Crawlspace and Basement Design: Treating Only Half the Foundation
A homeowner has a 2,400 sq ft house with a finished basement and an attached vented crawlspace with a bare soil floor. Pre-mitigation testing reads 11.4 pCi/L in the basement and 8.9 pCi/L in the first-floor living room. The crawlspace has open foundation vents and a soil floor; the basement has a sump pit connected to a foundation drain. The contractor proposes a single SSD suction point in the basement slab with 3-inch pipe and a 1/3 hp fan, and no treatment of the crawlspace because it is vented and not a living space. The homeowner wants the lowest bid and asks whether the crawlspace work can be skipped.
1. Critique the proposed design.
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The design treats only part of the foundation. A vented crawlspace with bare soil is not a harmless buffer: stack effect and pressure differences draw soil gas from the crawlspace into the living space above through floor gaps, penetrations, and the rim joist. The first-floor result of 8.9 pCi/L, nearly as high as the basement, is consistent with a crawlspace contribution.
A single basement suction point also leaves the sump as a potential short circuit and does nothing to reduce crawlspace soil-gas entry. Skipping the crawlspace would likely leave elevated radon in the living space and would not meet the intent of a mitigation standard that addresses all soil-contact areas.
The lowest bid is not the lowest risk. The design should be judged by whether it can achieve a verified result below the action level across the occupied spaces, not by whether it includes the fewest components.
2. What design would you specify, and why?
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Specify a sub-membrane depressurization system for the crawlspace: a reinforced polyethylene membrane, typically 6 to 10 mil or heavier where required, sealed to the foundation walls and piers, with a suction point and a fan sized for the crawlspace volume and soil. If the basement and crawlspace communicate, one fan with separate suction points and balancing may work, but separate systems are often more reliable and easier to verify.
Seal the sump pit with a gasketed lid and tie it in if it is a communication path. Keep the foundation vents if they are required for moisture control, but the membrane is the radon control barrier. Route discharge above the roofline and label the system.
The design must address both soil-contact areas because the measurement data show an elevated first-floor concentration, not just a basement problem.
3. How would you verify that the design is working?
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Verify with a communication test: measure vacuum under the membrane at the farthest point from the suction, confirm the manometer reading at the fan, and check that the membrane is sealed and not ballooning excessively. A smoke test can reveal leaks at seams and wall terminations.
Run a post-mitigation test in the basement and first floor after at least 24 hours of operation, ideally 24 to 48 hours, and compare with the pre-mitigation values. Target below 4 pCi/L, ideally below 2.7. Provide a label and operation and maintenance information, including a reminder to keep the membrane intact and the sump lid sealed.
If the post-mitigation result remains elevated, recheck pressure-field extension and look for untreated soil-contact areas or air leaks before adding equipment.