Vapor barriers are an important installation for all homes. However, vapor barriers in homes build on a slab or with a finished basement will not be visible or accessible. A vapor barrier should be made of 6-mil black polyethylene plastic and is designed to prevent moisture vapor in the underlying soil from reaching the homes building materials and causing issues, such as mold growth. Additionally, vapor barriers help to contain toxic gases that permeate through the soil, such as radon. The term vapor barrier, while commonly used, can imply that the material provides a full barrier for moisture and gases. However, vapor barriers do not completely stop these issues. Therefore, the term VDR (Vapor Diffusion Retarder) is more accurate.
Moisture prevention is vital for maintaining a healthy and structurally sound home. Damp air coming from the soil under a home encourages mold growth and can deteriorate the wooden components present throughout a home. Mold and damp wood can attract wood destroying organisms, such as termites and carpenter ants, as these pests are attracted to wood with an above average moisture content. Wood rot and decay caused by excessive moisture can lead to structural issues by weakening the affected framing components. Excessive moisture can also decrease the R-value (the capacity of an insulating material to resist heat flow) of your floor insulation and can even cause it to sag or fall.
Radon is a naturally occurring radioactive gas that is created by a breakdown of Uranium in the soil. Radon is the second leading cause of lung cancer in the US, and it is estimated that 1 in 4 homes in the Portland, Oregon metropolitan area has elevated levels of radon. VDR’s help to block radon from entering the home. VDR’s are not a cure all for radon gas, however, and should be accompanied by radon mitigation systems if radon levels in a home are high. The lack of a proper VDR can affect indoor air quality in general because they help to stop the transfer of mold spores and mildew, among other unhealthy pathogens.
Many older homes have VDR’s that are made of thin clear polyethylene plastic. The disadvantages of this type of plastic are that it is often more susceptible to damage and UV exposure, which can lead to accelerated degradation. Current building codes require VDR’s to be made of 6-mil black polyethylene.
VDR’s should completely cover the crawlspace floor, with no gaps or breaks. Adjoining sections can be taped to ensure they do not shift and expose areas of the underlying soil. If you are concerned about the integrity of your crawlspace VDR you should consider scheduling a home inspection with a qualified company to help ensure that it has been properly installed and is free of defects that can affect your home and its occupants.