Relative Humidity Revisited: The Case of the Soggy House

Although dry houses are the norm in the colder areas and seasons of Alaska, there are exceptions. Stan Jones of Fairbanks, for instance, wrote that in his new house windows sweated, crackers mildewed, and things generally felt so soggy that the family finally had to get a dehumidifier. How could this happen, considering what I said a few weeks ago about how little water really cold air could hold?

When cold air with a high relative humidity but very low actual water content is brought into a house and heated, the result is a very low relative humidity. Human activities inside the house, however, are continually adding water to the air. If there were no way to remove the water, and the air, once inside the house, stayed there, the relative humidity would gradually approach 100%.

In the real world, houses are not that tight and there are mechanisms to remove some of the water without exchanging air with outside. In a very well-built house, however, it is indeed possible to raise the humidity to uncomfortably high levels.

Consider the removal mechanisms first. Ever noticed the "sweating" of an iced drink glass? As air is cooled, its ability to hold moisture decreases. The temperature to which the air must be cooled so that it is 100% saturated is called the dew point temperature. If there is an object with a temperature below the dew point temperature, water will condense on it.

Naturally, the higher the relative humidity is, the higher the dew point temperature will be. In a house with a temperature of 68 degrees F, if the relative humidity were 70%, water would condense on any surface with a temperature below 58 degrees. At 50% relative humidity, the dew point would be 49 degrees. In order to get water to condense on an ice-cold glass of lemonade in a 68 degree room, the relative humidity must be greater than 26%.

If a really cold surface is available, a mere 5% relative humidity corresponds to a dew point of 2 below, while 1% gives a dew point of about - 30.

In a typical house, there will be parts of walls (the lower parts of house corners, for instance) and windows which will be a lot colder than the air temperature in the house. In Alaska, these areas will often be at temperatures below freezing, and if the dew point temperature of the air in the house is higher than the coldest surface around, the result will be steamed-up (or frost-covered) windows and frost or wet spots on the walls - a situation that can lead to mildew, peeling point and general sogginess even with a relative humidity of only about 25%.

If there is not a good vapor barrier, indoor air can get into the insulation, which will be even colder than the interior surfaces of the walls. When the temperature in the insulation is below freezing,so much frost can build up in the insulation that when a thaw comes, the insulation is soaking wet.

A large fraction of "leaking roofs" after a warming and snowfall in the colder parts of Alaska in fact have nothing to do with the roof. Rather, the frost that has condensed into the insulation from the relatively moist air in the house has melted and is dripping out. Walls and insulation can be damaged or lose insulating ability in the process, so it's generally wise to try to keep the relative humidity low enough that the dew point is colder than the coldest wall or window around.Axel Carlson (now retired) and Rich Siefort of the Cooperative Extension Service have published numerous reports relating to ways of keeping your insulation dry.

New superinsulated houses have very thick insulation and a tight vapor barrier. Wall and window surfaces stay relatively warm, and the indoor air is unable to get into the insulation, where temperatures are lower. Since there are no really cold surfaces available to remove water vapor from the air, the dew point temperature can be high enough to allow a fairly high relative humidity.

Relative humidity will be controlled by how fast water vapor is added to the air by breathing, cooking, washing, and showers, and how fast it is removed by exchange of wet indoor air for dry outdoor air. Since it is more energy efficient to recirculate air that is already warm, some newer heating systems are designed to do this, pulling any combustion air needed from outside the house. If indoor air is not regularly replaced by dry outdoor air, the amount of water vapor in the air may reach uncomfortable levels.

If the house is not heated by electricity, some air will be required for fuel to burn, and if this is taken from inside the house, fresh outside air will be pulled into the house to replace the air going up the stovepipe. Some modern systems, however, pull any combustion air needed from outside the house, eliminating even this exchange of air. The result can be a very soggy home.

Since water is not the only thing added by indoor activities, high humidity in the winter, being an indicator of poor air exchange, may be a danger sign that indoor air pollution could be a problem. If you really have to use a dehumidifier in the winter, I would suggest that you get your air tested, and perhaps consider redoing your air circulation system to bring in outside air via a heat exchanger.