Chapter 5 - Quality Assurance & Quality Control
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Active and Combination System Design

A) Sub-Slab Soil Depressurization [SSD] or Active Soil Depressurization [ASD]

The most common radon control and reduction technique designed to achieve lower sub-slab pressure relative to indoor air pressure by use of a fan-powered vent drawing air from beneath the concrete slab. When powered by a fan, these systems are called Active Soil Depressurization [ASD]. The fan’s velocity creates suction below the foundation and vents radon gas and its byproducts into the ambient air, typically to an area the above the highest eave of the house. Pipe routing may be interior or exterior and the discharge point must be high enough to prevent re-entrainment.

  • Rely on ventilation powered by a fan to draw radon from under the home to the outside.
  • Can originate as a passive system that is activated by installing a fan onto the existing vent pipe.
  • Often retrofitted to an existing building by installing a suction point in an unfinished area of the property. A fan is attached to a segment of the vent piping in an unconditioned area or exterior of the building.

B) Sub Membrane Depressurization [SMD]

Sub Membrane Depressurization is performed in crawl spaces and areas that are directly in contact with rock or soil. Suction is created under a specified polyethylene or equivalent flexible material (plastic sheet) permanently installed over exposed soil or rock. A fan drawing radon from beneath the plastic sheet exhausts the radon outdoors above the highest eave of the house.

C) Drain Tile Depressurization [DTD]

Drain Tile Depressurization means a type of active soil depressurization system where the suction point piping attaches to a drain tile or is located in the gas-permeable material near the drain tile. The drain tile may be inside or outside the footings of the building.

D) Block Wall Depressurization [BWD]

Block Wall Depressurization means a radon mitigation technique that depressurizes the void network within a block wall foundation by drawing air from inside the wall and venting it to the outside.

E) Crawlspace Depressurization [CSD]

Crawlspace depressurization is not ideal due to the great potential for hazardous back drafting and high energy loss associated with its operation during the colder and hotter months. Sub-slab and sub membrane depressurization are the crawlspace mitigation methods that should be used whenever possible.

  • When CSD is used for radon mitigation, cracks and openings in floors above the crawlspace, which would permit conditioned air to pass from the living spaces to the crawlspace must be sealed to the extent practicable to prevent re-entry.
  • Openings or cracks that are determined to be inaccessible or beyond the ability of the contractor to seal shall be disclosed to the client and included in the documentation.
  • CSD must not be used as a radon control system in the following circumstances:
    • When combustion appliances are installed within the crawlspace, or within an abutting crawlspace or basement.
    • Where the depressurized space cannot be adequately isolated between interior spaces containing one or more combustion appliances and the crawlspace.
    • When friable asbestos material exists in the crawlspace, or when work in the crawlspace would render non-friable asbestos material friable. If asbestos is to be removed from the crawlspace during the installation of a CSD system, the contractor must employ qualified asbestos removers in compliance with EPA and OSHA regulations.