The HVAC, IAQ, and comfort challenges of prefab classrooms, which house millions of students in the U.S.
Whether you are a general contractor, consulting engineer, supplier, manufacturer, or leasing company, it's a good time to be in the modular-and temporaryclassroom industry. Business is booming. The state of Florida illustrates how dependent upon modular-and temporary-classroom space communities and school districts have become. The Florida Department of Education conducted a survey of school districts in the state's 67 counties and published "The Use of Relocatable Classrooms in Public School Districts of Florida.".
The report concluded that, as of 1993, there were more than 16,000 relocatable classrooms in use in the state. The average age of the units was 19 years, while the expected service life was 23 years, on average. A large number were over 40 years old. In Florida, and many other states with rapidly growing student enrollments, these units are being replaced, frequently with new "temporary" units.
The Modular Building Institute (MBI) represents manufacturers, suppliers, and dealers of commercial factory-built structures. Typically, these units are 8 ft by 16 ft up to 18 ft by 84 ft. The standard construction is wood frame mounted onto a steel chassis with fixed or removable axles and hitches. While these trailers may be the norm, nearly 25 percent of the floor space sold and leased by MBI companies in 2002 is defined as either "one-story complex" or "multi-story complex."
"There are many misconceptions about the modular-classroom market," Mike Lehman, president of Bard Manufacturing Co., one of the largest suppliers of modular-and portable-classroom HVAC equipment,said. "Modular classrooms are light years beyond the 'trailer' design reputation that they were known for seven or eight years ago. Many of our customers are designing modular-classroom campuses that you would have to look twice at to realize that the classrooms were not built on site."
Modern modular classrooms have standard floor plans and features and often are built as permanent classroom space. The benefit here is that the site work for the school and factory construction of the structures occurs simultaneously, giving a school district what it often wants most after affordability: rapid occupancy.
The high-volume markets traditionally are found in California, Arizona, Texas, Georgia, and Florida. But there have been an increasing number of portable/modular schools going up in northern climates, according to Bard.
SCHOOLS BUCK THE TREND
MBI companies reported producing nearly 8 million sq ft of temporary space in 1998, 8.2 million in 1999, 7 million in 2000, and nearly 10 million in 2001, but only 5.6 million in 2002. 2 MBI estimates that the 2002 sales of modular space totaled $1.84 billion, 30.2 percent of which was for education. Despite the steep decline in temporary floor space produced in 2002, it is interesting to note that sales for education floor space increased in 2002 by over 24 percent from 2001.
The National Center for Education projects that the public-school student body (K-12) will reach an all-time high of 15.9 million students in 2005. The previous record was 15.7 million in 1976.
According to Triumph Leasing Corp., which leases portable educational space, the U.S. will spend over a billion dollars annually on portable school space until this enrollment bulge ends in 2012.
"Our estimates are that there are several hundred thousand modular classrooms currently being utilized in the United States," Lehman said. "We estimate that there were approximately 20,000 modular classrooms built in 2002."
According to the American School Board Journal, more than 2 million students attend classes in modular classrooms in California. 3 Cesar Chavez Elementary School in Corona is made entirely of modular units. Clark County, Nevada, is the fastest-growing area in the country, opening, on average, eight new schools annually. To accommodate the 10,000 new students that enter Clark County schools each year, the county relies on 4,500 portable classrooms.
The State of New Jersey provides a good illustration of why modular-classroom-space is so essential.
"The State of New Jersey is buying hundreds of TCUs (temporary classroom units," Edward H. Brzezowski, PE, a consulting engineer in New Jersey and member of HPAC Engineering's Editorial Advisory Board, said. In crisis areas involving billions of dollars of deferred maintenance, capital construction, and swing-space needs, "The state has no other options but TCUs," Brzezowski added.
In 2000, New Jersey legislators passed the Educational Facilities Construction and Financing Act in response to a 1998 state Supreme Court decision (Raymond Abbott et al. v. Fred G. Burke), the upshot of which mandates the state to assume responsibility for all costs necessary to repair and replace schools in poor districts, now known statewide as Abbott districts. Abbott districts will receive 100 percent of eligible costs for school construction and renovation. This means that most of the schools in Abbott districts need to be renovated or replaced; thus, TCUs are unavoidable in these areas because entire schools are being reconstructed.
