Chapter 5 - Quality Assurance & Quality Control
2 of 3

Passive Sub-Membrane Depressurization [SMD]

In buildings with crawlspace foundations, the following components of a passive SMD must be installed during construction.  Depressurizing beneath the plastic membrane is carried out in a similar manner to a concrete floor SSD system. This is typically performed by installing perforated pipe below the membrane or a wood ring or a combination of both.

Soil Gas Retarder (aka Vapor Barrier or Membrane)

*Remember, this section pertains to New Construction so this soil gas retarder goes below the slab.

A minimum 6-mil (or 3-mil cross-laminate) polyethylene or equivalent flexible sheeting material must be placed on top of the gas permeable layer prior to casting the slab or placing the floor assembly to serve as a soil gas retarder by bridging any cracks that develop in the slab or floor assembly and to prevent concrete from entering the void spaces in the aggregate base material.

  • The sheeting should cover the entire floor area with separate sections of sheeting overlapped at least 12 inches.
  • The sheeting should fit closely around any pipe, wire or other penetrations of the material.
  • All punctures or tears in the material should be sealed or covered with additional sheeting.

Membrane Installation

  • Any seams in soil gas retarder membranes must be overlapped at least 12 inches and sealed in a permanent airtight manner using compatible glues.
  • The membrane must also be sealed around interior piers and to the inside of exterior walls with furring strips and compatible glues.
  • Excellent Sub-Membrane Depressurization System
  • Sealed nicely at the joints/openings.
  • A plumbing tee or other approved connection must be inserted horizontally beneath the sheeting and connected to a 3- or 4-inch diameter fitting with a vertical vent pipe installed through the sheeting.
  • Vent pipes must connect to a single vent that must terminate:
  • at least 12 inches above the highest roof in a location
  • at least 2 feet above any window or other opening into the conditioned spaces of the building and
  • 10 feet from any window or other opening in adjoining or adjacent buildings.

Crawlspace Preparation

  • Crawlspaces must be provided with vents to the exterior of the building. The minimum net area of ventilation openings must comply with Section R408 of the 2012 International Residential Code for One- and Two-Family Dwellings (copyrighted 2011 by the International Code Council, Inc.;).
  • The soil in crawlspaces must be covered with a continuous layer of minimum 6-mil polyethylene soil gas retarder.
  • The ground cover must be lapped a minimum of 12 inches at joints and must extend to all foundation walls enclosing the crawlspace area.
  • Use an appropriate membrane to withstand damage or lay a sub-membrane underneath to protect from rips, tears or separation.
  • Remove any construction debris (glass, wire, rebar etc.).
  • Watch for asbestos-covered ductwork has been routed and/or discarded.
  • Protect skin and clothing from insecticides that may have been sprayed in these areas.
  • If unsafe conditions are discovered, cease all operations until proper procedures for dealing with hazards in accordance to state, federal, or OSHA guidelines can be implemented.