Maintaining Household Moisture Balance
Air sealing and ventilation give energy efficient homes greater control over humidity.

The air in houses needs to have the right amount of moisture in it to be comfortable and healthy. Too much or too little moisture causes problems. For overall health and comfort, relative humidity should stay in the range of 30 to 60 percent.

In leaky conventional houses, occupants have almost no control over humidity. By contrast, one of the major benefits of living in a tighter, energy efficient house is the ability to keep humidity within the preferred range.

In winter, a common complaint about moisture is the lack of it. When it's too dry, many people blame the heating system: "Wood stoves give off dry heat." But excess dryness is the rule in conventional houses, with or without wood heat. The true culprit: air leaks.

When air leaks out of a house, water vapor suspended in the air leaks out with it. People generate humidity by breathing, cooking, showering, and doing laundry. A typical family pumps about three gallons of water into the air every day. Yet, because of air leaks, the house can be too dry.

In winter, replacement air that leaks into the house from outside is cold, so it can't hold as much water vapor as warmer air. At 32 degrees F, air can hold only one-third of the water vapor it can at 70 degrees F. When cold air infiltrates the house and warms up, its water holding capacity increases dramatically. However, the actual amount of water stays the same. So, the relative humidity drops. Dry replacement air mixes with moist indoor air, reducing the overall relative humidity inside the house.

When relative humidity falls below 30 percent, discomfort increases. Nasal passages dry out and get stuffed up. Skin feels itchy and cracks. Eyes become irritated, especially for contact lens wearers. As relative humidity approaches 20 percent, static electricity and ozone increase. Wood furniture and floors shrink and crack. Asthma and allergies flare up. Less moisture means the air feels cooler, so occupants raise the temperature to maintain comfort—and heating bills go up.

Conventional houses leak the most in winter, often more than one air change per hour. Air leaks are driven by two factors: the weather and the heating system.

Both operate at their peak in winter. Wind speeds are highest then. The stack effect, caused by the temperature difference between indoors and outdoors, is at its greatest. Forced air heating systems pressurize rooms differently, increasing air leakage by as much as 50 percent.

Ducts running through attics, crawlspaces and other unconditioned areas also contribute to leakage. Supply ducts, under positive pressure, blow air out of unsealed joints and seams. Return ducts, under negative pressure, pull outside air into the house. Wood stoves add to leakage by drawing air up the flue. Replacement air leaks into the house through cracks and openings. It's not surprising that the house dries out.

None of these air leaks can be controlled by the occupants. Instead, They put a kettle of water on the stove or operate a humidifier—trying to vaporize moisture into the air faster than it can leak out of the house. This solution may work to keep the humidity in the comfort range, but the energy cost is high.

By caulking and sealing to reduce air leaks, eliminate drafts, and cut heating requirements, energy efficient houses are less likely to experience winter dryness.

Actually, tightly built homes may retain too much moisture. High moisture levels—70 percent or above—make the house feel stuffy and clammy. Worse, high humidity supports the growth of disease and allergy causing pathogens, such as bacteria, mold, mildew, fungi, dust mites and viruses. Annoying condensation appears on cool surfaces like windows. And structural damage caused by moisture-loving wood fungi becomes a real threat.

In this case, the ability of outside air to rapidly dry the house becomes useful. A ventilation system can control relative humidity indoors by replacing moist indoor air with drier outdoor air. Excess humidity can be vented out by exhaust fans. Fresh air supplies can bring in outdoor air without creating drafts or discomfort. The system can be set up to run automatically on a timer or a dehumidistat, or to run continuously at a low speed. Properly designed, the system distributes replacement air throughout the house.

The same ideas apply in summer. People feel cooler as the air becomes drier, because dry air evaporates more moisture from the skin and air with water vapor holds more heat than air alone.

Heat pumps and air conditioners remove moisture from air while cooling it. Their job is much easier if the cool, dehumidified indoor air isn't always leaking out and being replaced by hot, humid outdoor air.

By sealing air leaks and installing whole-house ventilation systems, energy efficient houses give their occupants control over humidity. This ability to keep humidity in the proper range is a key factor in maintaining a healthy, livable home.

This article appeared in Energy Source Builder Number 3, 1990,
©Copyright 2008 Iris Communications, Inc.