Key Points
- High humidity levels can lead to moisture and mold problems in a facility.
- Proper insulation, combined with temperature control and ventilation, help control moisture.
- Desiccant dehumidification systems may be viable options in a variety of settings.
Source: www.hud.gov
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When even small amounts of water are allowed to accumulate on surfaces inside a building, the resulting moisture and mold problems can become quite costly. Mold and fungi can lead to air quality problems, and even sick building syndrome. High humidity and condensation are frequent causes of moisture accumulation, and they can be better controlled once the cause of the moisture problem is identified.
Relative humidity and condensation
Temperature affects humidity, which is the amount of water vapor in the air. Relative humidity (RH) is the amount of water vapor in the air compared to how much moisture the air can hold at that temperature. Warmer air can hold more moisture than colder air. The dew point is the temperature at which air cannot hold any more moisture. As the temperature decreases, moisture will accumulate on colder surfaces, such as walls and windows. This is called condensation.
To illustrate the effect of temperature on moisture content, air at 70°F and 40 percent RH contains approximately 0.006 pounds of moisture per pound of dry air, while air at 50°F and 40 percent RH contains approximately 0.003 pounds of moisture per pound of dry air. Although both are at 40 percent RH, the 70°F air contains roughly twice as much moisture as the 50°F air.
The humidity level in a building can vary continuously. If moist air is removed to another area where a drop in temperature occurs, condensation may result. The first strategy for reducing condensation is uniform air temperature control. A second strategy is to heat the air to remove the moisture. Desiccant heaters and duct heaters can effectively remove the moisture from air, thereby lowering the relative humidity. The moisture can then be dispersed through a ventilation unit, or it may be redirected to other areas of the facility where moisture is needed.
If relative humidity reaches 70 percent, mold growth can occur. This often happens in poorly heated or ventilated areas. The following strategies can help reduce mold growth:
- Reduce humidity
- Improve air circulation
- Improve insulation
- Improve overall background heating
Temperature control or humidity control
Mold and mildew are commonly found on surfaces of the exterior walls of corner rooms. An exposed corner room is likely to be significantly colder than adjoining rooms, becoming a potential site for condensation. If mold and mildew growth are found in a corner room, then relative humidity at these room surfaces is above 70 percent. This can happen even though the RH is 30 percent and the temperature is 70°F in other areas. If temperature is the problem, increase insulation or improve heat flow to increase the temperature at the cold room surfaces.
If the RH at the increased temperature is higher, this indicates there is a relatively large amount of moisture in the air and control strategies should focus on decreasing moisture content. Possible solutions for reducing relative humidity levels include:
- Preventing hot, humid exterior air from contacting the cold interior finish. In other words, controlling the vapor pressure at the surface
- Eliminating cold spots by relocating ducts and diffusers
- Ensuring vapor barriers, facing sealants and insulation are properly installed, and maintained
- Increasing the room temperature to avoid over cooling
Desiccant dehumidification
Desiccant systems are commonly used to lower humidity levels in all types of buildings, including supermarkets, theaters, restaurants, ice rinks, schools and hotels. Restaurants are often over cooled during summer. Increasing the inside temperature of the facility is not an option, because it can cause comfort problems. In these circumstances, a more energy-efficient option is to remove the moisture content of the air, then cool the lower-moisture air with conventional air conditioning.
A desiccant dehumidifier can easily maintain 30 percent to 40 percent relative humidity within an ice skating rink, which helps eliminate fog and condensation all year, regardless of outdoor weather conditions. The average energy consumption of a desiccant dehumidifier is as much as 70 percent less than a comparable refrigeration system for this application.
Cooling air with high moisture content is considerably more expensive, and requires a lower air temperature to achieve proper comfort levels. Desiccant systems are designed to remove the moisture from the incoming air, so the lower RH air does not have to be over cooled to be tolerable. Occupants are more comfortable, and the facility manager is happy because energy costs are lower.
For further information
Mold Remediation in Schools and Commercial Buildings. U.S. Environmental Protection Agency. September 2008. Last accessed Feb. 7, 2013.
