Radon System Sizing, Routing, and the 4 pCi/L Line

Sub-slab depressurization sized from the floor area, the unit type, and the soil: suction points, the gravel layer and membrane under the code, fan CFM, exhaust routing, and the test-before-and-after sequence.

The code build is a 4 in permeability layer under a 4-mil membrane with a 4 in suction pipe, one pipe point per 1,000 sq ft, and a fan at 100 to 150 cfm. EPA asks for mitigation at 4 pCi/L and calls the system a sub-slab depressurization with the pipe through the conditioned space.

—Zone, reading, and the action line
—System type and suction points
—Gravel layer and membrane
—Suction pipe, length, and routing
—Fan CFM, power, and the run
—Sealing, testing, and retest
How the math works
  • The action line (EPA): 4 pCi/L is the action level, 2 pCi/L is the line where EPA says consider mitigation, and no level is a safe level. A short-term test is 2 to 7 days in the closed-house condition in the lowest lived-in level, a long-term test is 90 days and over, and a real estate test is a short-term test with the conditions met. A follow-up test after the system starts is the check that the system works.
  • The code build (IPC Appendix F): a radon gas-permeable layer of clean gravel at least 4 in deep under the membrane (F105.3), a 4-mil polyethylene membrane over the layer with the laps sealed (F105.2), a 4 in nominal gas-permeable pipe (3 in minimum) run to the fan (F105.3), a suction pit under the slab with gravel and the pipe, and the mechanical exhaust at least 10 ft from any opening and at least 2 ft above the roof surface (F105.4).
  • System sizing: one suction point per 1,000 sq ft of conditioned floor footprint, with two points on a slab over 2,000 sq ft, a 10 ft minimum pipe run, a 1.5 in static pressure target, a 100 to 150 cfm fan on a residential slab, and a fan on an unswitched circuit with the service access at the fan. A long run, a ribbed slab, a vapor barrier, and a clay subgrade move the fan to the top of the band.
  • Crawl units: the ground gets a Class I vapor retarder with the laps sealed (IRC R316.1), the permeability layer under the membrane, the suction pipe through the membrane, and the fan in the crawl with the discharge to the exterior, 10 ft from an opening. A sealed crawl with the wall insulation and the conditioned air is the standard, and a crawl vent is not a radon system.
  • Sealing and pressure: the system works by depressurizing the soil against the house pressure, so the sealing list drives the result: slab cracks, sump lids, drain tile, rim joists, band joists, utility sleeves, pipe penetrations, floor-to-wall joints, mud caps, and the access door. A house with a bath fan, a kitchen fan, a dryer, and a garage door is a house pulling slab gas in, and the make-up air question is part of the system.
Planning tool on the EPA radon guidance, the EPA radon-resistant new construction details, and IPC Appendix F as adopted locally. The EPA radon zone map is a county map, the zone in this calculator is a selection, and the county assignment, the state mitigation certification, the licensing, the permit, and the real estate disclosure form are jurisdiction calls. A radon-in-water test on a private well, a uranium and gross-alpha question, a granite basement with a stone wall, a slab with radiant heat, a ribbed slab, a TJI floor with a rim cavity, a stack-effect house, a tightly sealed home with a heat recovery ventilator, a barometric-pressure venting system, a sub-membrane depressurization on a crawl, and a home with a sump and a French drain are all system-design calls, and a mitigation contractor with the state certification signs the system. A manometer, a test port, a fan tag, and the retest numbers are the record.

How big a fan do I need for radon?

A residential slab gets a 100 to 150 cfm radon fan at 1.5 in of static pressure, about 60 W, on an unswitched circuit. A 1,200 sq ft slab with one suction point and a short run runs on the bottom of the band, and a clay subgrade, a ribbed slab, a vapor barrier under the slab, and a long pipe run with three elbows push the fan to 150 cfm at 3 in of static (IPC Appendix F and the fan manufacturer curve).

How many suction points does a slab need?

One suction point per 1,000 sq ft of conditioned floor footprint, so 1 point on 1,000 sq ft, 2 points on a 2,000 sq ft slab, and 2 points on a slab with a garage, a porch, and a wing. A 10 ft pipe run to the point, a 4 in Schedule 40 pipe, and a run with the fewest elbows are the standard (IPC Appendix F).

What is the radon action level?

4 pCi/L is the EPA action level, and EPA asks you to consider mitigation at 2 pCi/L. A short-term test is 2 to 7 days in a lowest lived-in level with the closed-house condition, a long-term test is 90 days and over, and mitigation is followed by a retest 12 to 24 hours after the fan starts and a long-term test in the heating season.

Does a crawlspace need a radon system?

A crawl with exposed ground gets the membrane over the ground with the laps sealed, a permeability layer under it, and a suction pipe through the membrane to a fan, one point per 1,000 sq ft of crawl floor, and the crawl gets sealed and conditioned (IRC R316.1 and R316.2, IPC Appendix F). Open vents are not a radon system, and a crawl with a water problem gets the water fixed first.

Related tools

Crawlspace encapsulationThe membrane the system pulls gas under Insulation R-valueThe sealed crawl wall assembly Mold containmentThe crawl with a moisture and a growth history Water damage dry-outDrying a wet crawl before the membrane Sump pump sizingSame pit, other system