The Radon Number That Climbed After the Basement Bath

Why this matters

A plumber adding a basement bathroom cuts a trench through the slab, lays drain line, backfills and patches. The work passes inspection and the bathroom is beautiful. Then the house's radon number, which had been comfortably under the action level, comes back high. Radon is a soil gas, and a slab is the lid that keeps most of it out; the bathroom job cut a long hole in that lid and added a fan that pulls on it. Neither the plumber nor the finish carpenter did anything wrong by their own trade's standards, and the owner still ended up with a house that tested three times its old number.

This case walks the candidate explanations one at a time, because the first two were plausible and would have sent the owner to a mitigation contractor before anyone looked at the slab.

The record

A 1970s two-story with a full basement, poured slab, block foundation walls. The owners bought it two summers ago. The home inspector's short-term radon test, run in the finished half of the basement in July: 2.6 picocuries per liter (pCi/L), a measure of how much radioactive decay happens in a liter of air, under the EPA's action level of 4 pCi/L. The EPA recommends fixing a home at or above 4 and suggests considering it between 2 and 4, so the owners noted it and moved on.

Last fall they had a bathroom added in the unfinished half: toilet, shower and sink, with a new exhaust fan ducted outdoors through the rim joist. The plumber cut a trench about 18 inches wide and 12 feet long through the slab to tie into the building drain, and the patch was poured over gravel backfill.

In January, with a new baby in the house, the owners bought a short-term test kit and ran it in the new basement bedroom next to the bath. 7.8 pCi/L.

Candidate 1: the two tests are not comparable

This one was real and had to be dealt with first. The two numbers differed in three ways besides the bathroom: season (July against January), location (finished half against the bedroom next to the new bath), and conditions (a real estate test under closed-house conditions against an owner's kit whose instructions they had followed loosely, with a window cracked one night). Radon in a house commonly runs higher in winter, when the stack effect pulls the lower floors negative and the windows stay shut. See related: Stack Effect and the Neutral Pressure Plane.

So 2.6 against 7.8 could not by itself say the bathroom did it. A radon professional the owners called suggested the cheaper next move before quoting a system: a continuous radon monitor, which logs hourly, in the same spot, under proper closed-house conditions (windows and exterior doors kept shut except for normal coming and going, starting at least 12 hours before a short test begins, per EPA's testing guidance), with one variable changed at a time. In the owners' state, as in Illinois and Pennsylvania, radon testing and mitigation for hire are licensed or state-certified activities; in states with no such program, look for a professional certified by one of the national radon certification bodies.

Candidate 2: it was always this high in winter

Possible, and the monitor could test it. If the house had simply always run high in January, the bathroom fan would make no difference to the reading. The professional ran the monitor 48 hours with the bath fan off, then 48 hours with it running continuously, which is how the owners had been running it on a humidity control. A manometer logged the basement against outdoors over the same four days.

With the fan off, the house still came in at 4.6 in January, well above its summer 2.6, so some of the climb was season. But turning the fan on raised the average from 4.6 to 8.1, a rise of 3.5 pCi/L, or about 76 percent over the fan-off figure, and moved the basement about 1.5 pascals more negative. The fan was not the season. Candidate 2 explained part of it and not the part the owners had noticed.

Candidate 3: the work opened the lid

A negative basement only pulls soil gas if there is a path. The professional and the plumber, who came back at the owners' request, looked at the slab together.

  SLAB SECTION THROUGH THE NEW BATH (not to scale)

  new bath fan -> duct -> outdoors
  (pulls the basement down)
  basement air, about -2.5 Pa with fan on
      ^             ^                 ^
      | crack       | open annulus    | crack
  ====|=====+=======|=====+===========|=====
  old slab  | patch | cleanout riser  | old slab
  ==========+=======|=====+=================
     cold joint   gravel backfill    cold joint
     at edge      in trench, open     at edge
                  to the soil
  ------------------------------------------
   soil and sub-slab gravel (soil gas)

• The patch's cold joints. New concrete poured against old shrinks away from it as it cures. Both long edges of the patch and both short ends showed a hairline gap: 12 feet plus 12 feet plus 1.5 plus 1.5, about 27 feet of joint, open from the basement to the trench backfill.
• The cleanout riser. A cleanout on the new drain came up through the patch, and the concrete had been finished around it with a visible gap, about 1/4 inch, all the way around the pipe.
• The backfill. Gravel in the trench is an open path from the soil to every gap above it, more open than the compacted soil it replaced.

