Construction techniques for minimizing radon infiltration into a crawl space are appropriate if there is a reasonable probability that radon is present.  To determine this, contact the state radon staff.

Figure 3-12 shows an example for a vented crawl space; Figure 3-13 shows a vented example. General approaches to minimizing radon include (1) evacuating soil gas surrounding the basement, (2) sealing joints, cracks, and penetrations in the foundation, and (3) venting the crawl space while creating a continuous air barrier at the crawl space ceiling.

For ventilated crawl spaces

  1. For ventilated crawl spaces, provide substantial outside air ventilation. Place vents on all four sides of the crawl space. A second more reliable radon control solution is to control and isolate the source as suggested for basement construction in Chapter 2.
  2. Place a 6-mil polyethylene vapor retarder over all exposed soil floor areas. Overlap edges 12 inches and seal with mastic. Seal edges to the interior face of the foundation wall.
  3. Construct floors above unconditioned spaces with a continuous air infiltration barrier. Tongue and groove plywood floor decking should be applied with butt joints continuously glued to floor joists with a waterproof construction adhesive. Seal all penetrations through the subfloor with caulk. Enclose large openings such as at bath tub drains with sheet metal or other rigid material and sealants.
  4. Avoid duct work in the crawl space if possible, but it may be installed providing all joints are securely sealed with mastic, and ducts are well-insulated.
  5. Make crawl space walls separating an attached vented crawl space from a basement or living space as airtight as possible.

For unvented crawl spaces

Sealing the Crawl Space Floor

  1. Use solid pipes for floor discharge drains to daylight, or mechanical traps that discharge to subsurface drains.
  2. Use a 6-mil (minimum) polyethylene film beneath the slab (if one is present) on top of the gravel drainage bed. This film serves as a radon and moisture retarder and also prevents concrete from infiltrating the aggregate base under the slab as it is cast. Slit an “x” in the polyethylene membrane to receive 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 rounded riverbed gravel if possible. The riverbed gravel allows for freer movement of the soil gas and also offers no sharp edges to penetrate the polyethylene. The edges of the film should be lapped at least 12 inches. The polyethylene should extend over the top of the footing, or be sealed to the foundation wall.
  3. Tool the joint between the wall and slab floor (if present) and seal with polyurethane caulk, which adheres well to concrete and is long-lasting.
  4. Avoid perimeter gutters around the slab that provide a direct opening to the soil beneath the slab.
  5. Minimize shrinkage cracking by keeping the water content of concrete as low as possible. If necessary, use plasticizers, not water, to increase workability.
  6. Reinforce the slab (if present) with wire mesh or fibers to reduce shrinkage cracking, especially near the inside corner of “L” shaped slabs.
  7. Where used, finish control joints with a 1/2-inch depression and fully fill this recess with polyurethane or similar caulk.
  8. 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 70 °F, and longer at lower temperatures. Use an impervious cover sheet or wetted burlap to facilitate curing. The National Ready Mix Concrete Association suggests a pigmented curing compound should also be used.
  9. 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.
  10. Do not install sumps within crawl spaces in radon-prone areas unless absolutely necessary. Where used, cover the sump pit with a sealed lid and vent to the outdoors. Use submersible pumps.
  11. Install mechanical traps at all necessary floor drains discharging through the gravel beneath the slab.
  12. Place HVAC condensate drains so that they drain to daylight outside the building envelope or to sealed sumps within the basement. Condensate drains that connect to dry wells or other soil may become direct paths for soil gas, and can be a major entry point for radon. At the very least make sure these condensate drains are properly trapped so that the full diameter of at least the section of an elbow is always filled.

Sealing the Crawl Space Walls

  1. Reinforce walls and footings to minimize shrinkage cracking and cracking due to uneven settlement.
  2. To retard movement of radon through hollow core masonry walls, the top and bottom courses of hollow masonry walls should be solid block, or filled solid. If the top side of the bottom course is below the level of the slab, the course of block at the intersection of the bottom of the slab should be filled. Where a brick veneer or other masonry ledge is installed, the course immediately below that ledge should also be solid block.
  3. Parge and seal the exterior face of below-grade concrete masonry walls in contact with the soil. Install drainage boards to provide an airway for soil gas to reach the surface outside the wall rather than being drawn through the wall.
  4. Install a continuous dampproofing or waterproofing membrane on the exterior of the wall. Six-mil polyethylene-lapped, taped and placed on the exterior of the crawl space wall surface will retard radon entry through wall cracks.
  5. Seal around plumbing and other utility and service penetrations through the wall with polyurethane or similar caulking. Both the exterior and the interior of concrete masonry walls should be sealed at penetrations.
  6. Install airtight seals on doors and other openings between an unvented and adjoining vented crawl space.
  7. Seal around ducts, plumbing, and other service connections between an unvented and a vented crawl space.
  8. Do not place air supply or return ducts under the slab or in the footing.

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 3-13).

Subslab (or sub-membrane) 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 subslab (or sub-membrane) 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 moisture retarder. 

A 3- or 4-inch diameter PVC vent pipe should be routed from the gravel layer through the conditioned portion of the building and through the highest roof plane.  The pipe should terminate below the slab or membrane 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 the 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 or membranes. A single vent pipe is adequate for most houses with less than 2,500 square feet of floor 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 to be nearly airtight so that collection efforts are not short-circuited by drawing excessive room air down through the air barrier and into the system. Cracks, penetrations, and control joints must be sealed. Sump hole covers should be designed and installed to be airtight. 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.

Another potential short circuit can occur if the subdrainage system has a gravity discharge to an underground outfall. This discharge line may need to be provided with a mechanical seal. The subsurface drainage discharge line, if not run into a sealed sump, should be constructed with a solid-glued drainpipe that runs to daylight. The standpipe should be located on the opposite side from this drainage discharge.

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.

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.

figure 3-12

Figure 3-12. Crawl Space Radon Control Techniques

figure 3-13

Figure 3-13. Soil Gas Collection and Discharge Techniques

For more information visit Water Managed Foundations within the Building America Solution Center.