Gas and vapor barriers, also known as membranes, consist of materials or structures used to prevent gas and other vapors from penetrating a surface. A prominent use of such barriers is to place them beneath a building to prevent gas and vapor intrusion. They are installed under new slabs during construction. When set correctly, they force soil gas vapor to move beyond the building cavities and stop it from further infiltration. If the likelihood of vapor intrusion is small, a precautionary liner can be installed and later upgraded in conjunction with more robust vapor mitigation measures.

Materials packaging is another industry that makes extensive use of gas and vapor barriers in the form of laminated vapor barrier films. The flexibility and durability of these barriers help keep food and liquids fresh by preventing exposure to oxygen, which impairs the flavor. Moisture sensitive equipment, used by the military and other industries, is also commonly shipped in containers lined with vapor barrier film. The film is also widely used by the museum industry for lining boxes being sent out for exhibition purposes to reduce tarnishing of the showpieces and for insect de-infestation.

Types of Gas and Vapor Barriers

There are a variety of barriers used to help prevent gas and vapor intrusion, including:

Sheet membrane: Commonly made from 40 to 60 mil high-density polyethylene (HDPE).

Fluid applied membranes: These cured-in-place membranes are sprayed on prior to pouring the slab. They should be at least 30 mils to avoid damage during the placement process and construction of the building, as well as to withstand ground motion or aging.

Peel and stick membranes: Commonly used for waterproofing, these membranes quickly and easily apply to a surface.

Vapor barrier in an SMD (sub-membrane depressurization) system: These membranes should have a minimum thickness of 6 mils, according to New York State guidance. The state also recommends the membrane be of polyethylene or similarly flexible sheeting. It needs to cover the entire floor area and be appropriately sealed.

Laminated vapor barrier films: Prevents oxygen and moisture from entering into and degrading the contents of the containers. Also known as a "bag in a box," they are available in a broad range of types for various uses. They are sealed against heat if necessary. The films provide efficient service for the money: a six mil sheet of laminated vapor barrier can resist the ingress of gasses and vapors more adequately than a ½ inch thick sheet made up of acrylic glazing.

Mechanism

Vapor barriers placed under a slab during construction are 30 mils or higher, as membranes of lighter weight tend to fail during the building process. The membranes should not be stretched too tight to avoid rips and tears. Common usage is in existing buildings that contain a crawlspace.

Vapor barriers installed under a building are not solely intended to keep out gas and vapors, except in cases where subsurface conditions are shown to accommodate natural venting. This is due to small imperfections in the membranes that allow soil gas to pass through the barriers. Therefore, vapor barriers need to be tested on a regular basis to identify and repair any flaws. If the barrier is installed along with a sub-slab depressurization system, the membrane should be located about one foot under the foundation slab and at a maximum elevation of six inches above the gas collection piping.

Laminated films feature a variety of materials for different uses. Their characteristics vary and depend on the film's intended use. Primary laminates have an inner layer that exhibits high resistance to the passage of water vapor. In most cases, this material consists of aluminum, or a transparent polymer such as Aclar or Saran. This water-resistant layer is located between layers of polyethylene and acts as a heat-sealable adhesive on the inside. The outside polyethylene layer allows for laminating the vapor barrier to a film on the surface. The outside material provides added benefits to the completed film, such as puncture resistance or printability. It protects the vulnerable inner layer and reduces permeability to elements, such as oxygen and nitrogen.

Polymer film is also used for an outer layer after it has been stretched to orient it. This helps align its molecular bundles into a tighter grouping, thereby making the film tougher and more difficult for atmospheric gasses to penetrate.

To ensure an adequate seal, heat-sealing a film barrier made from polyethylene is mandatory. Sealing is accomplished by joining two dull sides together, followed by the application of heat. Most transparent films with two identical sides have a note on the outside edge identifying the side that needs to be sealed.

Vapor barrier films are also used to treat insect-infected objects. The extermination is accomplished by using nitrogen to replace the majority of oxygen contained in a vapor barrier bag. An oxygen absorber functions to reduce the concentration of oxygen to less than 1%. An alternate technique uses high levels of carbon monoxide to achieve the same result.

Applications

Gas and vapor barriers have an array of applications across many industries, including:

• Construction
• Existing buildings
• Museums
• Materials packaging
• Waterproofing
• Automotive
• Military  

Selecting Gas and Vapor Barriers

Gas and vapor barriers consist of an extensive range of types, shapes, and sizes. To select the best barrier for a particular application, first determine the type that is suitable for the job. Membrane barriers come in different levels of thickness. As a rule of thumb, the thickness of the barrier increases with heavier duty applications. If an application requires heat sealing, check to make sure the material used is amenable to such treatment.

Specifications and Standards

Relevant ASTM standards for gas and vapor barriers include ASTM D1434, ASTM D543, and D412.

References

Image credit:

Polyguard Products, Inc.