A Loveland slab with a crawl-space addition: when one mitigation strategy is not enough

The starting point

A 1995 ranch in Loveland, 2,200 sqft of original construction on a poured slab, with a 1998 master-suite addition (about 600 sqft) framed over a 4-foot crawl space because the lot grade dropped at the back. Original sealed slab, original sump pit on the north wall of the main house. The crawl space under the addition had a thin polyethylene liner from the 1998 build, two access vents to outside, and exposed dirt under the liner.

The new owners had moved in three months earlier. A neighbor mentioned radon during a backyard conversation. They ordered a hardware-store charcoal kit, placed it in the master bedroom (lowest-occupied space), and got back a reading of 6.8 pCi/L four days later. EPA action level is 4.0 pCi/L. They called the partner-network dispatcher the next morning.

What the on-site assessment found

Two distinct soil-gas environments under the same roof. The original slab portion had a sealed sub-grade with one obvious entry path: the unsealed sump-pit lid. The 1998 addition's crawl space had multiple entry paths: gaps around the perimeter where the polyethylene liner had pulled away from the foundation wall, two ground-level access vents that pulled negative pressure during winter stack effect, and several inches of exposed soil between the liner and the foundation perimeter.

The partner-network mitigator measured the suction profile under the slab using a hand pump and pressure gauge: about 0.3 inches of water column negative pressure achievable at the sump pit, dropping to under 0.1 at 12 feet from the pit. Adequate for a single suction pit on the slab portion, but the crawl space needed its own treatment because the liner was not sealed enough to support depressurization on its own.

The plan

A combined system. One suction pit through the slab next to the existing sump (sealed lid + new sub-slab depressurization). One sub-membrane suction pipe under a new properly-installed reinforced 12-mil polyethylene barrier across the crawl space. Both stacks join in the basement utility room and run as a single 4-inch PVC stack out the side wall, up to roofline, terminated above the eave per International Residential Code requirements. One inline radon fan (Radonaway RP145) sized for the combined air volume.

Quote: $2,640 fixed, including: 1. Core-drill suction pit through original slab next to sump 2. Replace sump pit lid with sealed, gasketed unit; integrate into suction 3. Install reinforced 12-mil polyethylene barrier across crawl-space dirt floor and 8 inches up perimeter walls 4. Seal barrier perimeter with butyl tape and polyurethane caulk 5. Install sub-membrane suction line through the barrier 6. Run combined 4-inch PVC stack from utility room to roofline 7. Mount RP145 inline fan in exterior weather enclosure 8. Manometer + labeled fan-failure indicator at basement ceiling 9. Dedicated 15A circuit 10. 48-hour post-install verification monitor + 12-month follow-up

Install day

Crew on site at 7:30 AM. Slab core-drill done by 9:15. Sump pit lid swapped and sealed by 10:30. Crawl-space prep started after lunch: removal of the old failing liner took about 90 minutes, installation of the new reinforced barrier with proper edge sealing took another 2.5 hours. Suction pipe routed through the barrier and connected to the basement-side stack by 4:00 PM. Fan mounted, wired, and powered up at 4:45.

Initial manometer reading: 0.95 inches of water column of negative pressure on the slab side, 0.6 inches on the crawl-space side. Both healthy. The mitigator confirmed the system was pulling on both sides simultaneously, with no short-circuit between the two suction zones.

The 48-hour retest

A continuous radon monitor (Sun Nuclear 1027) was placed in the master bedroom and ran with closed-house conditions documented for 48 hours. Result:

0.9 pCi/L average. Down 87% from the pre-mitigation reading. Hourly readings stayed between 0.5 and 1.4 pCi/L, with no significant variation across weather changes during the test window.

Why this case is worth knowing

Three things that homeowners and even some mitigators get wrong on mixed-foundation Cincinnati homes.

One: treating it as a simple slab job and ignoring the crawl space. The crawl-space addition by itself was probably contributing 4-5 pCi/L of the total reading. A slab-only install would have brought the home to roughly 2-3 pCi/L, technically below the action level but still elevated and likely to drift back up over a few years as the polyethylene liner continued to degrade.

Two: using two separate systems with two fans. That's a common alternative quote that ranges $3,500-$4,500. It is not necessary when both zones can share a single integrated stack, and the redundancy is wasted. Fan failure is rare and shows on a manometer either way.

Three: skipping the proper sub-membrane barrier replacement. The cheap version of this job is to "patch" the existing 1998 liner. That fails within 18-36 months as the mitigated suction pulls air through the patch seams. The polyethylene barrier replacement is the part of the quote that the homeowner should never let a contractor cut.

What the homeowner paid

Total out-the-door: $2,640 fixed. Annual electricity for the radon fan: about $95. Resale-value impact for a documented mixed-mitigation system on a Loveland mixed-foundation home: typically $7,000-$10,000 favorable per local MLS comparables, especially relevant because mixed-foundation homes are otherwise harder to mitigate after closing and buyers know it.

What to do if you have a similar Cincinnati home

If your home has any combination of slab + crawl space + addition, ask the partner-network mitigator during the on-site assessment to confirm both zones are treated. Get the suction-pressure measurements written into the quote. Ask which fan model is being specified and confirm it is sized for the combined air volume of both zones (not just the slab portion). And ask explicitly about the polyethylene barrier work in the crawl space; that is the line item where contractors most commonly cut corners.