THE BENEFITS OF MODULAR
The cost of the all-modular Cesar Chavez school in California was $5 million, less than half the projected cost of conventional construction. A 1998 study by a California education-research organization, EdSource, determined that a conventional classroom costs between $115,000 and $177,000 to build in California. In contrast, a temporary/ modular classroom costs between $35,000 and $100,000. 4
Portable/modular-classroom space includes classrooms, restrooms, locker facilities, conference rooms, and offices. Individual units, especially those intended for stop-gap use, look like trailers with skirts covering the piers or blocks that serve as a foundation. However, many modular units are designed for long-term use, which is a trend that is expected to grow.
Water service and plumbing usually is not difficult to provide to temporary/ modular space, especially if it is adjacent to a brick-and-mortar school. It is just a matter of shunting off the system at the main structure. Temporary/modular HVAC runs the gamut from heat pumps, A/C with electric heat, both gas and electric, split systems (gas, oil, or electric), and packaged systems from 1 to 10 tons. However, the vast majority are electric systems. So, electricity, water, and sewer are the only utilities.
Bard sells exterior wall-mounted air conditioners, heat pumps, and gas/ electric units with cooling capacities ranging from 1 to 6 tons. Bard also provides interior wall-mounted air conditioners, heat pumps, and geothermal units with cooling capacities ranging from 1 to 5 tons. The company provides built-in ventilation packages ranging from barometric dampers to energy-recovery wheels, energy-management systems, and horizontal split-system air conditioners.
Delivering portable/modular space can be difficult, depending on local transportation limitations, bridge capacities, and highway widths.
A survey of modular space by the school board in Orange County, Florida, revealed rampant leakage, moisture, and structural problems. However, this may reflect poor maintenance, rather than poor construction.
Where there's water, mold and IAQ complaints are not far behind. The Environmental Working Group (EWG) in California studied the state's modularand temporary-classroom space and concluded that kids in these classrooms are exposed to higher levels of volatile organic compounds (VOCs) than their peers in site-built schools. 5
Of the nearly 1,000 temporary classrooms studied, nearly half failed to meet health guidelines for eight-hour indoor exposure to formaldehyde. This study found portable classrooms are more likely to have carpeted floors, pressed-wood bookcases, metal roofs, and suspended ceilings than is conventional school space.
Early last year, Papillion La Vista School District in Nebraska found mold in five portable classrooms at four different schools after a student came down with an allergy problem at school. The district closed the portable classrooms for repairs and to replace the carpeting.
However, maintenance, rather than construction, may be the real culprit. Mike Apte, PhD, a staff scientist with the Indoor Environment Department of Lawrence Berkeley National Laboratory (LBNL), has studied indoor environmental quality (IEQ) within portable schools for many years. He has found that whether temporary or permanent, portable/modular classrooms often get haphazard maintenance, at best.
Part of the maintenance deficiencies is access, Brzezowski said.
"Typically, there's a unitary unit on the side of these things," Brzezowski said. "You also have wood staircases and ramps as an ADA (Americans with Disabilities Act) requirement. Looking at it from a bird's-eye view, if you have a dozen units with all these stairs and ramps and all the electric supply lines running around, maintenance can be a mess.
"Instead of a primary plant or a couple rooftops units, you now have all these distributed units that are mostly electric and installed through the wall," Brzezowski added. "Many are just not that easy to service."
Portable classrooms, depending on the location and the type of HVAC system, can be quite loud. 6
James P. Waltz, PE, a consulting engineer based in Livermore, California, and a member of HPAC Engineering's Editorial Advisory Board, said the makers of modular classrooms need to work harder to limit reverberation time within the structures. Also, it's very difficult to remove HVAC equipment far enough from these classrooms to solve the noise problem.
Ideally, Waltz said, portable classrooms would use split systems so that the air-handling and refrigeration functions were separate. However, the driving factor for most school districts is cost, which puts these systems out of reach.
"I'm a former school board member and can tell you that the last place school boards spend money is on facilities," Waltz said.
