Schools Tackle Humidity Problems
March 26, 2006
From 1995 to 1997, the United States General Accounting Office (GAO) issued seven reports on the condition of American school facilities. At that point in time, the reports stated that it would cost approximately $112 billion to bring schools into good overall condition and that about a third (or 25,000) of all public schools needed extensive repair or replacement of at least one building.
The main report that was issued in 1995, School Facilities: Condition of America's Schools, detailed that 50% of the schools reported at least one unsatisfactory environmental condition. And, no surprises here, ventilation was the second most frequently cited unsatisfactory condition.
Since that report was issued, ASHRAE's requirement for 15-cfm/occupant of outside air has been routinely used as the basis of school design specifications. However, the increased ventilation rates prescribed by ASHRAE do not necessarily mean a school will have proper IAQ. Some schools have increased ventilation rates only to find that the problem is made worse by the introduction of humid outdoor air, which increases the latent load in the building, making occupants uncomfortable.
Compounding the problem are HVAC systems that weren't designed or installed correctly in the first place, or equipment that hasn't been maintained properly over the years. Hopefully, school officials and building designers realize that the relationship between the HVAC system, outdoor air, and indoor environment can have a negative impact on the building and its occupants if not designed to control humidity properly. However, tight budgets often lead to poor decisions being made regarding humidity control, and that ultimately affects the indoor environment in our schools.
POOR DESIGNThe simple reason why schools are suffering from humidity problems is poor design, according to Larry J. Klekar, vice president-sales for Munters Commercial Division in Selma, TX.
"Engineers are feeling the pressure to build schools inexpensively, which has caused them to search for inexpensive methods to treat makeup air. Code dictates the amount of makeup air they bring in, but frequently they choose inadequate methods of treating this air, which has resulted in several schools being shut down for costly mold remediation. Many of those schools have had to deal with even more expensive litigation. If they would just properly condition the makeup air, it would do so much to help prevent mold and structural/material deterioration."
Eric Althouse, an indoor environmental quality consultant for Air Intellect, LLC in Tallahassee, agreed, noting that, "Nearly all humidity problems in schools begin with poor design. Maintenance budgets are wrecked when a poorly designed building is opened."
The awareness of the engineering community concerning the long-term effects of moisture in buildings is improving somewhat with the publication of ASHRAE's Humidity Design Control Guide, said Bill Holder, vice president of Assured Indoor Air Quality, LP of Dallas. "However, most engineers design systems the way their grandfathers and fathers did and will continue to do so until they get nailed in a lawsuit. The performance expectations of the owners are increasing, but seldom are those expectations documented sufficiently before the system is designed."
Poor mechanical design combined with a lack of understanding by the temperature controls contractor can make the problem worse, according to John Bergman, principal with John E. Bergman LLC, in Columbia, MD. "If the engineer is not experienced with schools, which are different from any other system, he probably will not treat outside air properly. Problems also arise when the temperature control contractor is asked to provide controls for dedicated outdoor air system units instead of the unit manufacturer. Manufacturers thoroughly understand psychrometrics, and I have not found many temperature controls engineers who understand psychrometrics."
OTHER FACTORSPoor system design isn't the only reason why schools suffer from moisture problems, though. Brian Monk, vice president sales of Dectron Internationale in Roswell, GA, noted that improper maintenance of equipment is often a bigger issue. "We've seen instances where equipment is put in schools and things like loose bearings, broken belts, and other general maintenance issues are never addressed. We've seen too many school districts where the superintendent is stretched too thin and has to oversee multiple schools. Overall, we see schools failing to budget appropriately for long-term and routine maintenance."
Holder agreed, noting that operating budgets are seldom large enough for an adequate maintenance program, much less a PM program.
Moisture problems in schools have been around for a long time, said Michael A. Pinto, CEO of Wonder Makers Environmental in Kalamazoo, MI, and many of the problems fall into four major categories:
"In our experience, the type of problem varies a little bit by area of the country. For example, there are more condensation problems down south and more boiler leaks in the north, but the primary factor is whether or not the school has an effective maintenance program," stated Pinto.
Roof leaks or condensation problems often seem to happen during the two to three months that schools are not in operation, which coincidentally, are the most humid months of the year. "Problems seem to occur primarily during summer break, or during periods in which the primary cooling system (which would help to prevent such problems) is throttled back or shut down," said Gregg Dunn, director of marketing for Dehumidifier Corporation of America in Cedarburg, WI. "Then outdoor air is brought into the building in increasing volumes to provide cooling and/or circulation and that air is more moisture laden than that of the air introduced during other times of the year."
Problems also result from what occurs right before summer break: Schools are cleaned up, floors are mopped, and carpets are shampooed, then the building is locked up for three months. "Because air conditioning cannot be justified during the unoccupied summer, the next September finds the building with a mold infestation, damp books, and sometimes a damaged gym floor," said Bergman.
SOLUTIONSSchools are unique buildings in that they are seasonally occupied, and they have their air conditioning shut off after school hours, on weekends, and during summer months. This is done to save energy, but it also eliminates any chance for humidity control. Without controlling humidity, school buildings can become a breeding ground for mold and mildew. So what's the best way to control humidity? The solution is dependent upon the type of problem that is being addressed and the outdoor environment where the building is located.
