A Granville 1924 stone-rubble basement: when sealing is most of the job

The starting point

A 1924 American Foursquare in Granville, 1,950 sqft of original construction, original limestone-rubble basement walls (mortar joints between irregular stone, no poured concrete on the perimeter), original earth-and-concrete-cap floor in the unfinished portion of the basement, partially-finished family room added in 1972 with a 4-inch concrete slab over the original floor.

The new owners had bought the home eight months earlier as a renovation project. Their contractor included radon testing in the pre-renovation due diligence: the 48-hour test came back at 13.6 pCi/L, more than three times the EPA action level. The contractor referred them to our dispatch line.

Why pre-1950 Granville housing tests this high

The Devonian limestone and shale geology of central Ohio runs uranium-bearing rock at relatively shallow depth across much of Granville and the older Licking County town centers. Pre-1950 builders used local stone for foundations because it was cheap and abundant. The result is residential basements built directly into uranium-rich soil with hundreds of mortar-joint paths for soil gas to enter.

Granville and the surrounding limestone-area town stock show some of the higher documented pre-mitigation pCi/L averages in the metro. Local mitigators have records of pre-mitigation readings above 30 pCi/L on a meaningful number of pre-1940 stone-basement homes. The 13.6 pCi/L on this 1924 foursquare is on the high end of typical for Granville, not unusual.

The on-site assessment

The Columbus-area mitigator walked the basement for about an hour. Findings:

1. Hundreds of mortar joints in the limestone-rubble walls, most showing visible weathering and gaps where mortar had spalled. Rough estimate: 300-500 linear feet of joint length needing some level of sealing 2. Original earth-and-concrete-cap floor in the unfinished portion, with the cap broken in several places and partial dirt exposure 3. 1972 concrete slab in the finished portion, hairline-cracked but generally sealed 4. Original sump pit in the southwest corner with no liner, an unsealed lid, and a bypass float that had failed at some point in the past 5. Furnace and gas water heater in the basement, both natural-draft, both 12-15 years old (replaced from the 1924 originals at some point)

The mitigation strategy that emerged: active sub-slab depressurization on the 1972 slab portion, sub-membrane depressurization on the unfinished portion, aggressive mortar-joint sealing of the perimeter walls, sealed sump-pit lid integrated into the suction network. Single fan, two routing paths joining at the basement ceiling.

The quote

$3,840 fixed, broken out:

• Slab core-drill suction pit + lid for the 1972 slab portion: $400
• Sub-membrane suction pipe + reinforced 12-mil polyethylene barrier across the unfinished-floor portion: $850
• Mortar-joint sealing across the entire perimeter wall: $1,400 (about 35% of total quote)
• Sealed sump-pit lid + suction integration: $300
• Stack routing through closet chase to attic to roofline: $350
• Radonaway RP145 fan in attic enclosure + manometer + dedicated circuit: $400
• Combustion-safety pre/post test on furnace and water heater: $140

The sealing line item dwarfed every other component. That is normal for Granville pre-1950 stone-basement work; in some cases sealing reaches 50% of total labor.

Install (two days)

Day 1 (sealing-heavy): Crew of two on site 7:30 AM through 5:30 PM. Pre-install combustion-safety test on the furnace and water heater (both passed at acceptable margins). Mortar-joint sealing started at 9:00. The technique: clean each joint with a wire brush to remove loose material, apply a primer where the existing mortar was friable, then run polyurethane caulk into the joint and tool it flush. About 8 hours of joint-by-joint work. The two-person crew finished roughly 80% of the perimeter on Day 1.

Day 2 (mitigation install): Same crew 7:30 AM through 4:00 PM. Finish the remaining 20% of mortar-joint sealing in the morning. Slab core-drill done by 11:00. Sub-membrane barrier installed by 1:30. Stack routed through the closet chase, fan mounted in the attic enclosure, manometer placed and labeled by 3:30. Post-install combustion-safety test confirmed both natural-draft appliances still drafted properly. Power up and walk-through with the homeowner at 4:00.

End-of-Day-2 manometer reading: 1.1 inches water column negative pressure on the slab side, 0.7 on the sub-membrane side. Healthy.

The 48-hour retest

Continuous radon monitor placed in the 1972 finished family room (the same spot the original 48-hour test had been placed). 48-hour result:

1.6 pCi/L average. Down 88% from the 13.6 pCi/L pre-mitigation reading. Below the EPA-recommended 2.0 pCi/L post-mitigation target.

The key word in that delta: 88%. Granville pre-1950 mitigations typically hit 80-90% reduction even from very high starting levels, but only when the sealing portion of the work is done properly. Cutting the sealing scope can leave a home at 4-6 pCi/L post-install: technically a passing reading on average but unstable, with seasonal swings back into action-level territory.

What the homeowner paid

Total: $3,840. About 1.6x the typical Columbus basement install ($2,400 high end), reflecting the extensive sealing scope. Most of that incremental cost is labor; materials for sealing are $200-$300 of the quote.

Annual electricity for the fan: about $90 on AEP Ohio residential rates.

Why this case matters for other Granville and pre-1950 homeowners

If you have a stone-rubble basement (Granville, Clintonville, Bexley, Grandview Heights, older Worthington, older Lancaster), expect a quote that allocates significant labor to mortar-joint sealing. If a contractor quotes a typical $1,200-$2,400 install on a pre-1950 stone-basement home without acknowledging the sealing scope, the install will likely under-perform.

Things to watch for in the on-site assessment: - Did the mitigator actually walk the perimeter and count linear feet of joint? - Is sealing called out as a separate line item, or buried in "miscellaneous"? - What polyurethane caulk product is being specified? (Sika 1A or DAP Polyurethane are the typical Columbus-area choices; cheaper acrylic-latex caulks fail within 2-3 years) - How many crew-hours are budgeted for sealing? (Should be 8-16 hours for typical Granville-class work)

A credentialed NRPP mitigator who has done 30+ Granville or Clintonville stone-basement installs will give you those numbers without hesitation. A contractor who treats sealing as an afterthought is the wrong contractor for this kind of home.