Put a dinner plate in your freezer and let it chill. Put several ice cubes in a frying pan and turn on the heat. The cubes melt and the water evaporates. Hold the chilled dinner plate 10-12 inches above the water boiling in the pan. It will get moist as the water condenses.
This is what happens in a tropical storm: the sun warms the ocean. The water absorbs the heat and evaporates. The air above absorbs more and more of the evaporating water. As the air absorbs heat, it expands, becomes lighter and rises. Cooler air rushes in to take its place — the winds of the hurricane. Since the earth revolves around its axle, the whole system assumes a circular pattern centered around a core, where the warm moist air keeps rising.
Moving over warm water, the storm absorbs more energy, becoming more powerful. Over land or cooler waters its power moderates. When the warm air rising in the core reaches cooler heights in the atmosphere, it cools, releasing moisture it can no longer hold. The moisture condenses. Clouds form, become heavy with droplets of water, which fall back to earth as the storm’s torrential rains.
The whole process is driven by heat. Similarly in a heated house heat, air and moisture interact. The moisture from the wet cellar, your shower, your kitchen, your freshly watered houseplants, your sweat all evaporate and get transported by the warming air. What happens to that airborne moisture?
It escapes from a leaky house along with the warm air and all the heat that you paid for and it absorbed. But as you tighten your house — as you must to save energy and increase comfort — more and more of this moisture gets trapped inside. This is good to a point – no winter sniffles, the indoor air is comfortably moist – as long as the air doesn’t cool and drops its excess moisture. But the air does cool: it’s winter. And its moisture, like on your chilled dinner plate, condenses on the nearest cold surface it finds: a cold window glass, for instance, the metal frame of a sliding door – any cold surface the moist warm air may encounter. In the attic it condenses on the cold underside of the roof. And if the warm air can leak into the wall cavity – e.g.: through unsealed electrical outlets – moisture will condense on the inner surface of the wooden sheathing closest to the outdoors. Condensing moisture can cause mold and mildew. Over time it causes rot, damaging the structure.
This is why researchers have developed a five-part approach to energy efficiency in buildings: super-insulation, thermal bridge prevention, air-sealing and good windows to retard heat loss, and to remove excess moisture and provide fresh air, ventilation. Modern systems remove stale air and excess moisture and also transfer the heat from the outgoing air stream to the incoming fresh air. Well worth the investment. So seal those leaks, but ignore the need for ventilation at your peril.