figure 4-7

Figure 4-7. Slab-on-Grade Radon Control Techniques

Sealing the Slab

The following techniques for minimizing radon infiltration through a slab-on-grade foundation are appropriate, especially in moderate or high potential radon areas (zones 1 and 2) as designated by EPA (see Figures 4-7 and 4-8). To determine this, contact the state radon staff.

  1. Use solid pipes for floor discharge drains to daylight or provide mechanical traps if they discharge to subsurface drains.
  2. Lay a 6-mil polyethylene film on top of the gravel drainage layer beneath the slab. This film serves both as a radon and moisture retarder. Slit an “x” in the polyethylene membrane at penetrations. Turn up the tabs and seal them to the penetration using caulk or tape. Care should be taken to avoid unintentionally puncturing the barrier; consider using riverbed gravel if available at a reasonable price. The round riverbed gravel allows for freer movement of the soil gas and has no sharp edges to penetrate the polyethylene. The edges should be lapped at least 12 inches. The polyethylene should extend over the top of the foundation wall, or extend under a monolithic slab-grade beam or patio, terminating no lower than finished grade. Use concrete with a low water/cement ratio to minimize cracking.
  3. Provide an isolation joint between the foundation wall and slab floor where vertical movement is expected. After the slab has cured for several days, seal the joint by pouring polyurethane or similar caulk into the 1/2-inch channel formed with a removable strip. Polyurethane caulks adhere well to masonry and are long-lived. They do not stick to polyethylene. Do not use latex caulk.
  4. Install welded wire in the slab to reduce the impact of shrinkage cracking. Consider control joints or additional reinforcing near the inside corner of “L” shaped slabs. Two pieces of No. 4 reinforcing bar, 3 feet long and on 12-inch centers, across areas where additional stress is anticipated, should reduce cracking. Use of fibers within concrete will also reduce the amount of plastic shrinkage cracking.
  5. Control joints should be finished with a 1/2-inch depression. Fill this recess fully with polyurethane or similar caulk.
  6. Minimize the number of pours to avoid cold joints. Begin curing the concrete immediately after the pour, according to recommendations of the American Concrete Institute (1980; 1983). At least three days are required at 70F, and longer at lower temperatures. Use an impervious cover sheet or wetted burlap.
  7. Form a gap of at least 1/2-inch width around all plumbing and utility lead-ins through the slab to a depth of at least 1/2 inch. Fill with polyurethane or similar caulking.
  8. Place HVAC condensate drains so that they run to daylight outside the building envelope, or to a floor drain suitably sealed against radon penetration. Condensate drains that connect to dry wells or other soil may become direct conduits for soil gas, and can be a major entry point for radon.
  9. Place a solid block course, bond beam, or cap block on top of all masonry foundation walls to seal cores, or fill open block cores in the top course with concrete. An alternative approach is to leave the masonry cores open and fill solid at the time the floor slab is cast by flowing concrete into the top course of block.
  10. Do not place HVAC ducts under the slab.
figure 4-8

Figure 4-8. Soil Gas Collection and Discharge Techniques

Intercepting Soil Gas

The most effective way to limit radon and other soil gas entry is through the use of active soil depressurization (ASD). ASD works by lowering the air pressure in the soil relative to the indoors. Avoiding foundation openings to the soil, or sealing those openings, as well as limiting sources of indoor depressurization aid ASD systems. Sometimes a passive soil depressurization (PSD, with no fan) system is used. If post-occupancy radon testing indicates further radon reduction is desirable, a fan can be installed in the vent pipe (see Figure 4-8).

Subslab depressurization has proven to be an effective technique for reducing radon concentrations to acceptable levels, even in homes with extremely high concentrations (Dudney 1988). This technique lowers the pressure around the foundation envelope, causing the soil gas to be routed into a collection system, avoiding the inside spaces and discharging to the outdoors.

A foundation with good subsurface drainage already has a collection system. The underslab gravel drainage layer can be used to collect soil gas. It should be at least 4 inches thick, and of clean aggregate no less than 1/2 inch in diameter.  The gravel should be covered with a 6-mil polyethylene radon and vapor retarder.

A 3- or 4-inch diameter PVC vent pipe should be routed from the subslab gravel layer through the conditioned portion of the building and through the highest roof plane. The pipe should terminate below the slab with a “tee” fitting. To prevent clogging the pipe with gravel, ten-foot lengths of perforated draintile can be attached to the legs of the tee, and sealed at the ends. Alternately, the vent pipe can be connected to a perimeter drain system, as long as that system does not connect to the outdoor environment. Horizontal vent pipes could connect the vent stack through below grade walls to permeable areas beneath adjoining slabs. A single vent pipe is adequate for most houses with less than 2,500 square feet of slab area that also include a permeable subslab layer. The vent pipe is routed to the roof through plumbing chases, interior walls, or closets.

A PSD system requires the floor slab to be nearly airtight so that collection efforts are not short-circuited by drawing excessive room air down through the slab and into the system. Cracks, slab penetrations, and control joints must be sealed. Floor drains that discharge to the gravel beneath the slab should be avoided, but when used, should be fitted with a mechanical trap capable of providing an airtight seal.

While a properly installed passive soil depressurization (PSD) system may reduce indoor radon concentrations by about 50%, active soil depressurization (ASD) systems can reduce indoor radon concentrations by up to 99%. A PSD system is more limited in terms of vent pipe routing options, and is less forgiving of construction defects than ASD systems. Furthermore, in new construction, small ASD fans (25-40 watt) may be used with minimal energy impact. Active systems use quiet, in-line duct fans to draw gas from the soil. The fan should be located outside, and ideally above, the conditioned space so that any air leaks from the positive pressure side of the fan or vent stack are not in the living space. The fan should be oriented to prevent accumulation of condensed water in the fan housing. The ASD stack should be routed up through the building or an attached garage or carport, and extend twelve inches above the roof. It can also be carried out through the band joist and up along the outside of wall, to a point high enough so that there is no danger of the exhaust being redirected into the building through attic vents or other pathways. Because PSD systems rely on natural buoyancy to operate, a PSD stack must be routed through the conditioned portion of the home.

A fan capable of maintaining 0.2 inch of water suction under installation conditions is adequate for serving subslab collection systems for most houses (Labs 1988). This is often achieved with a 0.03 hp (25W), 160 cfm centrifugal fan (maximum capacity) capable of drawing up to 1 inch of water before stalling. Under field conditions of 0.2 inch of water, such a fan operates at about 80 cfm.

It is possible to test the suction of the subslab system by drilling a small (1/4-inch) hole in areas of the slab remote from the suction point, and measuring the suction through the hole using a micromanometer or inclined manometer. The goal of a subslab depressurization system is to create negative air pressure below the slab, relative to the air pressure in the adjacent interior space. A suction of 5 Pascals is considered satisfactory when the house is placed in a worst-case depressurization condition (i.e., house closed, all exhaust fans and devices operating, and with the HVAC system operating with interior doors shut). The hole must be sealed after the test.

PSD systems require near perfection in sealing of openings to the soil, since the system relies on a 3- or 4-inch pipe to vent more effectively than the entire house. Sealing openings to the soil is less critical for radon control with ASD systems, although it is highly desirable in order to limit the energy penalty associated with conditioned indoor air leaking into a depressurized subslab, and from there to the outdoors. ASD fans have service lives averaging about ten years, with a higher life expectancy if the fan is protected from the elements. Since an ASD system may be turned off by occupants, service switches are usually located in areas with limited access.