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Radon Vent Retrofit, continuedAdding a secondary vent. With a straightforward floor plan, a single slab vent is usually enough to do the job. In this case, though, the built-in garage was supported by its own continuous strip footing, effectively isolating that section of the sub-slab from the finished area served by the initial sump. (Radon isn't ordinarily a concern in a single-story garage, but if there's living space above — as there was here — the area must be treated.) To deal with that, we created an additional sump in the garage floor and connected it back to the main vent stack with a long run of 2-inch PVC (Figure 4).

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Figure 4. A second hole was hammered into the garage slab to create an additional vent sump (left). The vertical pipe was cemented in place with its open end extending just beneath the underside of the slab, minimizing the risk of blockage by an unexpectedly high water table (right). The uncemented cap on the vent line allows for later expansion, if needed.

Fortunately, we managed to route the pipe through a closet, under a flight of stairs, behind a bathtub in the downstairs bathroom, and back into the utility room, keeping it out of view and making life easier for the finish carpenter. To make sure that some future plumber doesn't connect a sink drain to the radon-reduction system, the piping is identified with stickers (Figure 5).

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Figure 5. The radon-reduction piping is clearly marked at regular intervals to ensure that it won't be confused with plumbing drains.

Vent Stack and Fan

With the inside piping done, we connected the vent pipe to the fan and ran it up the exterior wall and through the gable-end overhang (Figure 6). Purely by chance, an existing weatherproof outlet was perfectly positioned to bring power to the fan motor. After we finished up, the builder's carpenter built an enclosure around the fan and boxed in the exposed pipes.

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Figure 6.External vents always require a fan to provide a reliable draft. The vent pipe was secured to the fan housing with standard neoprene plumbing connectors before continuing up the wall of the house (left). The finished assembly — located on an inconspicuous outside elevation — was later boxed in for a more finished appearance (right).

Evaluating the Completed System

Before switching on the fan for the first time, we attach a U-tube manometer to an easily visible location on the vent pipe so we can confirm that we're getting the required vacuum. In this case, we found that the system was pulling 1 1/8 inches of water, which translates to about 80 cfm (Figure 7).

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Figure 7. A vacuum monitor installed in the vent pipe confirms that the fan is providing the required negative pressure, although further testing is needed to determine whether indoor radon levels have actually declined.

Where's the air coming from? That was quite a strong airflow, but a single vacuum reading tells relatively little about how well the system is operating. Without investigating further, there's no way to know whether it's depressurizing the sub-slab — as it should be — or drawing the entire volume of air through one or two concealed pathways near the vent sump. To rule out the latter, our usual approach is to drill a small hole in the corner of the slab farthest from the sump and install a second vacuum gauge. A solid reading there as well strongly suggests that the entire sub-slab is under negative pressure. At that point, we remove the second gauge, patch the hole, and prepare to test the post-remediation radon level to be sure it's been reduced to a safe level. Final test. In this case, though, the radiant heat coils in the slab prompted us to skip that step, because we didn't want to risk puncturing the tubing. Instead, we went directly to the final test, which involves setting up a pair of electronic monitoring devices, each of which prints out a record of the current radon level once every hour (Figure 8). If the numbers we came up with were still too high, we'd have to bore additional holes in the slab and add additional vent pipes to bring them down.

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Figure 8. An electronic radon-monitoring device continually tests the amount of gas present. Once each hour, it prints out the current radon level as well as an average that factors in all previous readings.

But four days later, when we evaluated the printouts (Figure 9), we found that both machines had recorded levels far below the 4 pCi/L action level. (One recorded a four-day average of .2 pCi/L, while the other, some distance away, showed .5 pCi/L.)

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Figure 9. After four days of continuous testing with two separate monitors, the highest average recorded came out at .5 pCi/L — comfortably below the 4 pCi/L action level.

While those figures may fluctuate slightly over time — they may rise slightly when the house is closed up tight during the winter months, for example — they'd never approach the action level, let alone the much higher levels seen originally.


Peter Beaupreis owner of Sunrise Home Inspection in Lyme, N.H.

For Further Information

EPA/625/2-91/032, Radon Resistant Construction Techniques for New Residential Construction, U.S. Environmental Protection Agency, 800/490-9198.