Chapter 5 - Quality Assurance & Quality Control
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Sealing Radon Pathways

The success of any mitigation approach, whether through pressurization or depressurization techniques, relies primarily on the ability to prevent soil gasses from entering the home from below the foundation. To accomplish this, the mitigation contractor must identify and seal off radon pathways.

However, sealing accessible radon entry points is only a piece of the mitigation puzzle and should never be used as a stand-alone approach. It is most effective when used in conjunction with ASD techniques to improve distribution of the pressure field surrounding the structure, which can dramatically enhance the performance of the system.

Caulks & Sealants

Selecting the appropriate caulk is key to the effectiveness and craftsmanship of the project. General caulk requirements include:

  • Long lasting
  • Sufficient elasticity to meet the expansion/contraction potential of the surface
  • Strong adhesion


Surface preparation is key to achieving long lasting adhesion. A good start is cleaning the surface with a wire brush and vacuuming any dirt or debris. Grounding small floor cracks with a hand grinder to assure a clean, adhesive surface may be required to adequately seal small cracks.  Note: workers should wear a fine particulate respirator to prevent inhalation of dust particles and subsequent silicosis.

A two-point bond is required to obtain a strong caulked joint that can flex and expand with the concrete. Using a bond breaker such as tape or leftover dirt from the groove will ensure the caulk does not adhere to the bottom of the surface. Floor-to-wall joints require a caulked bond with a break at the joint intersection. The expansion joint may serve as the bond breaker between the floor and the wall requiring the crack to be ground out twice as wide as the groove depth. The same ratio also applies to floor to wall joints.

Note: Caulks may contain toxic chemicals and off-gas toxic vapors warranting the use of respirators and latex gloves available to avoid skin exposure to the many (possibly toxic) solvents used with caulks. Material Safety Data Sheets should be made available to employees and homeowners as necessary.

Small Cracks and Joints

  • Cracks 1/4” wide or more, openings, and wall joints should be sealed with polyurethane or other expandable caulk.
  • Wall joints should be caulked at least 3/8” up the wall.

French Drains and Large Joints

French drains such as open basement dewatering systems are major radon entry pathways and can negatively impact the performance of an ASD. Also referred to as canal drains, this technique can be an adequate method to provide perimeter drainage without leaving an open airway to the living space. Flowable urethane has many advantages for sealing the perimeter edge of a canal drain. A backer rod should be installed in the drain approximately 1/2″ into the drain channel with the flat side up. Smaller sizes of backer rod can also be used to fill larger floor cracks before applying urethane sealant. This method leaves an air space below the backer rod for water migration from the base of the foundation into the sub-floor.  This gap also serves as an air channel to extend the pressure field of a sub-slab ventilation system.

Expansion Joints

Expansion joints between the floor slab and foundation wall are especially important in areas where expansive soils exist. Expansion material is typically 1/2″ wide fiberboard fastened to the foundation wall and adjacent concrete slab, allowing the slab to expand and contract in conjunction with control joints in the slab. The caulk should bridge the expansion joint covering 1/4” of the wall and floor, allowing movement of the slab without breaking the bond. Overflowing expansion material should be cut back 3/8” above the concrete surface. Gun-grade polyurethane that will not flow through expansion joint gaps should be used in lieu of flowable caulk that can run onto the floor.


Large holes that penetrate foundation surfaces and serve no functional purpose should be sealed.

Large Openings

Holes too large to be caulked should be sealed with urethane foam with good bonding characteristics. The primary drawback when using these sealants is adequately reaching hard to reach spaces. Mortar or non-shrink grout can also be used to fill holes, the best method for sealing penetrations into block walls.

Use of Sump Pits

Ideally, sump pits should only be used as a secondary suction point for mitigation systems unless radiant heat lines or other health, safety or economic issues must be considered. However, using the sump pit is accepted in many states and national radon standards. Sump pit covers should allow for homeowner servicing through a clear observation port. Sump pits should be sealed with a silicone or other non-permanent caulk or an airtight gasket.

