Vapor Barriers: Your High-Performance Guide
Last Updated: Apr 7, 2025There is nothing worse than coming home from a long vacation to a musty, humid home that smells like your great-grandmother's cold room where she would store canned tomatoes.
Excess humidity levels in homes can lead to uncomfortable heat levels and force homeowners to run their air conditioners more intensively. An 80-degree day in high humidity New Orleans is much less comfortable than those 100 degree days in the arid regions of the Southwest. Moisture in the home can also lead to mold and mildew growth that negatively affect your indoor air quality.
To lower humidity and moisture levels in homes, many homeowners focus on improved ventilation systems. These can include exhaust fans, attic vents, and even heat recovery ventilators (HRVs) or energy recovery ventilators (ERVs). Proper ventilation techniques are an essential element of a sustainable home and necessary in high-efficiency homes built to passive house standards. However, many homeowners fail to realize that the interior moisture levels inside their homes might stem from vapor that is rising from below their home foundation. This article explains how high-performance vapor barriers function, what they do, and where they can be installed.
Table of Contents
- What are the Dangers of Water Vapor Infiltrating a Home?
- What Does A Vapor Barrier Do?
- Is A Vapor Barrier Necessary?
- Where Should A Vapor Barrier Be Placed?
- Why Choose a High-Performance Vapor Barrier?
- What Is The Difference Between a Vapor Barrier and a Moisture Barrier?
- Are Vapor Retarders the Same As Vapor Barriers?
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.
What Does A Vapor Barrier Do?
Though we cannot necessarily see it, the moisture in the soil around our home is continually evaporating and becoming water vapor. Water vapor is inherently attracted to areas of warmth. The warm air of your home, then, essentially acts as a giant magnet. This warmth subsequently attracts vapor that evaporates from the soil underneath your house and around the foundation. As moisture evaporates from the earth and moves towards your home, it will tend to saturate the foundation walls and basement flooring before eventually moving into your home. Once inside the walls, this excess moisture will penetrate your wood flooring, insulation, and other home elements. In severe cases, this can lead to mold and mildew growth and leave your home smelling musty.
High-performance vapor barriers are typically made from polypropylene sheathing. This sheathing essentially acts as an impermeable barrier that does not allow vapor to seep into a home. As an added benefit, high-performance vapor barriers can also be fundamental in stopping radon gas from entering the house. According to one study by the National Cancer Institute, 15,000 to 22,000 lung cancer deaths in the United States each year are related to radon gas entering the home.
Is A Vapor Barrier Necessary?
Some form of a vapor barrier is essential in virtually every home where soil moisture levels can be high. If you live in an extremely arid region such as the desert southwest, vapor barriers might not be necessary. In these regions, the limited water vapor might be a welcome respite from the dry air. However, vapor retarders can play an essential role in protecting your home and improving your indoor air quality in any region with normal rainfall levels.
High-performance vapor barriers are also significant for any home that has a crawlspace or basement. Houses built on top of a concrete slab should include high-performance vapor barriers as cement can "suck up" moisture from the soil and filter that moisture in the form of water vapor into the home.
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.
What Is The Difference Between a Vapor Barrier and a Moisture Barrier?
Some builders and contractors might end up confusing their clients by using different construction jargon. Some might believe that a moisture barrier is of higher quality than a vapor barrier. But, these two terms primarily refer to the same building material. Both vapor barriers and moisture barriers are high-grade plastics designed to prevent water (either from moisture or vapor) from entering the home.
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.
Tobias Roberts
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.
