We get lots of questions about household problems, but one of the most common concerns is condensation. We don't want to bore you with technical terms like vapor pressure differential, so we will keep this simple. There are two rules at work here:
When warm moist air touches something cool, condensation will form. Warm air can hold a lot of moisture; cold air cannot. (FYI: While warm air can hold a lot of moisture, it doesn't necessarily have to. Think of the dry air in Arizona.)
Despite the threats of global warming, it's still pretty
cold outside in the winter. Consequently, windows
are cold, too. If the inner glass surface is extremely
cold, then condensation (in the form of water or ice)
will form. It will form even in a house which has
normal indoor humidity. This is the principle reason
for storm windows. The exterior pane of glass and
the air between the two layers of glass provide
enough of a buffer zone that the surface tempera-
ture of the interior pane of glass stays warm and
condensation is less likely to form. After doing what
we can to raise the temperature of cool surfaces,
we should turn our attention to reducing the
moisture in the indoor air.
The easiest way to maintain low humidity levels is
to buy an old house that is not particularly well sealed. Admittedly the house might be drafty, but the drafts mean that cold outside air is sneaking into the house. When that cold air warms up, it will have very low humidity. Similarly, warm air that has picked up moisture from cooking, bathing, etc. is flushed out of the house. Unfortunately, this approach flies in the face of current thinking. Modern homes are sealed tightly because every bit of cold air which leaks into a house means that warm air must leak out, and this is not energy efficient.
Another way to get cold dry air into your house is to use up the warm moist air within. In many houses, air from within the house is used by the furnace, hot water tank and fireplace to create combustion and maintain proper draft up the chimney. This warm moist air escapes up the chimney causing cold dry air to enter the house and make up the difference.
Energy efficient homes don't want to waste this inside air (which you have already paid to heat) by letting it go up the chimney. Consequently, most modern furnaces and fireplaces bring in outside air for combustion, which increases efficiency. Taken to the extreme, the most efficient house imaginable would not allow any cold outside air to leak inside nor would it use any inside air for combustion. While the heating bills would be low, the windows would be dripping with condensation and the indoor air quality would be terrible.
The high tech solution is to put in a heat recovery ventilator (also known as an air-to-air heat exchanger). As you exhaust the stale contaminated air from inside the house, you replenish it with fresh air from the exterior. While the fresh air and the contaminated air are not allowed to touch one another, the heat from the exhaust air is transferred to the fresh air coming into the house. Some furnaces have the option of adding humidity control. This benefits you in the summer when you want to lower the humidity in your home. While lowering the humidity in the winter may help with condensation, in the cold months we generally want enough humidity in our house to keep our hands and skin from drying out.
In conclusion, condensation within houses requires two major ingredients - humid air and cold surfaces. If you increase the temperature of cold surfaces by adding storm windows and reduce the humidity levels by venting clothes dryers to the exterior, using bathroom and kitchen fans etc., you should be fine. If you still get a little condensation, go to the low tech solution: Open a window!