Sump holes are typically sealed as part of a drain tile depressurization system in which the sump acts as a suction plenum maintained at a negative pressure. Sump pits can be sealed with metal, heavy plastic, or custom sump covers and gaskets.  The sump lid must be strong enough to withstand 155 lbs. and sealed to the floor. If the sump pit is designed to drain water it must include a trapped floor drain or a new, trapped floor drain must be installed adjacent to the pit with sub-slab plumbing draining into the sump. The lid should be sealed with silicon caulk for easy servicing and inspection.

Sump covers are typically made of durable plastic materials such as PVC, fiberglass, and polypropylene sheet including holes for anchors, gaskets, view ports, fittings for sump pump discharge pipes, electrical power lines, and sump hole depressurization pipes.

Floor Drains

Floor drains attached to pipes open directly to the soil (for example, a dry well) serve as pathways to entry. Grab samples or smoke sticks can be used to determine if the drain is a source of concern for radon entry. If necessary, the floor drain can be sealed with a proprietary drain insert, such as a small unit retrofitted into the existing drain and sealed with a weighted ring trap or a small rubber ball. Another option is a full-scale water trap, which is more expensive than the retrofit water trap but a more permanent installation.

Hollow Block Walls

It is extremely important to seal open cores at the top of block foundation walls in order to properly extend the pressure field of a BWD or ASD up into the wall.  If the block openings are accessible, they can be sealed with urethane foam, mortar, extruded styrene foam, lumber caulked to the block and existing sill plate, or urethane caulk if the gap is small enough. Two-part foam has a tendency to expand and cure faster than the one-part foam, and should be used to seal the void spaces with non-combustible backing material to prevent the foam from falling down through the cores before it sets.

If the open block tops are difficult to reach or if the void spaces need to be sealed in the middle of a block wall an alternative sealing method will be required. A tedious but effective approach is spraying two-part urethane foam into small holes drilled into each voice space of the block walls. This is the most effective approach when brick or stone veneer tops the course of open core block; when the block wall construction extends from footing to eave; or when the block walls are behind finished interior walls requiring blocks to be sealed from the exterior.

Porous Surfaces

If the block foundation wall is a major contributing radon source, it may be necessary to seal the porous block surface to extend or enhance the vacuum from a block wall or sub-floor suction system.  Good foundation sealant or several coats of waterproof paint will tighten the porous surface of the block face. Using a fan to pressurize the basement while the wall is being coated forces the coating into the block wall or open all windows and doors in the home to neutralize the negative pressure in the basement during application.

Exposed Soil [i.e. crawlspaces, unsealed floors, etc.]

Occasionally, areas of exposed soil may be encountered.  These areas may be larger than a hole and be a significant part of a crawl space or basement floor.  The areas may be closed with a concrete or mortar patch (depending on size) and urethane sealant applied at the edge where the new patch meets the original slab.  If the area is large, it may be appropriate to cover it with a membrane and apply a suction beneath the membrane (see SMD).

  • Crawl Space Membrane
    • Typically, a durable, high-density polyethylene. Minimum of 6-mil (3-mil cross-laminated) polyethylene or equivalent flexible material.
    • Seams in membrane should be overlapped at least 12 inches and sealed in a permanent airtight manner.
    • Membrane should be sealed around interior piers and the inside of exterior walls with furring strips and sealant.
  • Concrete
    • It is common to have dirt or unsealed brick floors in basements or crawlspaces of older homes where the ‘cellar’ was not an occupiable space. Because these homes were typically built before the turn of the century before the invention of central heating and cooling systems in which circulate air throughout the home, the unfinished floor was not a concern. However, in modern homes with mechanicals, that circulate indoor air, exposed soil within the envelope of the home is a health hazard and energy concern that must also be addressed for radon mitigation efforts to be successful.