Should You Put Plastic Under Your House in the Crawl Space?

Updated February 21, 2017

Plastic sheeting, also called a vapour barrier or a moisture barrier, is often installed in crawl spaces to help keep moisture out of a home. If you are experiencing moisture problems, often indicated by the growth of moulds or a musty smell, consider installing a vapour barrier in your crawl space.

Moisture Problems

Excess moisture can cause a host of problems inside your home. Ground moisture evaporating up through a crawl space and into the subfloor above can cause hardwood floors to rot and vinyl sheeting to peel. Excess moisture can also cause paint and wallpaper to peel, increasing the likelihood of insect infestation, or produce mould and mildew growth. Mold and mildew growth is perhaps the most serious of these problems because exposure to mould and mildew can result in significant health problems, including eye irritation, asthma attacks, and other respiratory problems. A vapour barrier can help abate this spread of moisture on your home's interior.

Vapour Barrier Types

Vapour barriers are made from several different materials, the most common being polythene plastic and roofing paper. For crawl spaces, choose a vapour barrier with a permeability value of 1.0 or less. The best materials for crawl spaces include polythene plastic and asphalt-laminated paper.


Vapour barriers should be laid directly on the floor of the crawl space to stop moisture from coming in at its source--the ground. If you have hardwood floors above the crawl space, install the vapour barrier over 2/3 of the crawl space floor; this proportion keeps the majority of moisture away but lets enough in to prevent the hardwood from drying out. For carpet or vinyl floors, lay the vapour barrier across the entire crawl space floor. Once the vapour barrier has been laid, use electrical tape to secure the sheets together. Finally, consider spreading a 1- to 2-inch layer of gravel on top of the vapour barrier. This helps prevent condensation and wear and tear from shifting.

Additional Solutions

Installing a vapour barrier is only one way of controlling moisture. Other measures include running a dehumidifier to keep the relative humidity inside your home between 30 and 50 per cent, installing ventilation ports, storm windows, and insulation, managing external water so that it drains away from the home, and reducing moisture-generating activities inside the home.

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About the Author

Thomas King is a graduate of the University of Pittsburgh School of Law where he served as managing editor of the "Pittsburgh Journal of Environmental and Public Health Law." He currently lives in Aberdeen, Washington where he writes and practices law.