Old house radon mitigation in Clintonville: a stone basement

If your Clintonville house dates to the 1920s and the basement walls are stone laid in mortar, radon mitigation will not look like the tidy one-pipe job a newer house gets. The walls, the floor and the lot all push the design in different directions.

What follows is an illustration of that house type, not a real job or a real homeowner. It shows where an old stone basement forces choices a poured-wall basement never raises.

The house in this illustration

Picture a two-story frame house on a narrow Clintonville lot, with a driveway down one side and a neighbor's house a short walk across the other. The foundation is rubble stone: irregular pieces set in lime mortar that has gone soft and sandy in spots. The basement floor is a thin concrete layer, patched over the years around old plumbing and a former floor drain, and it pulls away from the stone at the edges.

The brick homes of German Village and Old North tell a similar story: old masonry below grade, floors poured or replaced long after the house went up, and walls never asked to hold back gas. An old basement was built to carry a house, not to seal out soil gas, and every decision below starts from that fact.

Suppose the first short-term test, placed in the basement the way the Ohio protocol requires, reads around 12 pCi/L. At 8 or above, the state's strategy calls for a second short-term test before acting, and in this illustration that second test confirms the result.

Why stone walls leak soil gas

A poured concrete wall is one continuous piece with a few cracks. A stone wall is hundreds of joints. The mortar between the stones is porous and wears away, and behind the visible face the wall often holds smaller stones, rubble and soil. Air moves through that maze easily.

In winter the house works like a chimney. Warm air rises and escapes through the upper floors, which leaves the basement at slightly lower pressure than the soil around it, and soil gas is drawn in through every open joint. A thin, patched floor adds more paths: hairline cracks, patches that never bonded to the old concrete, and the open seam where floor meets wall.

On a stone foundation the walls can be as large an entry path as the floor, which changes where the suction has to reach.

What sealing can and cannot do

The Ohio Radon Mitigation Standards (ORMS) expect the floor-wall joint to be sealed with urethane caulk, backed with foam rod where the gap passes half an inch, plus sealing of cracks and utility penetrations. EPA's Consumer's Guide to Radon Reduction treats sealing as something normally paired with other techniques, not a fix on its own.

On a rubble wall, that means sealing what can be sealed and being honest about the rest. The seam between the thin floor and the stone can usually be closed. Individual open mortar joints can be pointed. But the voids behind the face of the wall cannot be reached, and a coating on the inside face cannot be counted on to seal a wall that leaks through its core. Repointing or parging the whole wall can be worth doing as its own masonry project, but the radon design should not rely on it.

ORMS requires openings that cannot be sealed to be disclosed to you in writing, so a list of them is a normal part of an honest old-house proposal, not a red flag.

More suction points, proven by the communication test

With a thin slab over fill nobody can see, ORMS requires a communication test before installation. In an old house the result can show suction fading within a short distance, because the fill may be soil instead of gravel and the patches may split the floor into sections that barely connect.

The answer is often more than one suction point, spaced so the pressure field reaches each section of floor and, above all, the base of the stone walls where gas enters. EPA lists sub-slab suction at 50 to 99 percent reduction, and the width of that range reflects how much the material under the floor matters.

In an old basement, the number of suction points comes from the test holes, not from a rule of thumb. Old houses also bring combustion questions. If the furnace or water heater is a natural draft appliance venting into an old masonry chimney, ORMS calls for recommending a qualified backdraft inspection, and a fan system does not go in where spillage has been confirmed until that is fixed. A backdraft test also follows installation when those appliances are present.

Routing the pipe on a tight lot

The fan cannot go in the basement, so the pipe has to leave it. On this house there are two realistic paths. An interior route climbs through a closet stacked above the basement into the attic, where the fan sits, and exits through the roof. An exterior route leaves through the wall above the foundation, puts the fan outside, and runs the stack up the side of the house.

The outlet rules decide a lot on a narrow lot. The discharge has to be above the highest eave, as close to the ridge as possible, at least 12 inches above the roof and at least 10 feet above the ground. It must be 10 feet or more from any window, door or opening less than 2 feet below it, and the same 10 feet from openings into an adjacent building. A side-wall stack runs vertical and ends at least 12 inches above the edge of the highest roof, with no rain cap.

On a narrow Clintonville lot, the neighbor's windows can matter as much as your own. An exterior fan is hard-wired with an outdoor disconnect within sight and within 4 feet. For how these installs are scoped here, see radon mitigation in Clintonville.

A guarantee, or an honest "no guarantee"

Every Ohio proposal must include one of two things: a guarantee to bring the average below 4 pCi/L, with its conditions spelled out, or a statement that there is no guarantee and why. ORMS also says plainly that complying with the standards does not guarantee any particular radon level.

On a newer house with poured walls and clean gravel under the slab, a guarantee below 4 pCi/L with stated conditions is an easy promise to write. On a rubble-stone basement over unknown fill, a legitimate no-guarantee statement looks like this:

• It names reasons specific to your house: the stone wall, the unsealable joints, the communication test results.
• It lists the openings that could not be sealed, which Ohio already requires in writing.
• It explains what the next step would be if the post-mitigation test comes back at 4 or higher, such as an added suction point.
• It still carries everything else a proposal must include, from license numbers and scope to the installed price and an estimate of yearly operating cost.

A no-guarantee proposal is legitimate when its reasons are specific to your house; a bare "no guarantee" line is not. Our guide to the Ohio mitigation proposal walks through every required item, and if a finished system still reads high, what to do when radon stays high covers what to check next.

If you own an older house in Clintonville, call or text us with your test result and a photo of the basement walls. The wall alone says a lot about how the system will be designed.