The Visual Arts Building (VAB) at the University of Iowa in Iowa City, Iowa, is a marvel of both aesthetic design and mechanical engineering. Six years in the making and completed in October 2016, the award-winning, 126,000 square-foot structure blends the artistry of the renowned Steven Holl Architects (New York) with an array of innovative mechanical systems that smoothly integrate with Holl’s aesthetics while meeting the numerous, industrial-like requirements of this challenging space.
Thanks to these mechanical systems, the VAB is projected to consume 60% less energy than a building in compliance with ASHRAE 90.1. A Verification Report prepared by a local utility, MidAmerican Energy, converted that percentage into an annual cost savings of nearly $300,000. The utility calculated the final payback on the “incremental costs associated with energy conservation strategy investments” at a comparatively swift 2.1 years. For this project performance, the university earned an incentive payment of more than $550,000 for the building, which recently achieved LEED Gold certification.
“Every time we talk about this building and the incredible level of system integration throughout the space, I’m amazed we pulled it off,” says Amy Infelt, P.E., LEED AP — and only half-jokingly. Infelt is managing principal for the Cedar Rapids, Iowa-based Design Engineers (DE), the mechanical and electrical engineering consultant on the VAB project, for which she served as project manager. She was assisted by mechanical engineer Tim Lentz, P.E., LEED AP; and electrical engineer Eric B. Bruxvoort, P.E.
Among the highlights of the VAB, none is more architecturally striking than its atrium, spanning the entire 150-foot depth of the five-story structure and extending 70 feet vertically. Ductwork, piping and conduit for mechanical, electrical and plumbing systems typically are concealed above the ceiling in commercial buildings, but the VAB has very few ceilings, and the design intent was for the atrium to remain clear of any exposed MEP systems.
That meant its thousands of feet of piping and conduit had to be meticulously coordinated, so systems were arranged in fastidiously neat and precisely parallel runs, rather than the less rigorous style of most above-the-ceiling construction.
“We created what we called ‘utility routing zones’ — specific areas within each room through which all ductwork, pipe and conduit could be routed, with a special hanging system for each zone,” Infelt says.
Thermally activated slab system
As Infelt remarks in her firm’s own literature on the VAB, “The installation of the thermally activated slab heating and cooling system required especially close coordination.” In part, that’s because the slab itself is not solid concrete, but what is called voided biaxial slab, or bubble deck, construction.
The VAB bubble deck consists of hundreds of empty, but watertight, plastic balls — or the preferred term, “voids” — 7-1/2 inches in diameter and sandwiched between layers of criss-crossing rebar, and all of it buried in 12-1/2 inches of concrete along with the Uponor Wirsbo hePEX. The latter, like most radiant installations, is laid out six inches on center, in a serpentine fashion. (See above illustration).
The special challenge for DE and the installing contractor: The Wirsbo hePEX had to be looped carefully into the slab in a way that avoided the voids (as well as all the rebar).
The rationale for this type of slab is straightforward enough: All those spherical voids allow for less concrete to be poured, without lessening the thickness nor sacrificing the integrity of the slab itself. Less concrete means a smaller load, which, in turn, permits weight-bearing column spacing to be far wider — an absolutely critical need inside the expansive, SHA-designed atrium.
More frequently seen in Europe, this type of void slab remains a rarity in North America, most especially in the Midwest. Equally rare in the Mid-west is a TAS system that provides not just heating, but also cooling. Concerns about condensation leave many building designers reluctant to specify radiant cooling. However, inside the VAB, which uses TAS construction on six different levels, “The system allows you to provide cooling in a way that keeps the slab surface temperatures above the point where condensation is produced,” Infelt says.
“What makes a TAS different is its use of the full concrete mass to store its heating and cooling longer,” she continues. “The six slabs at VAB contain no insulation, so a portion of the energy radiates upward. But, the Wirsbo hePEX is positioned five-eighths of an inch from the bottom of the slab. As a result, most of the radiation is downward, conditioning the space below.”
Radiant was an obvious fit for the new VAB with its large, loft-like, atrium space. Whatever misgivings the university construction team and Controls and Maintenance Group might have had about radiant were largely resolved by an off-site visit to Chicago and to the Loyola University Information Commons, which uses a similar TAS system.
“The extended, 25-year warranty provided by the Wirsbo hePEX was also a key factor in ensuring the university’s comfort with the TAS,” Infelt says. “Uponor played a key role in bringing us up to speed on activated-slab heating and cooling technology and then worked closely with us on the VAB design.”
Of particular value were the thermal modelings on the heating and cooling capacity of the slab, performed by Uponor Sales Engineer Him Ly. Using the slab depth, the tubing diameter, the depth of the Wirsbo hePEX inside the slab and the temperature of the water moving through the tubing, Ly calculated how many heating or cooling Btu per square foot the slab could provide.
“Once we understood the slab’s true capacity for heating and for cooling, we could best determine what supplemental systems were needed, and what type of system would work best in a given space inside the VAB,” Infelt says.
