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A variety of steps can be taken to reduce radon levels in homes under construction. These steps can be implemented with less expense and with greater effectiveness during construction, than they can after construction is completed. Steps that can be taken to reduce the risk of elevated radon levels in a new home fall into one of the following three categories:
- Reduction of soil gas entry routes including, for example, steps to avoid cracks in the concrete slab, sealing around penetrations through the slab and foundation walls, capping the top of hollow concrete block walls and sealing the tops of sumps.
- Reduction of building envelope depressurization and house air exfiltration that can increase soil gas entry including for example, avoidance of thermal bypasses throughout the house, providing outside air supplies for combustion appliances and providing adequate crawlspace ventilation.
- Provisions during construction enabling effective subslab suction after the house is built, if radon levels turn out to be elevated despite the preventive steps referenced above. These provisions include a 4" deep layer of clean crushed rock under the slab with an exterior of interior drain tile loop, which drains into a sump for which is stubbed up and capped outside of the house, or through the slab. Alternatively, one or more 1' lengths of PVC pipe can be embedded into the aggregate through the slab and capped at the top. These standpipes can later be uncapped and connected to a fan (or to a passive convection stack) if needed.
Techniques for radon mitigation which can be inspected during construction include the following:
- Installation of a layer of gas permeable material, placed under concrete slabs, including a uniform layer of clean aggregate 4" thick, or a layer of sand 4" thick, overlain by a layer of geotextile drainage matting.
- A minimum 6 mil polyethylene or equivalent flexible sheeting placed on top of the gas permeable layer prior to pouring the slab.
- The creation of an air-tight seal, such as non-shrink mortar, grouts, expanding foam around all large openings in contact with the soil.
- Sealing of smaller gaps around pipes, wires and other penetrations, with elastomeric joint sealant.
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- Sealing of all control joints, and construction joints in concrete slabs or between slabs and foundation walls, with continuous formed gaps and a subsequent layer of elastomeric sealant.
- Sealing of all french drains with backer rods and elastomeric joint sealant.
- Installation of continuous solid piping for all floor drains.
- Installation of gasketed or sealed lids on all sumps or sump openings.
- Sealing the top of all hollow concrete masonry foundation walls with solid material.
- Sealing of all joints, cracks and other openings in foundation walls below grade both interior and exterior surfaces.
- Waterproofing of the exterior surfaces of below grade foundations walls.
- Sealing of all ductwork which passes through crawlspace areas and beneath slabs.
- Sealing of air handling,units located in below grade areas.
- Sealing of all penetrations through floors above crawlspaces.
- Sealing of access doors.s between basements and adjoining crawlspaces.
- Ventilation of crawlspaces with passive foundation vents.
- Covering exposed soil in all crawlspaces with continuous layer of 6 mil polyethylene sheeting.
- Installation of perforated pipe under the aforementioned sheeting connected to a "T" fitting with a vertical standpipe installed through the sheeting, to the outside of the building.
- Labeling of all vent pipes with the wording, "radon reduction system".
- The installation of electrical junction boxes necessary to activate subslab depressurization systems.
Further information on radon reduction techniques is available from The United States Environment Protection Agency, State Departments of Health, and various code authoring organizations.
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