High or Low Interior Humidity
High Humidity
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends that interior relatively humidity (RH) be kept below 65% “to reduce the likelihood of conditions that can lead to microbial growth.” Also, RH above this can be perceived by occupants as being ‘sticky’ or too humid. It can also lead to odors and mold growth. Depending on your local climate, several factors can contribute to high interior humidity.
High Humidity due to Problems with AC
In warm climates with high humidity, modern buildings are air conditioned. The key functions of an AC system in such areas is to reduce both temperature and humidity. This is accomplished by drawing or pushing interior air across evaporator coils. The coils are cooled by refrigerant as it changes state from liquid to vapor. This same refrigerant is then compressed and condensed at the exterior, where heat is released. As interior air is drawn across the evaporator coils and cooled, moisture is removed and the interior humidity is reduced. A sufficient air flow must occur in order to properly reduce interior humidity.
‘Short cycling’ is a term used to describe an AC system that is cycling too quickly, or the compressor and evaporator are being called back into service after being stopped for a relatively short period of time. Short cycling reduces an AC system’s ability to remove humidity. This is because a short cycling AC system will not run long enough to draw water from a sufficient quantity of air. Short cycling causes additional wear and tear on the AC system’s components, and may lead to premature failure. This is because of more frequent stopping and starting. Also, under normal conditions the compressor discharge pressure is allowed time to dissipate through the expansion valves at the evaporator. If the compressor starts before this pressure dissipates, it must work against the higher pressure, resulting in increased power usage, operating temperatures, and mechanical stress. Higher electric bills are sometimes noticed in conjunction with high interior humidity.
Causes of short cycling include an over-sized AC unit, too much refrigerant in the system, inadequate movement of air through the home, improperly placed thermostat, condensate blockage or leaks, etc. Any of these conditions, acting alone or in combination, can disrupt the proper operation of an AC system and contribute to higher interior humidity.
Clogged filters, the lack of return vents, and inadequate undercut at bedroom doorways can also contribute to inadequate air movement through a building. These factors result in a deficiency of the home’s relative warm, moist air being drawn across the evaporator coils. The ideal is a continuous flow of relatively warm, moist air from conditioned interior areas to the evaporator coil; and the relatively cool, dry air returning back to the entire area. If this air movement through the building and across the evaporator coil is insufficient, the evaporator coil may not be given the volume or circulation of air flow required to properly remove moisture and evenly cool the interior.
Other Potential Causes of High Humidity
Roof leaks or other water intrusions, cooking, laundry, and other sources of moisture can cause or contribute to high humidity. Take, for instance, the case of mold or mildew on a bathroom ceiling. This is often caused by bathing activities combined with a lack of adequate ventilation. A lack of stains at the ceiling board and roof or floor structure above such a bathroom would be evidence that the problem was not caused by another source.
Low Humidity
Low interior relative humidity is a common issue in colder climates, especially during winter months. Colder air holds less moisture, and its relative humidity decreases even further when it enters a home and is heated. ASHRAE recommends maintaining indoor relative humidity between 30 percent and 60 percent for optimal occupant comfort. Falling below this range can lead to dry skin, respiratory irritation, etc.
A significant contributor to low indoor humidity is the use of wood burning stoves. This is because wood combustion consumes a significant volume of interior air relative to other heaters. Forced air or boiler heaters fueled by propane or natural gas are provided with an intake and exhaust, which reduces or eliminates interior air used in the combustion process. And electric heaters obviously do not use combustion. So the interior air that goes up a wood stovepipe needs to be replaced by drawing the relatively cold and dry exterior air into the building to replace it. This results in a continue replacement of the more humid interior air with drier air from the exterior.
In such a scenario, building occupants are usually inclined to add moisture to the air by using humidifiers, placing water pots on the stove, etc. However, this should be done with moderation and awareness. Adding too much moisture to interior air can lead to moisture related problems with the structure.
Nature seeks a balance: heat travels towards areas of lower temperature, and moisture travels towards areas of less moisture. Therefore, both heat and moisture at the interior tend to drive towards the exterior. The building’s insulation is intended to slow this heat transfer. However, vapor drive, or water in the form of vapor, traveling outward through the building envelope will condense into liquid water when its temperature falls below its dew point. This is particularly true when a vapor barrier or retarder exists at the outside face of a wall for floor asssembly. The accumulation of moisture from vapor drive can lead to eventual mold and decay within the structural envelope.