How to Select The Correct Vapor Retarder for Your Home
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How to Select The Correct Vapor Retarder for Your Home

How to select the correct vapor barrier or vapor retarder for your home based on the climate zone and type of construction.

Moisture trapped inside a home is bad for the building materials and the building occupants. To help slow moisture diffusion through roof, wall, and floor assemblies, many experts-and in some parts of the country, building codes-demand the use of vapor retarders.

The term vapor barrier is commonly used in place of vapor retarder, but the two terms are not equal.

Until 2007, the International Residential Code (IRC) treated the country like a single cold climate and provided only one solution to limit vapor intrusion through exterior surfaces. The code required that a vapor retarder be installed on the interior or warm side of assemblies. However, most of the country has both heating and cooling seasons, so sometimes the cold side is the inside of the wall or roof instead of the outside. The old codes created a situation that could cause problems in the summer, especially if the local code official insisted that a vapor retarder meant a polyethylene sheet. When you fill a wall with a highly vapor-permeable insulation, such as fiberglass batts, and cover one side of it with a virtually non-permeable vapor retarder, such as polyethylene sheeting, the vapor retarder will be on the wrong side of the assembly for part of the year and inhibit the wall’s ability to dry.

The IRC now breaks the country into eight climate zones and recognizes three classes of vapor retarders that have different levels of permeability. Generally, the calls for a class-I or -II vapor retarder be installed on the interior side of homes in climate zones 5 and above, and in marine 4. However, if you’re building in a humid climate in zone 4, 5, or 6, and you air-condition your house in the summer, you may be concerned about having a vapor retarder in the “wrong” position for part of the year. If this is the case, just be sure to use a class-II vapor retarder on the interior of the wall. You also can use closed-cell spray foam in the cavity or a layer of exterior rigid foam, with a class-III vapor retarder on the interior.

When building in hot, humid climates (zones 1 to 3), you shouldn’t have a vapor retarder on the interior side of the wall. This allows any water vapor that makes its way into the wall to dry to the interior.

Permeability Ratings

Permeability, which is the amount of moisture that can pass through a material, is measured in perms. The lower the number, the less permeable the material and the more moisture it will block. Contact the manufacturer to get the perm-rating information for the product you are thinking about using.

Vapor Retarders or Vapor Barriers

The International Residential Code (IRC) defines a vapor retarder as a vapor-resistant material, membrane, or covering with a perm rating of 1 or less. However, the 2007 IRC supplement recognizes some materials that have ratings of 1 perm and higher as vapor retarders. Based on their perm rating, building products fall into one of three classes of vapor retarder. Many jurisdictions in the U.S. are still using local codes based on the 2006 or earlier versions of the IRC and IECC (International Energy Conservation Code). Vapor retarders are now separated into three classes:

• Class I: Less than or equal to 0.1 perm [e.g., polyethylene or non-perforated aluminum foil]

• Class II: Greater than 0.1 perm but less than or equal to 1.0 perm [e.g., kraft facing on fiberglass batts and extruded polystyrene over 1-in. thick];

• Class III: Greater than 1.0 perm but less than or equal to 10 perm [e.g., latex paint, 30# building paper, and plywood].

The IRC does not use the term vapor barriers, but some manufacturers and some people in the building industry use the term “vapor barrier” to describe a Class-I vapor retarder, or an impermeable material.

As building science improves and influences the way buildings are constructed it is critical that designers and builders select the appropriate materials for each situation. A building element or system is often referred to as an assembly. Assemblies may call for a vapor barrier designed to stop moisture on one surface, such as under a concrete slab, while more permeable vapor retarders allow some movement of moisture. If walls, roofs, or floors are configured with the wrong vapor-retarding products, a structure can trap moisture. Misusing these terms leads to confusion in product choice, which ultimately can lead to failure where it matters most-in your home.

Builders and architects must be aware of the permeability ratings for various vapor retarding materials and use them in the appropriate building assembly and in the correct climate zone of the country. As a homeowner, you need to be aware of these methods and details to improve indoor air quality and prevent mold growth when renovating or remodeling.

Building Resource Center

Climate Zone Map

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Comments (5)

Very well written Dan, a few building inspectors out there are missing out on the various classes and perm ratings, and are requiring the wrong perm rating for the designated climate zone. Your article explains this in very simple terms.

Is it retarded or retardant?

This is very important information Daniel.

Thanks for sharing

Many thanks for this article