Moisture Barriers

Moisture control is an important factor to consider in a building since obviously, moisture can lead to issues such as mold or pests. This is means it’s important to set up moisture barriers to prevent these problems from developing. A well designed moisture barrier will limit the amount of water that enters your home, while also allowing material that does get wet to properly drain. Examples of moisture barriers would include the siding on a home or polyethylene sheeting in a crawl space.

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Vapor Vs. Air

Moisture barriers can be divided into vapor and air. As you would guess, a vapor retarder reduces the rate that water vapor can move through a material. On the other hand, air retarders resist airflow and air pressure. Some materials can act as both air retarders and moisture retarders, but this is not always the case.

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Setting Up A Barrier

To work properly, a moisture barrier needs to be comprised of multiple layers, with at least one air barrier and one vapor retarder. For instance, the moisture barrier of a home could be, first, the siding, which will keep out of the majority of the water. Next, you would have the house wrap, which will shed any water that does get through the siding layer. House wrap is also useful because it will limit how much air gets through but still let in water vapor. This will prevent the water vapor from condensing on the framing. After that, you would have the sheathing, which needs to be kept dry. Finally, a vapor retarder would be used to keep moisture out.

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The Effect of Climate

Because of the thermal gradient, moisture will flow from warm to cold. This means that climate will play a factor in what moisture barrier works best for a building. In a hot climate, moisture from outside the home will flow towards the colder interior. In this case, the air and vapor barriers should be installed on the exterior of the building. In a cold climate, the warmer interior moisture will flow toward the cooler exterior. This means that air and vapor barriers should be installed towards the interior, warm surface. The example used in the previous section would apply to cold climate.

In a mixed climate, there are a couple different strategies that can be used. One option is to have permeable materials on both the interior and exterior surfaces. This will allow vapor to simply flow through. Alternatively, you could install the vapor barrier in middle of the wall assembly. In either case, the air vapor can be on the interior or exterior wall.

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Vapor Retarder Classifications

Vapor retarder is divided into 3 categories based on how much moisture passes through it, known as the permeability. Class-I vapor retarders have a permeability of .1 perm, or less. The lower the permeability rating, the more moisture it will block. Thus, Class-I blocks the most moisture of any class. Examples of Class-I vapor retarders include polyethylene sheeting or sheet metal.

Class-II vapor retarders have a permeability rating of 0.1-1.0 perms. Class-II will let in more moisture than Class-I. Examples would include plywood or extruded polystyrene.

Finally, Class-III vapor retarders have a permeability rating greater than 1.0 and less than or equal to 10 perms. Examples of Class-III retarders include gypsum board and fiberglass insulation. Obviously, materials in this class will block less moisture than Class I or Class II.

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Barrier vs. Retarder

The terms vapor barrier and vapor retarder are often used interchangeably. Technically, the International Residential Code only recognizes vapor retarders and vapor barriers are not mentioned. What many people think of as vapor barrier, is actually a Class-I retarder.

Air barriers are considered to be materials that resist 50% or more of the air pressure in a building. On the other hand, air retarders reduce air flow but do not resist 50% or more of the air pressure.

It’s crucial to have a properly installed moisture barrier to protect a building. The moisture barrier should protect against both air and vapor and include multiple layers. The climate should be factored into designing a moisture barrier, as well.

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Lstiburek, Joseph. “Moisture Control for Buildings.” ASHRAE Journal, Feb. 2002.

“Moisture Barrier Systems.” FEMA, US Department of Energy,

“Vapor Barriers or Vapor Diffusion Retarders.” Department of Energy, US Department of Energy,

Yagid, Rob. “What's the Difference: Vapor Barriers and Vapor Retar.” Fine Homebuilding, 5 Mar. 2009.