Vapor Barriers - Exterior Wall Assembly

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What about moisture in the form of water vapor? Water vapor moisture is produced both outside (humidity) and in- side (steam from cooking and cleaning).  e second law of thermo- dynamics says that stuff moves from areas of greater concentration and higher energy to areas of less concentration and lower energy. With regard to vapor, air carrying the moisture (vapor) always moves from high pressure toward low pressure, and water moves from wet toward dry and from warm toward cool. When water vapor hits a cool surface, it condenses and changes from vapor to a liquid. If this is within the wall assembly, that becomes a problem, leading to mold growth and issues with rotting assemblies and poor indoor air quality. Vapor barriers (like vinyl, polyethylene or the asphalt-coated Kraft paper face of our insulation batts) can either help to prevent this from happening, or can actually contribute to the problem, depending on the climate.

So you may or may not need or want a vapor barrier. The goal is to control or stop condensation.  ere are two ways to do this. One is to stop the warm moist air from coming into contact with the cold surfaces.  e other is to warm the surfaces so that they are too warm for condensation to occur.

In the past, we primarily used the idea of stopping the moisture using vapor barriers in our wall assemblies.  is is the concept behind the plastic vapor barrier covering the studs. Getting a perfect plastic vapor barrier installation is very hard and detailed work, and there have been far too many instances where small overlooked holes have caused major problems, so now there is another option. With the advent of rigid foam insulation board, we can now warm the wall surfaces to prevent condensation. And this provides the additional benefits of increasing the total insulation of the wall assembly and reducing thermal bridging.

When we do use a vapor barrier, where we place it is determined by which direction the wall will dry towards. Remember, our goal is to stop the water vapor from  finding a cold surface and condensing while still allowing the wall to dry in the other direction. Strange as it might seem, northern homes dry out while southern homes dry in.

For example, if your home is in a very humid, cooling-dominated climate like Dallas, Texas, (or in a mixed-humid climate like in the Midwest), the direction of water vapor drive is from the warm, humid outside air toward the dry, cool air inside of the air conditioned home. As moist air comes in contact with the backside of cool conditioned wall surfaces, condensation and related problems can occur.  is is especially true if the owners have kept the house at a temperature below the outdoor dew point temperature. If we had placed the vapor barrier on the inside of an exterior wall, it would have exacerbated the problem by halting the ability of the water vapor to dry to the inside. Many a builder . has removed vinyl wallpaper and found the drywall under it covered with mold and not understood the source of the excess moisture. So the right place for the well-sealed vapor barrier/vapor retarder is to the outside of the wall assembly, allowing the moisture to dry to the inside.

Now, let’s consider a home in a heating-dominated climate like Minneapolis or Toronto.  The warm/humid side of the house walls most of the year is the inside, and the cool/dry air is on the outside of the house. In this climate, the water vapor is driven from the inside toward the outside through the building assemblies in long winter. As this warm, humid air reaches the backside of the cold exterior sheathing, it again causes a condensation problem.  e place to put the vapor barrier would be on the inside of the wall assembly.

Except for extremely cold climates, we can skip the vapor barrier entirely and opt for installing the exterior foam sheathing, which keeps the wall assemblies warm enough to prevent condensation. We call this putting a coozie on your house.  e thickness of the foam sheathing required depends on your climate zone. In the mild winter areas, one-half inch to one inch will do the trick. In mixed climate zones (fairly equal heating and cooling seasons), you should use an inch to an inch and one-half of rigid foam on the outside of the wall. In areas with very cold winters, you will need to install one and one-half-inch or two inches of rigid foam board to ensure that you keep the wall cavity warm enough to be trouble free. If you check with your local building code official, they can look up what is recommended in their code books.

You can paint the inside wall with two coats of latex paint and that acts as an interior wall vapor retarder, slowing the rate of vapor diffusion. When combined with the exterior foam, this is an excellent system that works very well in any climate zone.36  is has been called “the perfect wall” by the building science community.37 It also works as the perfect  floor when rotated ninety degrees and the perfect roof when sloped properly. It controls water vapor condensation, temperature and thermal bridging and allows drying to the inside.

In the following drawings you can see the direction of water vapor  ow and therefore the direction of drying that occurs in heating- versus cooling-dominated climates.  e fact that the way a wall dries is not the same in all parts of our country has led to many poor decisions and confusion about where to place the vapor barrier in a new home.  e diagrams also illustrate how the vapor barrier reduces the moisture load on the wall assembly, thus protecting it.  e rule of thumb is to place the vapor barrier on the side of your wall that is more humid and warmer for the majority of the year. In hot, dry climates, walls that have no vapor barriers at all, o en called breathable walls, are a good option. Also, avoid using moisture-stopping drywall products, as are commonly used around bathtubs and showers, in areas where direct water contact is not an issue.

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