Vapor Barriers

The function of a vapor barrier is to retard, or mitigate the migration of water vapor. Vapor barriers are not intended to retard, or mitigate the migration of air, which is the function of air barriers. The confusion arises because air often holds a great deal of moisture as a vapor. So, in the strictest sense air barriers are also vapor barriers when they control the transport of water vapor. The two are similar but not the same.

So what is the problem?

Using a vapor barrier incorrectly can lead to an increase in moisture related problems. Vapor barriers were originally intended to prevent structure internals from getting wet, but in some cases actually prevented a structure from drying. This problem happens in air-conditioned spaces, below grade spaces, and where brick is installed over vapor permeable sheathing, and some building papers. The issue then, is twofold: keep moisture out and let moisture out should it gets in.

Remember Chemistry 101? Putting aside bubble points and dew points, Boyle's Law tells us moisture can come in several phases: liquid, solid, vapor. The liquid (rain) and solid (ice) phases have their own problems, but the vapor phase is where pesky ground water, moisture molecules make their way into the structure and do their worst causing mold, mildew, rot, and odor. These can be an economic disaster for homeowners.

Not all homes are equal however. Moisture vapor is managed on an individual basis and on a regional climatic basis, so moisture, to be controlled most precisely, becomes a "size-to-fit" situation. Give us a call and we can help control of moisture vapor in your home.