving into the pipe instead of into your house, so turning it off lets levels rise again. These fans are designed for continuous duty and draw about as much power as a light bulb.

Can one system handle both radon and foundation heave?

No. They are separate systems with different equipment, different sizing, and different installation locations, and each is designed around a different target. A moisture system may lower radon as a side effect, but confirming that still requires a radon test.

Can sub slab depressurization work if my home has no gravel under the slab?

Often yes, though it may take more than one suction point or a different design. A communication test tells the mitigator how far the pressure field reaches through whatever material is actually down there, which is why that diagnostic comes before installation.

My new home already has a pipe from the slab to the roof. Am I protected?

That is likely a passive stub from radon resistant new construction, and it helps, but passive systems perform inconsistently in a climate like ours. The only way to know your level is to test. If it comes back elevated, adding a fan to the existing pipe is usually straightforward.