What are the Dangers of Water Vapor Infiltrating a Home?

Before explaining the benefits of high-performance vapor barriers, it is crucial to understand the hazards of excess humidity in the home. The Environmental Protection Agency (EPA) states that "indoor relative humidity (RH) should be kept below 60 percent - ideally between 30 percent and 50 percent, if possible." Everyday household activities such as taking a shower, cooking, boiling water for tea, and washing your clothes are all sources of water vapor building up within a home. However, proper home ventilation systems will be able to maintain relative humidity levels. They do this by exchanging this moisture-filled air with drier air from the outside.

Excess water vapor in the home that leads to mold growth occurs when excess moisture is prevalent. Leaky pipes, cracks in foundations, and unsecured roofs can contribute to high indoor moisture levels. According to the University of Minnesota, signs of excess moisture in a home include:

• Condensation on windows in winter
• Excessive bathroom mildew
• Mold growth on interior surfaces, like in the corner of a closet, kitchen, or bathroom
• Peeling, cracking, or blistering paint on the exterior or interior finishes
• Excessive dust mite populations, which are a cause of dust allergies

Water vapor from underneath and around your home can also be a constant, menacing source of moisture that can negatively affect your home's air quality.

Where Should A Vapor Barrier Be Placed?

The most common area for vapor barriers is directly on top of the exposed soil in crawl spaces. However, high-performance vapor retardants can also be helpful in below-grade walls and floor slabs. Concrete walls or slabs certainly provide a strong foundation for a home, though they also tend to transmit ground moisture. Of course, vapor barriers are also an excellent addition around the entirety of cement slabs that make up basement walls and floors.

In humid and colder climates, builders must also place vapor retarders on the walls of homes. Generally, these barriers are placed on one side of an insulation sheet and must be placed on the wall's warm side.

Vapor barriers in the walls, slabs, or foundation of a home will only be effective if there is a focus on a sound, airtight building envelope. Studies have shown that almost 98 percent of the moisture transfer in homes occurs through cracks and gaps in walls, baseboards, and foundations. Even if you wrap your entire home in a high-performance moisture barrier, a leaky building envelope will still allow moisture and water vapor to enter your home.

Many home builders and contractors will attempt to cut costs using cheap polypropylene vapor retarders along the same lines. The rolls of this barrier are measured by their thickness. They generally come in standard thickness grades of 2-mil, 4-mil, or 6-mil. Unfortunately, many polypropylene vapor barrier products are manufactured from low-density resins, often scrap or reprocessed resins from other building components. These cheaper plastics can lead to holes and leaks in the material. In some cases, polypropylene can degrade due to the alkalinity in soils. It is close to impossible to replace a vapor barrier located underneath your home's concrete slab, so choosing a quality product is of utmost importance.

Why Choose a High-Performance Vapor Barrier?

Investing a little bit more money in high-performance vapor barriers is well worth the investment. High-performance vapor barriers are made from high-density polypropylene and are cross-laminated. The two plastic sheets are molded together with hot resin, similar to the way plywood is made. This cross lamination technique is much more efficient and effective at resisting water vapor and radon gas transmission.

For example, let's look at this 15-mil below-slab vapor barrier made by Stego Industries. It will not only ensure that water vapor doesn't seep into your home, but it can also help control the infiltration of soil gases. These can be naturally occurring gasses, like radon and methane, and can also include gases that result from soil contamination (from oil by-products, sulfates, and others) that affect the health of the indoor air.

Are Vapor Retarders the Same As Vapor Barriers?

No vapor barrier will be 100 percent effective at stopping moisture or vapor infiltration. So, the US Department of Energy prefers the term "vapor diffusion retarders" for a more accurate description.

Unless you live in Tucson, Arizona, the chances are that your new home construction will benefit from using high-performance moisture barriers. These barriers will only slightly increase the cost of your home construction. However, in the long-term, they can help protect your home from excess moisture and even lower your cooling bills in the summer and heating bills in the winter.