None of these is a plumbing defect. A slab patch over a trench is ordinary work, and the plumbing code is not written to keep soil gas out. But together they are a 27-foot crack and a ring around a pipe, sitting a few feet from a fan that pulls the room down 1.5 pascals more than before. See related: Radon and Soil Gas Entry Paths.

The fix: seal first, then decide about the fan

Seal the paths. The plumber's shop routed the cold joints to a clean, sound edge and filled them with a polyurethane sealant rated for concrete, and closed the cleanout annulus with backer rod and the same sealant. Routing concrete raises respirable crystalline silica; the crew used a grinder with a vacuum shroud and a HEPA-filtered vacuum, following OSHA's construction silica standard, 29 CFR 1926.1153 (its Table 1 entry for the tool, or the shop's exposure assessment), with respirators under the shop's written program wherever that entry or assessment calls for them. Anyone else stayed out of the basement until cleanup was done.

Change how the fan runs. The bath fan is required for the bathroom and stayed. The owners moved it from continuous running to a timer and humidity control, so it runs when the bath is used and not around the clock.

Retest the same way. The professional reran the monitor, same spot, closed-house conditions, fan on continuous to test the worst case: 3.0 pCi/L over 48 hours, with the basement at about -2.4 Pa against outdoors. The pressure barely changed, because sealing does nothing to the fan's pull; what it changed was what the pull had to work with. With the fan on its normal timer, a follow-up short-term test read 2.8 pCi/L.

That 2.8 is a January figure, so it is not strictly comparable to the July 2.6, any more than 7.8 was. What it does say is that a January house with the bath in normal use is back under the action level, and with the fan running continuously, the worst case, it is still under it at 3.0.

What the owners were told

The result is in the range where the EPA suggests considering mitigation, 2 to 4 pCi/L, and short-term tests vary. The professional recommended a long-term test, 90 days or more through the rest of the heating season, to see the year rather than the week. If it comes back at 4 or above, the standard fix is sub-slab depressurization, which installs a fan that pulls soil gas from under the slab and vents it above the roof; see related: Radon Mitigation Basics Reference. A house with a newly sealed slab is also a better candidate for that system, since the seal is what lets the system's suction reach across the slab.

What each trade took from it

The plumber's shop added to its slab work: a sealed joint at every patch edge and a sealed annulus at every pipe through a slab, with a photo, and a note to the owner recommending a radon retest after any slab cut in a house that has been tested. It costs a tube of sealant and fifteen minutes.

The remodeler added the basement exhaust fan to its conversation with the owner: a basement bath fan should run on demand, and in a radon-tested house the owner should know the fan can change the number.

What would have changed the answer. If the fan-on and fan-off monitor runs had come out the same, the slab would still have been worth sealing, but the climb would have been season and the conversation would have gone straight to a long-term test and probably a mitigation system. If the house had already had a sub-slab system, a trench through the slab can cut into its suction field and a retest of the system's performance, not just the house, would be the first step. And if the slab had shown no new openings at all, the next places to look would be the ones every basement already has: an open sump pit, the joint where the slab meets the block walls, and the open cores at the top of a block wall, any of which a newly negative basement will draw on.

References

• See related: Radon Mitigation Basics Reference; Radon and Soil Gas Entry Paths
• See related: Stack Effect and the Neutral Pressure Plane; Which Trade Jobs Change House Pressure and Moisture
• EPA, A Citizen's Guide to Radon (action level and the 2 to 4 pCi/L guidance)
• OSHA 29 CFR 1926.1153, respirable crystalline silica in construction
• State radon programs for licensing and testing protocols, where your state has one