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Illinois Country Living

Doug Rye, licensed architect and the popular host of the "Home Remedies" radio show

Energy Solutions:

Homes Shouldn’t Sweat

With the right insulation you may not need a vapor barrier

Never a week goes by that I don’t receive at least one call asking about vapor barriers. The question has been around so long that I sometimes just assume that everyone knows the answer, but that is not the case. So here is the answer. As always, let me keep it as simple as possible. A vapor barrier, as related to house construction, is anything that retards or prevents the moisture in the air from penetrating the adjacent surface.

Consider these facts:

• Fact 1 – Warm air can retain more moisture than cold air. (Now you know why summer is so humid.)

• Fact 2 – There are two forces, temperature and humidity levels, that cause the moisture in the air to move. If you place a damp washcloth on the bathtub, it will probably dry within a few hours. The moisture moved because the room air was drier than the washcloth. If you place that damp wash cloth outside on a 25-degree day, it will freeze dry because the water in the cloth is drawn out of the cloth and into the air by the lower temperature and lower humidity.

• Fact 3 – Vapor barriers are not needed in a Doug Rye house.

Are you with me thus far? This really isn’t rocket surgery (one of my favorite sayings).

Let’s take a look at a house wall. Let’s say it is 80 degrees outside and a constant 75 degrees inside the house. Outdoor relative humidity is 60 percent and indoor relative humidity is 50 percent. Not much difference in either, so the moisture in the air is just hanging around.

Then comes winter and the outdoor temperature is 30 degrees and the outdoor relative humidity is 40 percent. Since the moisture in the house wants to move toward a colder temperature and lower humidity, the moisture moves toward the outside. It starts moving toward any cold surface and any air cracks it can find to get outside. Bingo. It hits a cold window and it turns to “sweat,” I mean, condensation. The glass is a great vapor barrier and the moisture simply can’t go any farther.

This is exactly like a cold glass of iced tea sitting in a warm room. I already know what your mother said: “Don’t put that on the coffee table!” And you know the rest.

Have you noticed that the condensation stops as the iced tea warms to room temperature? Or have you noticed that you can prevent condensation by pouring the iced tea into an insulated foam cup? If there is no cold surface, there is no condensation.

We have been told for many years that walls in houses and buildings need a vapor barrier and that it should be installed toward the warm side of the wall. I have asked many times this question — “What if my house is in Miami, Fla., and it is warmer outside most of the time than it is on the inside of the house? Should I place the vapor barrier on the outside of the house?”

I never received an answer, but I know what it is. Houses in the South really don’t need a vapor barrier at all because typically there are not great differences between indoor and outdoor temperature and humidity levels, even in the winter. And houses in the North really don’t need a vapor barrier if cellulose or foam insulation is used, which will prevent the wall or cavity from ever being cold. By the way, two coats of latex paint is a pretty good vapor barrier.

Most code officials and inspectors now understand the principle. However, you should check the codes in your area prior to any construction.


More Information:

Doug Rye, a licensed architect and the popular host of the "Home Remedies" radio show, works as a consultant for the Electric Cooperatives of Arkansas to promote energy efficiency to cooperative members statewide. To order Doug's video, call Doug at 1-888-Doug-Rye. More energy-efficiency tips can also be found at www.ecark.org.

 

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