Apte and his colleagues at LBNL have been studying the ventilation and indoor-air-quality issues related to portable classrooms for many years. While portable/modular classrooms pose some maintenance-related IAQ challenges because of their status as non-permanent structures, Apte believes these IAQ and ventilation problems are the same as those found in site-built classrooms.
"In California, there's been a lot of press on IEQ issues in portable classrooms and schools in general," Apte said. "One of the big issues we identified is ventilation. The portable-classroom study mandated by the California state legislature identified a startling lack of ventilation in portable classrooms."
According to Apte, the two major ventilation problems associated with portable classrooms are ventilation control coupled to thermal controls, which causes ventilation only to occur when heating or cooling is being supplied, and HVAC-system noise. Noisy units force teachers to turn off ventilation fans.
Apte's lab recently conducted a study in which portable classrooms made with conventional materials were compared with those made with lower-VOC materials (carpets, ceiling tiles, and wall panels). 7 Furthermore, both types of portable classrooms with standard wallmounted heat-pump A/C units and a new hybrid indirect/direct evaporativecooling (IDEC) unit with gas-fired hydronic heating were tested.
IDEC significantly improved energy efficiency relative to vapor-compression air-conditioning systems because cooling is provided through the evaporation of water. Unlike a swamp cooler, IDEC supply-air humidity is low because most of the cooling occurs indirectly across a heat exchanger.
"We made a hybrid IDEC that has a heating component as well," Apte said. "We integrated a large fan coil into the plenum of this unit and plumbed it up into a hydronic loop, where we provided heat from an instantaneous water heater that fired up every time there was a demand for heat." (Figure 2)
Like standard portable-classroom units, the hybrid IDEC is an electric system and roughly the same size as HVAC units for this application. Because the unit has no compressor and relies on a highly efficient fan motor, the IDEC system required 30- to 50-percent less energy than the standard heat-pump A/C unit in the LBNL tests. As for noise, the IDEC unit is 7- to 12-dB quieter than standard equipment in unoccupied classrooms.
Interestingly, Apte's team found that VOCs also were significantly reduced with the IDEC systemboth in the standard design classrooms and the low-VOC classrooms. It seems ventilation has more to do with VOC reduction than the building materials used, the team found (Figure 3). VOCs were reduced by more than half in the IDEC classrooms, regardless of building materials.
CO2 levels also were reduced in the classrooms during IDEC hybrid operation compared with standard HVAC operation, but this finding was inconsistent. ASHRAE Standard 62-1999,Ventilation for Acceptable Indoor Air Quality, specifies a minimum ventilation rate of 15 cfm per person in classrooms. Ventilation delivered at this level typically reduces indoor CO2 levels to below 1,000 ppm.
IDEC did reduce CO2 levels during the cooling season compared with standard portable HVAC (960 480 ppm vs. 830 530 ppm, respectively). However, the IDEC system has the same problem as standard portable-classroom HVAC: Teachers choose to turn off both systems when outside temperatures are comfortable. With no ventilation, CO2 levels far exceeded 1,000 ppm in the LBNL test, going as high as 3,000 ppm, regardless of the ventilation system.
Apte said this human-factors problem is common with many conventional portable-classroom HVAC systems. Some manufacturers are considering offering separate thermal and ventilation control to prevent the problem.
"Ventilation is very dependent on the operator using the equipment," Apte said. "Teachers are not really trained in the importance of providing ventilation. They use the equipment just for thermal needs. We found this to be true even with the IDEC equipment."
Lehman also thinks user error is a significant problem: "The IAQ issues that modular classrooms face are no different than those that site-built schools face. Adequate ventilation air, low sound levels, and maximized energy savings are all areas where a properly designed HVAC system can provide benefits to a school district utilizing modular classrooms. It's up to the HVAC industry (i.e., equipment manufacturers, distributors, sales representatives) to educate school administrators on the environmental-control issues and how our industry can help them address their specific needs within their budget guidelines."
According to Apte, the IDEC system soon will be marketed for the portable/modular-school market under the trade name "Oasys."
As for new products from Bard, "We cannot divulge specifics," Lehman said. "But it's safe to say that we are making great strides in the areas of ventilation, operating sound levels, energy efficiency, and energy management."