What Retrofit Sub-Slab Depressurization Teaches You About Existing Slabs

By Jake Lusby, NRPP-certified radon professional, IEMA Radon License #RNM20232346

New construction gives you the luxury of designing for radon before anything is poured. You specify the aggregate, lay the loop, sleeve the riser, and the passive stack is in the wall before drywall. Retrofit work offers none of that. You arrive at a finished house, often with a slab poured in 1968 over whatever fill was on hand, and you have to find out what is under there by measurement rather than by specification.

That constraint turns out to be instructive. A retrofit crew learns things about slab construction that a new-construction spec never has to confront.

Start With a Communication Test, Not an Assumption

The single most useful diagnostic in retrofit work is the sub-slab communication test. You drill a test hole, apply a known vacuum with a shop vac or a diagnostic fan, and then read the pressure differential at points several feet away through additional small holes. What you are measuring is whether the material under the slab will actually move air.

The results are frequently counterintuitive. A house on clean washed gravel may show good communication at fifteen or twenty feet from a single suction point, which means one well-placed penetration can depressurize the entire footprint. A house on compacted clay or on sand that has silted in over decades may show almost nothing at four feet, which means the job needs multiple suction points, or an entirely different strategy such as a sub-membrane system in an adjoining crawl space.

In the St. Louis metro, this varies street by street. Homes in the Metro East sit on glacial and loess deposits that behave very differently from the limestone-derived soils west of the river. Two houses of the same vintage, ten miles apart, can require completely different suction point counts. Testing communication before you commit to a design is not optional, and skipping it is the most common reason a retrofit system underperforms.

Sealing Is Not the System, But It Determines the System’s Size

There is a persistent idea that sealing cracks reduces radon entry enough to matter on its own. It generally does not. What sealing does is change the economics of the fan you need.

Every unsealed opening, the perimeter floor-wall joint, an open sump pit, a plumbing chase, an abandoned floor drain, is a short circuit. It gives the fan an easy path to pull conditioned indoor air instead of soil gas. Seal those and the same fan produces more vacuum where you want it, under the slab, which often lets you specify a smaller, quieter, lower-wattage fan. The sealing is in service of the depressurization field, not a substitute for it.

Open sump pits deserve particular attention. An uncovered pit is both a major radon entry route and a very large parasitic opening. A sealed lid with a gasketed access port and a clear condensate path usually improves system performance more than any other single sealing measure.

Design the Stack for the Building, Not the Code Minimum

Discharge location gets treated as a checkbox and it should not be. The exhaust needs to terminate above the roofline and away from windows, doors, and any air intake, so the soil gas re-entrainment risk is low. On a complex roof, or on a house with a deep dormer or a nearby second story, the geometry matters more than the minimum dimension in the standard.

Routing also determines whether the homeowner keeps the system running. A riser through conditioned space keeps the stack warm, which prevents condensation from freezing in the run during a Midwest January. An exterior riser is cheaper and faster but needs to be pitched correctly back to the slab so condensate drains rather than pooling. Both work. Choosing without thinking about the winter is what causes callbacks.

Verification Is the Part That Cannot Be Skipped

A radon system is one of the few building assemblies whose performance is trivially measurable, and it should be measured every time. The EPA action level is 4.0 pCi/L, and the point of the work is to get and keep the house below that number.

Post-mitigation testing should happen after the system has run continuously for at least twenty-four hours, under closed-house conditions, with the test device placed in the lowest lived-in level. A manometer on the riser tells the homeowner the fan is pulling, but it does not tell anyone what the indoor concentration is. Those are different questions and only one of them is the one that matters.

Retesting every two years, and after any work that disturbs the slab or changes the building’s pressure regime, is reasonable practice. New windows, a new furnace, air sealing work, or a finished basement can all shift the balance in a house that previously tested fine.

What This Means for Remodelers

If you are opening up a basement, you have leverage that a mitigation contractor arriving later will not have. Runs can be concealed, suction points can be placed where they belong rather than where they fit, and sealing can be done while the assemblies are open. Testing before the remodel, rather than after, is what makes that possible.

Air Sense Environmental performs radon testing, radon mitigation, and indoor air quality work across the St. Louis metro and the Illinois Metro East, including Edwardsville, Chesterfield, and St. Charles. If you have a project where slab conditions are an open question, a communication test early in the schedule will tell you more than any assumption about the year the house was built.

Questions about a specific project can be directed through https://airsenseenvironmental.com/contact/ or by phone at 314-664-9807.