Sub-slab ventilation systems in Cincinnati basements, explained

If a quote in your inbox says "sub-slab ventilation system" and another says "active soil depressurization," you are looking at the same idea described two ways. Both mean a fan-driven pipe that pulls soil gas from beneath the basement floor and discharges it above the roof before it can drift indoors. Here is how that system is laid out in a typical Greater Cincinnati basement, what Ohio requires of it, and how to tell a careful install from a careless one.

One system, several names

Sub-slab ventilation, sub-slab depressurization, sub-slab suction, active soil depressurization, and radon mitigation system all describe the approach EPA's Consumer's Guide to Radon Reduction calls subslab suction. EPA credits it with typical reductions of 50 to 99 percent and notes that it works best when air moves easily through the material under the slab. The passive version, a pipe with no fan, is rated at 30 to 70 percent.

The principle is simple. The fan keeps the space under the floor at slightly lower pressure than the basement, so air leaks down through cracks rather than up. Soil gas that would have seeped in is drawn to the pipe and sent outside instead.

How the pieces fit in a basement here

The suction point. A hole is cored through the slab, usually in a utility area or an unfinished corner, and a small pit is dug out beneath it so the pipe draws from open space rather than packed soil. Where the sub-slab material is unknown, Ohio's Radon Mitigation Standards require a communication test first, which checks how far suction spreads under the floor and tells the designer whether one point will reach the whole slab.

The pipe. Ohio sets a minimum main run of three inches in diameter from the primary suction point to the exhaust, with joints permanently glued except for removable couplings at the fan. The pipe has to be pitched so condensation drains back under the slab.

The fan. The Ohio standards allow the fan in an attic, in a garage that is not beneath living space, or on the outside of the house. They prohibit it below ground, in conditioned space, or in a basement or crawl space, because a leak at the fan would push radon indoors.

The discharge. The exhaust must sit above the highest eave and as close to the ridge as possible, at least 10 feet above the ground, at least 12 inches above the roof surface, and 10 feet or more from any window or door that sits less than two feet below it. It points straight up, and a rain cap is not allowed.

The monitor and labels. Every active system needs a monitor that shows the fan is working or warns when it fails, most often a U-tube manometer on the pipe, marked with the pressure reading from the day the system started. Ohio also requires RADON REDUCTION SYSTEM labels on the exposed pipe on each floor and on the circuit breaker feeding the fan.

Variations for the foundations you find here

Greater Cincinnati basements are not all poured concrete, and the design has to follow the foundation.

• Hollow block walls. EPA describes block-wall suction for homes with hollow concrete block foundations, and Ohio's standards require the open tops of block walls to be closed and sealed so the system is not drawing air from the living space.
• Sump pits. Ohio's standards say a sump pit that holds a working pump should not be the primary suction point, and any sump cover must be sealed airtight around its pipes and wiring. A sealed lid still matters, because an open pit is a wide-open door for soil gas.
• Interior drain tile. Many hillside homes have a perimeter drain added during a waterproofing job. EPA recognizes drain-tile suction, and where that loop drains to daylight, Ohio requires a one-way valve so the system does not pull outdoor air through the drain.
• Crawl space additions. An addition built over a dirt crawl needs its own treatment: a soil gas retarder membrane of at least six mil polyethylene, with seams overlapped by at least 12 inches and sealed.

What happens before any pipe goes in

Ohio requires a thorough visual inspection before a written proposal, with the findings recorded on a drawing of the floor plan that shows walls, drains, the HVAC system, entry points, pipe layout, and the fan and warning device locations. That drawing becomes part of the file and is available to you and any future owner on request.

If backdrafting is a concern, the standards have the mitigator recommend that a qualified person inspect the natural-draft appliances, and no fan system goes into a house with confirmed spillage until that is fixed. It matters most in older houses where a gas water heater vents into a masonry chimney a few feet from the planned suction point.

The written proposal itself has to state the installation price and an estimate of annual operating cost, describe any maintenance you will be responsible for, and either guarantee to bring the average below 4.0 pCi/L with its conditions or explain why there is no guarantee.

A good install and a careless one

The quickest tell is paperwork: a careful installer gives you the floor plan drawing, the startup manometer reading, and a written guarantee statement without being asked. Beyond that, walk the job with this list:

• the floor-to-wall joint is sealed with urethane caulk, with foam backer rod in gaps wider than half an inch, and any perimeter channel still drains water
• the pipe is strapped to the structure, not hung from plumbing or ductwork
• the fan is in the attic, a garage, or outdoors, never in the basement
• the exhaust is above the eave, open to the sky, with no cap
• the manometer is marked with the startup reading, and the breaker is labeled
• an exterior fan is hard-wired with a disconnect within sight of the fan and within four feet of it

Every install here is performed by an NRPP- or NRSB-credentialed, Ohio-licensed mitigator working to ANSI/AARST SGM-SF, the consensus standard for existing homes, but the checklist above is worth running on any system, including one that came with the house.

After it starts running

Ohio requires you to be told in writing that the post-mitigation test should happen no sooner than 24 hours and no later than 30 days after the system is switched on, run either by the specialist who installed it or by an independent licensed tester. EPA suggests the independent route to avoid any conflict of interest, and recommends retesting at least every two years after that.

To see what a system like this typically costs on your foundation, start with our pricing by foundation type or the radon cost calculator. The radon mitigation systems page covers the full scope, and our answer on why sealing alone falls short explains why the fan does the heavy lifting. When you have a quote in hand, call or text us and we will go through it line by line.