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
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
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
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
CPEO "Stakeholders Guide to Vapor Intrusion" http://www.cpeo.org/pubs/SGVI.pdf
http://nepis.epa.gov/Adobe/PDF/P100AE72.pdf for a full description
of vapor intrusion technologies.
http://www.itrcweb.org/Documents/VI-1.pdf for regulatory guidance
on vapor intrusion.
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.