Checklist for Soil Gas Control and Mitigation — New Construction
Provide drainage layer under slab: Place a layer of crushed stone at least 4” (100 mm) thick under a concrete slab-on-grade or basement floor slab. Alternatively, a layer of sand can be provided that is overlaid with a layer or strips of manufactured geotextile drainage matting designed to allow the lateral flow of soil gases.
Install sub-slab moisture barrier: Install a rugged polyethylene soil gas/moisture barrier under the slab-on-grade or basement floor slab. The poly should be a minimum of 6 mils (0.15 mm) thick, 3 mils (0.08 mm) if cross-laminated. Separate sections should overlap a minimum of 12” (300 mm), and the barrier should be tightly fitting around pipes and other penetrations. Any tears or punctures should be sealed or covered with additional sections of the sheeting.
Install crawlspace membrane: For crawlspace or combination foundations, install the polyethylene sheeting described above to cover the entire crawlspace area. In this case, however, joints in the poly should be taped. The poly should also be taped to any piers, other penetrations, and foundation walls to ensure adequate depressurization of the sub-membrane area.
Provide mesh or fiber in slab: Include steel mesh or fiber in the floor slab to reduce cracking.
Use monolithic pour: With slab-on-grade foundations, use a monolithic pour to eliminate gaps and cracks at the junction between slab and frost walls.
Provide expansion joint: Where a basement floor slab meets the foundation walls, provide an expansion joint—this can be done by providing an intentional gap with a spacer, then caulking the gap. All other control joints, isolation joints, and construction joints should also be sealed with caulk or sealant.
Install vent pipe for passive sub-slab depressurization: This is a strongly recommended strategy in EPA’s Radon Zone 1 and on any agricultural or previously developed lands that are being built on. In other areas, it is a moderately recommended strategy. Install a 4”-diameter (100 mm) plastic pipe into the aggregate layer beneath the floor slab and poly soil gas barrier. This should terminate under the slab with a T-connection to provide firm support for the pipe. The vent pipe should extend up through the heated envelope and through the roof, terminating at least 12” (300 mm) above the roof surface in a location at least 10’ (3 m) away from any window or other opening or adjacent buildings. Also, provide an electrical junction box in the attic or wherever an in-line fan might later be installed, if needed. Ventilation pipes should be labeled at each floor level to avoid future confusion.
Install multiple vent pipes when necessary: On large buildings or buildings with interior footings that divide the floor slab into separate sections, provide multiple vent pipes for passive sub-slab depressurization (a minimum of one pipe per section of isolated sub-slab aggregate). Multiple vent pipes can be connected to a single vent that extends up through the building and terminates above the roof. A design alternative is to install “connect- ing” pipes through interior footings when they are poured, which allows the migration of soil gases between the sub-slab sections to a single vent pipe.
Caulk cracks and joints: Seal any cracks, gaps, and joints in the foundation floor and walls with caulk.
Remove grade stakes: Wooden grade stakes and screed boards used in pouring concrete floor slabs should be removed as the slab is being finished; otherwise they will eventually rot out, leaving open channels to the sub-slab aggregate.
Seal penetrations: Seal around all penetrations through foundation walls or floor (soil gas ventilation pipe, water supply pipes, fuel supply pipes, sewage pipes, drainage lines, wiring, etc.).
Seal sumps: Any basement sump should be fitted with a tight-fitting, sealed lid. Special sump covers for radon control are available from specialty suppliers.
Provide proper drainage: A continuous loop of perforated drainage tile or pipe should extend around the outside of the footings. Floor drains and this perimeter drain should extend to daylight, storm sewer pipes, or a sump.
Prevent backflow through condensate drains: Condensate drains should either include traps or be routed through non-perforated pipe to daylight.
Use care with CMUs: Avoid using standard concrete masonry units (CMUs) for foundation walls if possible, because CMUs are very difficult to keep airtight. Tight-fitting blocks with a continuous surface-applied, fiber-reinforced stucco coating on both sides are preferable. Hollow blocks should be capped at the top of the wall.
Dampproof foundation walls: Coat the outside of foundation walls with a low-VOC dampproofing coating.
Provide free-draining layer against foundation walls: Apply a drainage mat or board on the outside of foundation walls and/or provide a layer of free-draining aggregate against the wall. Silt should be kept out of the drainage layer with a filter fabric.
Seal crawlspace and basement ducts: Seal any ducts that extend through a crawlspace or basement with quality mastic. (It is preferable to avoid ductwork outside the heated envelope.)
Install sealed-combustion appliances: To avoid negative pressure in buildings, install only sealed-combustion furnaces, boilers, and water heaters.
Provide make-up air: Provide make-up air for clothes dryers and very large kitchen range hood fans.
Prefer balanced ventilation: Prefer a balanced ventilation system over an exhaust-only system to avoid depressurizing the building —though an exhaust-only system with effective sub-slab depressurization is acceptable and may be simpler than a fully ducted balanced system. In cold climates, use heat-recovery ventilators to reduce heat loss.
Ensure airtight construction: To minimize the stack effect from depressurizing the basement, keep the building as airtight as possible—particularly high up in the building.
Test for radon: Every home should be tested for radon. Tests should be conducted post-occupancy with an EPA-listed testing device, following established protocols.
Retrofit vent pipe with a fan if necessary: If measured radon levels in the building ever exceed 4.0 pCi/l, or if evidence emerges that significant levels of other soil gases are present, retrofit the passive sub-slab depressurization vent pipe with a high-quality, energy-efficient, in-line fan. To avoid possible leakage of sub-slab air into the buiding, install the fan outside the conditioned space and so that the pipe does not enter conditioned space past the point where the fan is installed.
