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Moisture is
allowed to penetrate your wall cavities because traditional insulation
products do not prohibit the movement of air. There
are some attempts
to control air movement, such as a plastic vapor barrier under the
interior gypsum board and house wrap over the exterior sheeting, but
these products only work if there are no seams or holes in them. If
holes exsist, the laws of physics will take over, and moisture will
find its way into the wall cavity. The reason air is constantly trying
to infiltrate or exfiltrate your home is because the laws of physics
dictate that temperature, and moisture will always try to equalize.
You may want your home to be 70degrees F. and 40%RH in the summer,
but exterior conditions could be 90degrees F.
and 70%RH. This creates an imbalance, and heat and humidity will try
to infiltrate the cracks in your home's thermal envelope to equalize
the conditions. Often times, as the hot, humid air moves into the wall
cavity, it comes into contact with the plastic vapor
barrier and condenses. It does this because as you cool your house,
the gypsum board and the vapor barrier are cooled. Just like a cold
glass of iced tea on a summer day, hot humid air meets a cool surface
inside your walls, and water dropplets form. When this occurs,
you have the three essential conditions for mold to grow: moisture,
temperature, and food. |
