Vapor barriers (also known as vapor membranes) are materials or structures installed below a building to block the entry of vapors. The most common application for vapor barriers is installation under new slabs during construction. If installed properly, sub-slab liners cause soil gas that would otherwise enter the building to migrate laterally beyond the building footprint. Where there is a sub-structure vapor depressurization system, a vapor barrier often enhances its performance. Where there is only a slight possibility of vapor intrusion, a vapor barrier is often installed as a precautionary measure that can be upgraded to a mitigation measure if needed.
In practice, vapor barriers are difficult to properly install due to the likelihood of punctures, perforations, tears, and incomplete seals. Since a single opening can defeat the function of the entire barrier, vapor barriers, by themselves are not an acceptable vapor intrusion mitigation system. Most regulatory agencies consider barriers helpful, but not reliable in the long run as stand-alone mitigation.
There are several types of barriers that are used to mitigate or help mitigate vapor intrusion. The most common types are sheet membranes made of 40 to 60 mil high-density polyethylene (HDPE), and fluid applied (cured-in-place) membranes that are sprayed on before the slab is poured. The membranes should be durable enough (at least 30 mil) to prevent damage during placement, building construction, remodeling, or maintenance, or to resist failure due to earth movement and age.
In buildings with a crawlspace foundation, a vapor barrier may be used in conjunction with the installation of a sub-membrane depressurization (SMD) system. New York State Guidance (2005) recommends a membrane of polyethylene or equivalent flexible sheeting with a minimum thickness of 6 mils. The barrier should cover the entire floor area and be sealed at the seams and penetrations. During the installation the sheeting should not be pulled too tight, because when the depressurization system is turned on, the membrane will be drawn down which may cause strain on the seals.
Limitations and Concerns
Vapor barriers should not be used as stand-alone mitigation for vapor intrusion, unless subsurface conditions are demonstrated to be conducive to natural venting.
Vapor barriers should be at least 30 mil and higher. Lighter weight membranes have a tendency to fail during construction. Vapor barriers should be more robust if only a passive ventilation system is anticipated.
Small imperfections in the barriers (e.g., due to holes, tears, or incomplete seals at the footings or pipe penetrations) may provide a significant migration route for soil gas when buildings are under negative relative pressure (compared to soil gas pressure). Such imperfections easily occur: For example, some studies of flexible membrane liners used for liquid containment in impoundments have shown that even placing sand and other earth materials are likely to cause puncturing. In some circumstances, a vapor barrier may exacerbate a vapor intrusion problem by directing vapors that have collected under a large slab through one puncture into a smaller airspace (room). Thus, vapor barriers should be tested before occupancy and periodically afterwards, usually by blowing smoke or some tracer gas under the membrane. If smoke is detected inside the building, the vapor barrier is not protective. Additionally, methods should be developed to pinpoint imperfections and repair them after installation.
There is a need to include thorough quality control procedures, including training of construction workers, to minimize barrier damage during installation and subsequent construction.
Institutional controls should warn future owners and occupants of the importance of maintaining the integrity of the barrier—that is, not taking action likely to penetrate it.
Studies of homes with crawlspaces have revealed that the most frequent problems involve tears or rips in the membrane or inadequate sealing around the edges of vertical pipes. Vapor barriers laid over the ground in crawlspaces do not reliably prevent vapor intrusion.
Obtaining a good seal around pipes and other protruding objects can be problematic. Most vendors of spray-on type membranes do discourage mixing the two types (sheet and spray-on) of barriers.
In all cases, cracks or holes in the slab (not just the barrier) should be sealed with impermeable, but flexible material.
If a vapor barrier is to be installed in conjunction with a sub-slab depressurization system, it should be placed a maximum of one foot below the foundation slab and a maximum of six inches above the gas collection piping.
Vapor barriers are primarily used in new construction as a relatively inexpensive aid to vapor intrusion mitigation. Vapor barriers may also be used in some types of existing structures, typically those with a crawlspace, but only when they are used in conjunction with another mitigation measure.
Technology Development Status
Several types of vapor barriers are commercially available.
Because vapor barriers are not viewed as a stand-alone approach for mitigating vapor intrusion, there are no dedicated websites for them except for vendor information. Some of these are located in the next section. As a policy, CPEO does not endorse vendors.
Other Resources and Demonstrations
See CPEO "Stakeholders Guide to Vapor Intrusion" http://www.cpeo.org/pubs/SGVI.pdf
See http://nepis.epa.gov/Adobe/PDF/P100AE72.pdf for a full description of vapor intrusion technologies.
See http://www.itrcweb.org/Documents/VI-1.pdf for regulatory guidance on vapor intrusion.
See http://www.dtsc.ca.gov/sitecleanup/upload/VI_Mitigation_Advisory_Apr09.pdf for California's 2009 Vapor Intrusion Mitigation Advisory.
See also http://www.epa.gov/tio/download/citizens/a_citizens_guide_to_vapor_intrusion_mitigation_.pdf and http://www.serdp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Emerging-Issues/ER-200423.