Vapor barriers and insulation are two of the most important components of a wall when it comes to the prevention of moisture inside the wall. There are many different opinions on the location and necessity of vapor barriers. Part of this is due to the fact that the proper placement varies by climate type. This article will deal with the climate of the northern United States, which has a wide swing in both temperature and humidity between summer and winter.
What is a Vapor Barrier?
First, we should clarify what is meant by the term vapor barrier. Though vapor barrier is the more commonly used term, a better term is vapor retarder. This is because nothing is a perfect barrier, there are always some gaps and cracks that will allow some amount of water vapor through the wall. For purposes of this article, we will use the more common term of vapor barrier. A vapor barrier is a plane of building material that is meant to stop the passage of water vapor. Water vapor does not necessarily mean steam, like that in your bathroom after a nice hot shower. A better way to think of water vapor is as humidity. Warmer air has the capacity to hold more humidity. As air cools it loses the capacity to hold humidity, and the water vapor condenses into droplets of liquid water. The temperature at which the liquid water forms is called the dew point.
Some materials, such as the paper backing on most fiberglass insulation, have a material that is meant to be a vapor barrier built in. Others, such as closed cell foam insulation, are by their very nature a vapor barrier. Another method is to add a material like plastic sheeting to the wall assembly to act as a vapor barrier. Whatever the method, what is important is the location of the vapor barrier. What is even more important is to understand when not to use a vapor barrier. It should be noted that most commercial house wraps, such as Tyvek, are not meant to be vapor barriers. The purpose of this material is to stop water in its liquid form from getting into your walls. These materials are usually designed to let water vapor pass through.
Where are vapor barriers typically used?
Conventional wisdom in the building community in a northern climate is that the vapor barrier should go on the warm in winter side. In the winter time, the temperature difference between the interior air and the exterior air can be very large. On a 30 degree winter day, if your thermostat is set to 70 degrees, the difference is 40 degrees. This means that the dew point is located somewhere inside the wall. The theory is that if a vapor barrier is put as close to the warmer air as possible, the vapor will be contained inside the home and will not enter the wall. Therefore at the dew point there is not sufficient water vapor in the air to condense, and the wall stays moisture free.
There are two problems with this theory. First, as mentioned above, there is no such thing as a true vapor barrier. Outlets, switches, gaps and cracks around windows and doors, and other construction imperfections will always occur in a wall. These breaches in the barrier are bound to let some moisture into the wall. If it is allowed to condense, moisture will form in the wall. Second, since the advent of air conditioning, the summertime condition is reversed. Air conditioning not only reduces the temperature of the interior air, it dehumidifies it. In this case, the hot, humid air is on the outside, and the cooler, drier air is inside the home. The dew point is still located inside the wall, but now there is plenty of moisture to form liquid water.
Why are vapor barriers no longer a good idea?
In both cases listed above, the vapor barrier takes away about half of the wall’s opportunities to dry out. In the days before air conditioning was common, the wall was still able to eventually dry out. However, as air conditioning becomes more and more common in homes, the moisture problem becomes extremely complicated by the vapor barrier. The moisture becomes trapped between the vapor barrier and the hot humid air outside the building. Therefore, in areas with wide swings in seasonal temperature and humidity such as the northern United States, it is probably best to omit the vapor barrier altogether. It is better to assume that there will be some moisture infiltration, and maximize the opportunities for the wall to dry itself out.
All building professionals should understand the concept of vapor barriers. Knowing where and when they should be installed is essential to the long life and comfort of buildings. In general, there are two rules of thumb that will help. Heat will always try to move from hot to cold, and moisture will always want to move from wet to dry. Hopefully this article has helps facilitate understanding of this complicated building issue.
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