If it is designed well, the thermal envelope of your home should control the flow of heat, air, and moisture. Unfortunately, the floors, walls, and ceilings of older buildings are often leaky: they leak heat, they leak air, and they leak moisture.
If you are building a new house, you have the opportunity to control the flow of heat, air, and moisture through your home’s building envelope. The result will be a durable, comfortable building that doesn’t cost much to heat and cool.
Of course, we use insulation to control the flow of heat and we use air barriers to control the flow of air. In almost all cases, it’s a good idea to limit the flow of heat and air through a home’s thermal envelope.
Controlling the flow of moisture is a little more complicated than controlling the flow of heat and air, however.
When your indoor air is too humid…
There are lots of ways that moisture can cause problems in a home — almost too many to list. Moisture entry can cause basement puddles, sheathing rot, and drywall mold. Because the list of problems is so long, I’m going to limit the focus of this article to just one issue: indoor air with elevated relative humidity (RH).
Signs that the RH of your indoor air is too high include mold, condensation on windows, and clammy-feeling air. If you’re not sure whether your indoor air is too humid, you can measure RH with a hygrometer. In general, indoor RH above 40% in winter or above 60% in summer is considered high.
The moisture that causes elevated indoor RH might be generated indoors — for example, it might come from a plumbing leak — or it might be…