The HVAC Factor: Moisture Management
By Doug Garrett, CEM
Originally published in the February 2010 issue of Today’s Facility Manager
Moisture can be very difficult to understand and even harder to manage. Sometimes, it is advised to humidify, and at other times it is advised to dehumidify. Sometimes, facility managers (fms) may read that ventilation is good, while another source may say it is part of the problem. Examining some basics related to these topics may help to clarify the issue.
It is also important to realize that the right answers regarding how to manage moisture often differ, depending on where a facility is located. For instance, from a moisture perspective, a building in Minneapolis cannot be managed in the same way as a building in Houston. The climates are very different.
Why Manage Moisture?
Moisture is a major cause of damage to buildings, and it typically leads the pack as a cause for facility repair costs. Moisture is also a major cause of occupant comfort complaints. High relative humidity makes people uncomfortable, and the first response is often to turn the thermostat to a cooler temperature. If the relative humidity is too high, this leads to mold growth, which is another leading cause of indoor air quality (IAQ) complaints.
In their duties, fms must respond to the needs of the building from a durability perspective as well as to occupant needs from a health and comfort point of view. To do this, fms must control the moisture content in their facilities, and in order to do that, they should know where moisture is coming from.
Moisture enters and affects a building in one of four ways. Some source points contribute a lot of moisture, while others contribute less. In order of the amount of moisture that each generally contributes (from most to least), these are:
- Air movement: This occurs via heating, ventilation, and air conditioning (HVAC) systems, as well as from drafts.
- Bulk or liquid: This occurs when water leaks in the roof, walls, or windows.
- Capillary: Water is absorbed into, or through, wood or concrete into the facility.
- Diffusion: This is the ability of water vapor in the gas state to move through seemingly solid, but vapor permeable, building materials (such as drywall).
It should be noted that air movement is a primary contributor to moisture levels in buildings. Air always contains water vapor, and air movement occurs all day, every day, while bulk water leaks are, hopefully, rare and repaired quickly. Further, air entering through a small gap will contribute 30 times more moisture than that which enters via diffusion through 32 square feet of drywall.
In properly constructed buildings, capillary breaks and water vapor diffusion barriers/retarders are incorporated into the structure. The function of capillary breaks is to separate materials that absorb water (and are not harmed by it) from those that would be harmed by water. An example is metal or asphalt impregnated building felt between concrete and wood. Meanwhile, vapor diffusion retarders/barriers slow or stop transmission of water in the vapor state through materials. Examples are polyethylene, foam insulation, and Kraft faced paper on insulation in the walls.
Vapor retarders/barriers present an example of how geographic location affects design and operation best practices. In colder climates, these items are recommended for placement on the inside of the wall (codes often refer to “the warm in winter side of the wall”). In warmer, humid climates, it is recommended the retarders/barriers be placed toward the exterior side of the wall (toward the humid air) or to construct a breathable wall, without any vapor barrier).
Wherever the location, to keep moisture at a level that does not cause harm to facility conditions, fms should aim to maintain interior relative humidity (RH) between 40% and 60% in the summer months and between 20% and 30% RH in the winter months. Why the difference? Fms can allow higher humidity levels in the summer than in the winter, because building surfaces are warmer (so there won’t be condensation). However, if the RH is allowed to exceed 60%, fms will run the risk of causing mold to grow; also, occupants will most likely become uncomfortable with that RH level. In the winter, fms must limit the RH to less than 30% in order to control condensation, which can lead to mold and rot.
HVAC And Moisture Management
The operation of the HVAC equipment has a significant impact on the moisture balance in a facility. There are two performance aspects of HVAC important to moisture levels. One is moisture removal through dehumidification, and the other is moisture introduced by ventilation air.
Setting the temperature of the air that leaves the evaporator coil or cold deck water too high will reduce or totally eliminate the dehumidification capacity of an HVAC system. Operators can set the temperature of the cold water circulating through the cooling coils, or they can vary the airflow across the coil. Changing either or both of these parameters will directly affect the dehumidification an HVAC system performs.
The air must be sufficiently cooled to cause enough condensation to control the moisture in the facility. To a great degree, the leaving air temperature of the HVAC coil determines the amount of dehumidification the HVAC system contributes. When investigating the reasons for mold and poor comfort in buildings, it is not uncommon for fms to find that the cold deck set point has been set to 62°F or higher. The aim was to save energy, but this practice inadvertently causes IAQ and comfort complaints from occupants.
The next issue is moisture introduced by ventilation air. ASHRAE Standard 62 requires ventilation with fresh outdoor air to maintain acceptable IAQ. In most parts of the United States, especially the southern region, summer ventilation air will introduce massive amounts of moisture into a building. Generally, air conditioners are designed to expend about 80% of their energy cooling the air and about 20% removing humidity—in other words, air that is already at conditions typical for conditioned indoor air. Meanwhile, ventilation air from outdoors contains two to three times as much humidity.
In these cases, there is an inherent mismatch between the moisture level of ventilation air and the design capacity of the equipment tasked with the job. Ventilation air needs 65% dehumidification, and HVAC equipment may only have 20% dehumidification capacity to bring it to acceptable indoor conditions.
The result is that the HVAC often cannot sufficiently dehumidify the ventilation air. As a result, in many areas, facilities with high ventilation loads require additional dedicated dehumidification and/or pretreatment of incoming air to maintain healthy indoor air moisture levels. It is not recommended to use an electric reheat method as a means to accomplish this.
ASHRAE Standard 62 allows fms to vary the amount of ventilation air based on several variables, including occupancy. However, many facilities are constantly ventilated as if they are fully occupied, while in fact they may only be lightly occupied much of the time. Fms can discuss with their HVAC professional or mechanical engineer about code approved ways to reduce ventilation air into the facility while still maintaining acceptable IAQ. One possibility is variable ventilation based on the level of carbon dioxide in the indoor air. This can also serve as a marker for occupancy levels, since it varies based on how many people are in a facility. This practice greatly reduces HVAC moisture removal load.
Another possibility is the use of HVAC economizer cycles. When outdoor air is cooler than indoor air, this control cycle shuts down the HVAC cooling compressor and brings in outdoor air to save energy using the free cool air.
The problem occurs when there is no consideration of the humidity level of the outdoor air being introduced. It is entirely possible to introduce so much moisture into a building during the economizer cycle that the building can’t be dried out by the HVAC unit during the next day or more. Therefore, in a humid climate, it is wise to reconsider the use of economizer cycles. This is especially true if it causes the indoor RH to exceed 60%.
Many building energy codes now give humid areas of the country a variance on the general requirement to include economizers in new construction. Just because an economizer is present doesn’t mean it is wise to employ it.
Controlling moisture is critical to durable, comfortable, and healthy facilities. Proper maintenance and use of cooling and ventilation aspects of an HVAC system are essential to controlling indoor RH. ASHRAE Standards allow for various control regimes to ensure fms do not overventilate their buildings. A comprehensive and informed approach is crucial to ensuring ideal moisture levels.
Garrett, CEM, is president of Building Performance & Comfort, Inc. (www.bldgperformance.com), a full service building science consulting company in Leander, TX. He spent nine years as manager of the award winning Austin Energy utility’s conservation programs and six years with the Austin Green Building Program focused on energy efficiency, sustainability, indoor air quality, codes, and moisture management. He regularly provides consultation and forensic diagnostic services for buildings and teaches seminars on building science related topics, energy codes, HVAC, and green building.
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