Technical Papers

NJ Homeowner's Guide to Vapor Barriers

Thursday, January 28th, 2016 by Bill Cowley

crawl space before and after vapor barrier

A vapor barrier, also known as a vapor diffusion retarder, is any material used to resist the diffusion of moisture, that is, to prevent moisture from moving from one area to another. These barriers are commonly adhered to walls, ceilings, and floors. Vapor barriers are sometimes confused with moisture barriers. Vapor barriers are installed on the inside of a house whereas moisture barriers are installed on the exterior of a frame wall.

What materials are used in vapor barriers?

A vapor barrier is typically made of polyethylene sheets, but can be made from other materials such as fiberglass or a rubber membrane.

How are vapor barriers classified?

Every material has a greater or lesser ability to allow water vapor to diffuse through to the drier side. In construction, you’ll often see a “perm” rating, which is short for permeance, that measures how easily water vapor moves through a material. The better the material’s ability to retard water vapor movement, the lower its perm rating. If the perm rating is low enough, we call it a vapor retarder, and if it’s really low, we call it a vapor barrier. The International Residential Code has three classes of vapor barriers based on perm ratings. Class I materials, with a perm rating of 0.1 or less, are impermeable vapor barriers that stop the movement of water vapor. Class II materials have a perm rating between .1 and 1, exclusive, and Class III materials have a perm rating of 1.0 to 10 perms. Class II and Class III materials are semi-permeable vapor retarders. Any material with a perm rating greater than 10 is a permeable material that is ineffective in restricting water vapor movement.

Vapor barriers can also be classified by their thickness. A “mil” is a unit of thickness equal to one-thousandths of an inch. As thickness (mil) increases, water vapor has a harder time making it through. Polyethylene sheets are commonly used as vapor barriers in crawl spaces. Because the material is flexible, it is well-suited to cover an already existing structure. Standard polyethylene sheets commonly used in crawl spaces is 6 mil. These sheets, typically sold in most home improvement stores, are approximately the same thickness as a standard tarp. They are easy to tear and may begin to disintegrate in a relatively short amount of time. Recognizing the limitations of a 6 mil material, the better contractors are using thicker, more durable polyethylene vapor barriers, even as thick as 20 mil, which is about the thickness of an industrial strength pool liner.

Where are vapor barriers commonly used?

Crawl spaces commonly use vapor barriers because they are subterranean and plagued with high moisture levels.  Most New Jersey foundations were built with cinder blocks, which are extremely porous, allowing moisture to penetrate into the home. Depending on the size of the structure, hundreds of gallons of ground water can seep into a home in a single year by either migrating up through the ground or by entering as humid air through open vents.

Why is it important to use a vapor barrier to stop moisture?

Moisture is an enemy of the home. In advanced stages of moisture damage, wood in building structures can experience wood rot and wood decay, losing their structural integrity. Moist wood also encourages a variety of pests including wood-destroying termites, causing further damage. Also, with excess moisture comes mold growth. Mold growing in the crawl space will produce spores that spread in air currents throughout the home affecting the health of the home’s occupants. Mold can cause a variety of respiratory problems including nasal and sinus congestion, difficulty breathing, cough, throat irritation, and sneezing, especially in the elderly and children.

Why do vented crawl spaces contribute to moisture problems?

Moisture is not just in the soil; it is also in the air. In the past, it was erroneously thought that the best way to remove crawl space moisture was through ventilation systems. Unfortunately, homes are still being built with vented crawl spaces. Vents are added to crawl spaces under the mistaken belief now disproved by modern building science that the vents would allow humid air to leave the crawl space and dissipate into the outside air. The underlying problem with venting a crawl space is that air moves up, not across. This upward, vertical air movement is referred to as the stack effect. When there is moisture in a crawl space the damp, mold infested air is drawn upward into the living area of the house, not outward through the vents. Vents actually exacerbate the problem by allowing outdoor moisture and humidity inside the house.

Will a vapor barrier in my crawl space solve my moisture problem?

A vapor barrier is an important first step in addressing your crawl space moisture problems. But other steps must be taken as well. Airtight doors and vent covers should be installed to keep outside air and moisture from entering the space. A crawl space drain and sump pump system must be installed if there are standing water issues. Finally, a professional, commercial grade, self-draining dehumidifier will help to keep your space dry and the air in your home healthier.