Sub-Membrane Depressurization (SMD)

SMD’s are the most effective system for homes built on crawlspaces.

  • As of January 1, 2014
    • New Residential Construction requires:
    • In buildings with crawlspace foundations, the required components of a passive SMD must be installed during construction. (See Section 3 of this presentation).
    • This allows for the conversion to an active system via installation of a fan by a licensed professional.
    • 32 ILAC 422.150(h)(9)

Soil Gas Retarder / Vapor Barrier

  • A soil gas retarder membrane must be installed in basement or crawlspace areas without a concrete floor.
  • Crawlspace depressurization without the use of a soil gas retarder membrane should only be used when the crawlspace is inaccessible.
  • Note: The use of an HRV in a crawlspace does not exempt the mitigation contractor from installing a soil gas retarder.
  • 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.
  • Any seams in soil gas retarder membranes must be overlapped at least 12 inches and sealed in a permanent air tight 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.
  • 32 ILAC 422.150(h)(9)

Example of a Non-Compliant Crawlspace

  • This 1800 sf crawlspace had a small 150 sf area that was inaccessible.  The soil gas retarder was omitted and an HRV installed in lieu of a sub-membrane depressurization system and mitigation fan. 
  • This application is not only high maintenance and expensive for the homeowner, HRVs are typically not capable of reducing radon levels more than a pico curie or two.  Waste of time and money.

Example Of Correct Installation of Vapor Barrier

  • Seams overlapped properly
  • Vapor barrier is brought 12″ up the wall and sealed with compatible glue and furring strips
  • Membrane sealed correctly around interior piers