Holder stated that when developing design criteria for a school, all the constants and variables must be considered and factored into the final equation. He has found that most school buildings constructed since the early 1980s have varying degrees of infiltration problems ranging from "horrible to barely tolerable." Combining infiltration problems and a humid climate with an HVAC system that does not manage moisture effectively for its location almost always results in poor performance, accelerated building deterioration, and poor indoor environmental conditions.
"Given this situation, I would develop the design criteria using a dedicated outdoor air system for ventilation, a demand control ventilation strategy for energy conservation, and building pressurization using active desiccant technology for dedicated outdoor air conditioning units to deliver neutral temperature air (72° to 75°F) at a 45° dewpoint," said Holder.
He added that a 45° dewpoint is low enough to keep most of the moisture off of the cooling coils in the air-handling systems that are controlling temperature, which will reduce the amount of moisture that could be reintroduced into the supply airstream during part load operation. The resultant relative humidity in the school will be somewhere in the range of high 40% to low 50%. Historically, Holder noted, the costs to design and retrofit existing buildings using the above scenario have usually been in the range of $25 to $30/cfm of outdoor air.
Klekar agreed, noting that dedicated makeup air systems can be retrofitted onto existing air handlers to treat the makeup air required. "In many instances, the existing ductwork can be used, reducing the cost of installation. When the dedicated makeup air system is combined with the existing A/C system, the dedicated system controls the humidity and is operated off the humidistat, and the existing A/C systems remains controlled off the thermostat."
According to Althouse, it is wise for school HVAC systems to be designed with one system that will handle cooling and a separate system to handle dehumidification of the outdoor air. "The dehumidification can be run by itself during a long summer when much of a school may be unoccupied. Or after a water event, the dehumidification system can be run to quick-dry a building. In addition, many school buildings do not meet guidelines for having a positively pressurized space. Active desiccant systems are the most energy-efficient way to dry the outdoor air and to positively pressurize a building."
Bergman believes that the basic solution for schools is to ensure that all outside air is chilled and therefore dehumidified to a dewpoint that is equivalent to the design point (for example, 75° db, 50% rh is equivalent to 55° dewpoint) minus any additional reduction in the supply dewpoint temperature sufficient to offset any internal or external humidity gains. Internal gains include showers, people, etc., while external gains include infiltration due to an inadequate building envelope.
"The simplest retrofit is to add a pretreatment device to the outside airstream," said Bergman. "This can be a precool/reheat dehumidifier, a mechanical dehumidifier, or an energy recovery unit (so long as it has adequate chilling and reheating capability). Retrofits are not inexpensive, but they pale in comparison to legal damages."
In a perfect world, perhaps the best way to combat moisture problems in schools would be for districts to get away from capital costs quotas, where they allocate a certain amount of funds for equipment, which is probably based on past school construction projects. "Instead of capital cost budgets, they should lean toward life-cycle cost budgets, which includes money for on-going maintenance, which in the long run, would be less expensive because the equipment will have a longer lifecycle," said Monk.
While this scenario is not likely in most of our cost-conscious school districts, system designers must make sure the HVAC components are able to control moisture at all times: during peak sensible, and latent load conditions, as well as during both occupied and unoccupied periods. It may be easier said than done, but given the litigious nature of our society, it's best to find a way to make it happen. ES
Sidebar: School Learns To Control Humidity The Hard WayMold in schools is getting a lot of attention at local, state, and federal levels - and it's getting expensive. For example, in 2002, an Austin, TX, school district passed a $49.3 million bond issue to pay for mold removal and preventive maintenance in 91 schools. That same year, the Texas Department of Health said at least 10 school districts had reported mold problems in the last year.
In addition to climate, changes in how schools are built are a contributing factor. Gypsum wallboard, now widely used in place of plaster, allows a fertile environment in which mold can grow. While older school buildings did not have carpeting, it is common today, and that too can be a breeding ground for mold. Flat roofs, which make it harder to detect leaks, also contribute.
Wide variations in temperatures and humidity levels in South Texas make it difficult to maintain internal atmospheric conditions to minimize the formation of mold or mildew. For example Pharr-San Juan-Alamo School District's Memorial High School, a $23 million project, had been open less than three years when toxic mold issues reached crisis stage.
Mold grew on ceilings, walls, equipment, and books, and students and staff complained of allergic reactions and respiratory infections. Unconditioned and uncontrolled hot, humid outside air from ventilation and infiltration had brought in so much moisture, the building had reached a saturation point. Small roof and window leaks also contributed to the problem, and mold proliferated. Desks and floors were wet, and humidity inside the building was as high as 90%.
Assured Indoor Air Quality, LP of Dallas, came up with the solution, which included bringing large volumes of fresh outside air into the building to meet health and ventilation codes. Since this outside air was also the main source of humidity, temporary desiccant dryers from Munters were brought in to supply dry air to the building. While the mold was removed and materials replaced, a permanent system was designed around the desiccant dehumidifiers, which condition the outside air and pressurize the building to keep humid air from being drawn inside.
Over a three-month period, the desiccant dehumidification system dried the building. Today, the school is kept at 45% rh and 75°F, and the building is mold free.