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Indoor airPLUS

Technical Guidance to the Indoor airPLUS Construction Specifications

1. Moisture Control

Sections 1.1 - 1.4: Water-Managed Site and Foundation

1.2 Capillary Breaks

Capillary breaks shall be installed as follows:

Beneath concrete slabs, including basement floors:

  • Install a 4-in. layer of ½-in. diameter or greater clean aggregate, covered with 6-mil (or thicker) polyethylene sheeting, overlapped 6 to 12 in. at the seams, and in direct contact with the concrete slab above; OR
  • Install a 4-in. uniform layer of sand, overlain with a layer or strips of geotextile drainage matting installed according to the manufacturer’s instructions, and covered with polyethylene sheeting overlapped 6 to 12 in. at the seams.

Crawl space floors:

  • Cover crawl space floors with a concrete slab over 6-mil (or thicker) polyethylene sheeting overlapped 6 to 12 in. at the seams; OR
  • Cover crawl space floors with 6-mil polyethylene sheeting (10 mil recommended), overlapped 6 to 12 in. and sealed or taped at the seams and penetrations. The sheeting shall be attached to walls and piers with adhesive and furring strips.

Exceptions:

  • In areas of free-draining soils — identified as Group 1 by a certified hydrologist, soil scientist, or engineer through a site visit — a gravel layer or geotextile matting is not required under concrete slabs.
  • Polyethylene sheeting is not required in Dry (B) climates, as defined by IECC Figure 301.1, unless the sheeting is required for radon resistance (see Specification 2.1).

Detailed Illustrations

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Basement slab w/capillary break - gravel and geotextile mat (inset)

Basement slab w/capillary break - gravel and geotextile mat (inset)

Crawl space - vapor retarder over soil

Crawl space - vapor retarder over soil

Crawl space floor - "Rat Slab" over polyethylene vapor retarder

Crawl space floor -

Best Practices

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Moisture-resistant basement flooring system (1 of 2)

Moisture-resistant basement flooring system (1 of 2)

Moisture-resistant basement flooring system (2 of 2)

Moisture-resistant basement flooring system (2 of 2)

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Why

Liquid water can be wicked up through the pores of a concrete slab by capillary action. This water may cause the failure of vulnerable building materials such as flooring adhesives, gypsum board, wood, and carpets that touch the damp concrete. However, a gravel layer beneath a slab provides a capillary break; the large spaces between the individual stones prevent liquid water from moving up to make contact with the slab. Geotextile matting provides the same capillary break and can be used as a substitute for gravel. The waffle-like or dimpled high-density plastic sheet or the matrix of plastic wire forms an air gap, and a synthetic filter fabric keeps poured concrete from clogging the matting materials.

Polyethylene sheeting, a vapor retarder, also provides a capillary break. A vapor retarder is necessary in crawl spaces to prevent the migration of water vapor from the earth into the crawl space and then into the house. A damp crawl space that has no vapor retarder over the earth floor can be the dominant water vapor source, humidifying the entire house. A vapor retarder installed beneath a basement floor slab allows low-vapor-permeability floorings such as vinyl tile or sheet goods to be applied over the slab. Without a vapor retarder beneath the slab, water vapor that migrates through a concrete slab can accumulate under low-perm floorings until the flooring adhesive fails. Sheeting is not required in dry climates because the ground generally contains less moisture so there is less potential for capillary movement.

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How

Preparations for pouring a concrete slab should include making sure the soil that will be beneath is level so that when the gravel is added, a consistent 4-in. minimum thickness is maintained. Polyethylene sheeting is then placed over the entire gravel area and touches each perimeter wall. Lengths of sheeting laid side-by-side must overlap at least 6 in. (12-in. overlaps are often easier because it can be difficult to cut long lengths of polyethylene in straight lines) and the two sheet surfaces sealed or taped together. A continuous bead of acoustical sealant, butyl rubber or butyl acrylic caulk forms the most durable bond. Seams may also be sealed with tape manufactured to seal or patch polyethylene, such as some builder's tapes and tapes used to repair polyethylene greenhouses. The final step is to pour the concrete slab over the sealed polyethylene sheeting.

EPA’s Indoor airPLUS Construction Specifications require crawl spaces to be designed to be "conditioned" (see Specification 1.4). Polyethylene sheeting shall be applied directly over the soil, with all lapped sheets sealed or taped as described above. However, the polyethylene must lap up each perimeter wall far enough to be sealed to the wall with a ½-in. wide bead of acoustical sealant or butyl caulk. Treated wood furring strips or other insect-resistant material must then be nailed or screwed to the wall to provide a permanent mechanical attachment of the up-turned polyethylene to the foundation walls. The polyethylene should also be sealed at any penetrations through the sheeting, such as steel columns or piers within the crawl space. Crawl space floors can also be covered with a thin concrete slab (sometimes referred to as a "rat slab") to protect the polyethylene sheeting. This slab prevents any tearing of the polyethylene (sustaining its vapor-retarding qualities) and resists entry of rodents. The thin slab is poured approximately 2 in. thick over the aforementioned polyethylene preparation.

See also 1.4 Best Practices - Crawl Spaces in Flood Zones

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References/Additional Information

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