Hilliard split-level radon mitigation: basement plus crawl space

Your Hilliard split-level has more than one kind of ground contact under a single roof: a partial basement under the family room, a crawl space under the kitchen wing, and a garage slab off to the side. When a radon test comes back high, the real question is whether one system can cover all of it.

This walk-through is an illustration, not a record of a real job: a common house type taken through the order of work Ohio requires, showing where the crawl changes the plan.

The house in this illustration

Picture a split-level in one of the Hilliard subdivisions built from the 1990s onward. You enter at grade, go up half a flight to the bedrooms, or down half a flight to a family room whose floor is a basement slab set partly below the yard. The kitchen wing rests on a crawl space whose old plastic sheet has slid off the soil in places. The attached garage sits on its own slab next to the lower level.

Each surface meets the soil differently: the basement slab through its floor-wall joint, sump pit and plumbing penetrations, the crawl through bare earth wherever the plastic has shifted. Central Ohio's clay-rich glacial till drains slowly, and when it is saturated, soil gas takes the easiest path up, which is often the crawl.

A split-level behaves like two or three small foundations sharing one set of walls, and each one needs its own evidence.

Testing each level the way Ohio's protocol expects

The Ohio radon measurement protocol speaks to split-levels directly: test each foundation type. In this house that means one device in the lower family room over the basement slab and a second in the den that sits above the crawl. The crawl itself is never a test spot, and neither are sumps, bathrooms, kitchens, laundry rooms or closets.

In both rooms the device sits 20 inches to 6 feet off the floor, clear of exterior walls, doors, windows, vents and heat, with the house closed up from 12 hours before any test shorter than four days.

Suppose the family room reads around 9 pCi/L and the den over the crawl reads around 5. Those are two results for two areas, and Ohio never lets readings from different locations be averaged into one number. Under the state's strategy for homes that are not being sold, a first reading of 8 or more calls for a second short-term test before deciding, while a reading from 4 up to 8 can be confirmed with either another short test or a long-term test that captures the year-round average. Our explainer on short-term versus long-term tests covers when each one is the better tool.

Inspection, floor plan and the communication test

Before a written proposal, the Ohio Radon Mitigation Standards (ORMS) call for a visual inspection, with findings drawn on a floor plan that shows the foundation, drains, mechanical equipment, entry points, diagnostics, pipe layout, and fan and warning device locations. You or a future buyer of the house can ask for it.

In a split-level the drawing shows where the basement slab stops and the crawl starts, whether the two connect through an open knee wall, and where a riser could climb without slicing through finished rooms.

Next comes the communication test, which ORMS requires whenever the material under the slab is unknown. A small hole is drilled, a vacuum is applied, and a sensitive gauge at test holes elsewhere in the slab shows whether suction spreads through the fill. This test is what turns "one suction point ought to do it" from a hunch into a measurement. Tight fill or bare clay under the slab can mean a second point. And because this basement has a working sump pump, the pit is not used as the primary suction point.

Sub-slab suction plus a sealed crawl membrane

The basement half is a conventional sub-slab system: a suction pit under the slab, pipe at least 3 inches in diameter in Schedule 40 PVC, foam core or ABS with glued joints, urethane caulk along the floor-wall joint, and an airtight sump lid with a view port so the pump can still be serviced.

The crawl half is sub-membrane depressurization, and ORMS sets clear minimums for it:

• Polyethylene sheeting of at least 6 mil (or 3 mil cross-laminated), heavier if the crawl holds storage or gets frequent trips.
• Seams lapped at least 12 inches and sealed.
• The sheet sealed to the foundation walls and to every pier with mechanical fasteners.
• A suction point under the sheet, tied into the pipe system.

The access door is typically closed off and weatherstripped too, so the crawl acts as one sealed volume. EPA's Consumer's Guide to Radon Reduction lists both sub-slab suction and sub-membrane suction at 50 to 99 percent reduction, while just ventilating a crawl is listed at 0 to 50. In a mixed house the membrane is not an accessory; it is half the system. Our page on crawl space radon mitigation goes deeper on the membrane.

One fan or two, and where it can live

Whether one fan can pull on both legs depends on the diagnostics. When suction spreads well under the slab and the membrane draws down easily, the two suction points are often joined into a single riser and a single fan. When one leg starves the other, the answer is a separate leg or a second fan.

Where the fan sits is not a matter of taste. ORMS permits an attic, a garage with no living space above it, or the outside of the house, and never a basement, crawl or conditioned room. If a bedroom sits over part of the garage, as it does in some split-level plans, that spot is off the list, so the practical routes are a closet chase to the attic or a run up an exterior wall.

The outlet has rules of its own:

• Above the highest eave, as near the ridge as practical, and at least 12 inches above the roof surface.
• At least 10 feet above the ground.
• At least 10 feet from any window, door or opening less than 2 feet below it, and 10 feet or more from openings into a neighboring building.
• No rain cap.

On a split-level, the stacked rooflines usually make the highest-eave rule the one that picks the route.

Labels, the monitor and a retest in each area

Every active system has a monitor that confirms the fan is running or warns when it stops. A manometer is marked at its start-up reading, and you get that pressure in writing. Labels reading RADON REDUCTION SYSTEM go on exposed pipe on every floor, on an exterior fan and pipe, and on the breaker.

The post-mitigation test goes back into the same two rooms: the lower family room and the den over the crawl. You must be told in writing to run it at least a full day after the fan starts and within 30 days, by the mitigation specialist or an independent licensed tester. If the den stays high while the basement drops, look at the membrane before you blame the fan.

For budgeting, a house like this lands in the crawl-space tier on the site's price list, $1,800-$3,500, because the membrane work drives the scope.

If you own a split-level in Hilliard with a high reading on either level, see how crawl space radon work in Hilliard is scoped, then call or text us with both results and a quick note on which rooms sit over what.