All the Wirsbo hePEX was installed in the six slabs before any interior walls were set. This tubing was routed out of the slabs through 71 different manifold cabinets with 634 circuits or loops of tubing. Built into the walls, these cabinets contain isolation valves, balancing valves and other hydronic accessories supplied by Uponor.
“The tubing is distributed via a supply-and-return manifold to a maximum of 12 loops per cabinet,” Infelt says.
“All the tubing was pressurized, and each of the 71 manifold cabinets was equipped with its own pressure gauge for its particular set of loops,” she continues. “Once in the morning and once at the end of the workday, the installation team would inspect each of the 71 gauges, comparing the two readings. Wherever there was a pressure drop, the contractor knew something happened that particular day to damage the tubing. Doing these inspections daily made it easier to isolate and immediately fix any problems.”
Supplemental HVAC systems
The VAB TAS is a “passive” system with minimal active control whose mass can be counted on to retain heating or cooling capacity over long periods. But that consistency also prevents it from reacting quickly to changing loads — such as a busy classroom with large numbers of students regularly moving in and out of the space.
“The capacity of the slab to heat or cool is what it is — a constant,” Infelt says. “That is why we designated it the baseline heating and cooling system for the building. We then created extra heating or cooling capacity with our supplemental systems, which can react more quickly — and actively — to changing loads.”
The “changing loads” can be triggered not only by student and faculty movements about the building, but also by the various specialized “industrial” processes taking place in its busy workshops. Another equally important factor is the weather extremes in Iowa where outdoor ambient temperature can range from subzero Fahrenheit in the winter to several days, even weeks, above 90° F in the summer. To help Transsolar understand the temperature ranges for the project area, Infelt would email them each time the area experienced temperature extremes.
Not surprisingly, DE ultimately chose to enhance Transsolar’s innovative TAS concept by adding supplemental HVAC systems that would support the teaching mission of the facility, while fitting in with its structure and climate. These systems include:
Outdoor and exhaust air systems: This system provides general exhaust for the building occupants, as well as exhaust from the industrial equipment and processes. In addition, these systems introduce outdoor air to provide ventilation air for indoor air quality, as required by ASHRAE 62.1 and to replace the exhaust air. These systems also maintain the building at a positive pressure relative to the outdoors. The outdoor air is delivered via variable air volume (VAV) terminal units with reheat coils and provides supplemental cooling and heating for the areas served.
Because of the number and type of industrial processes that occur inside the VAB, a considerable amount of makeup air from the outside is required for both replacement and ventilation air. This outdoor air is pretreated by a custom air-handling unit that maintains separate air streams: one running through a total energy, desiccant recovery wheel; and the second through a sensible heat pipe.
Located in the VAB’s lower-level mechanical room, the AHU pulls heat from some of the building’s exhaust before expelling it outdoors. At the same time, the air handler transfers that recovered heat to the incoming outdoor air before it moves into the building. This transfer is done through a slow-spinning desiccant wheel, handling the incoming interior exhaust on one side of the wheel and outgoing outdoor air on the other.
“We call it an ‘energy wheel,’” Infelt explains, “because it allows us to transfer both latent and sensible energy.”
But not all exhaust is permitted to move through the wheel. Exhaust from the various industrial processes is, of course, “dusty, smelly, even hazardous,” Infelt says, and therefore may not be transferred to the incoming outdoor air. This exhaust-air stream routes through the refrigerant-charged heat pipe where its heat is recovered and transferred to the incoming outdoor air.
“The heat pipe is a heat-recovery device only,” Infelt says. “It can capture only sensible energy, while keeping contaminants and odors on its side of the air-handling unit.”
While less efficient than the wheel, the heat pipe is still able to recover the heat from the exhaust from the industrial spaces, she adds.
Fan coil units: These units provide supplemental heating and cooling where the demand is beyond the capacity available from the TAS and the ventilation air system.
Radiant heating, installed in designated places on the building perimeter: The TAS structural slab at the VAB is covered by a 3-inch-thick topping slab. Wherever insulated channel glass has been installed around the exterior, a 3-foot-wide strip of radiant heating — again using Wirsbo hePEX — was installed inside the topping layer. Controlled separately from the TAS system, this perimeter radiant install provides heating only during the colder months, shutting down in the summer. No insulation is used because the 12-1/2-inch structural slab directs all the heating upward, where it is needed.
Single-zone displacement ventilation system: This energy-efficient system serves the 76-seat classroom by supplying air at a low — and therefore quiet — velocity, befitting an academic environment. The desired comfort temperature where students and faculty congregate at floor level (the “occupied zone”) is maintained, while the air space above is permitted to become progressively warmer as you near the ceiling.
Embracing the new
Post-construction tests have yet to be run comparing the actual performance of the VAB with its ambitious, 60% savings target. But Infelt reports the UI Controls and Maintenance Group, which is responsible for the building’s operation, has found the TAS and its supplemental HVAC systems to be “very robust, providing a very comfortable environment for the occupants.”
Given that the activated-slab system is such a good fit for large commercial projects with high-ceilinged, open areas, is DE looking to apply what it learned on the VAB to other commercial jobs?
“We would sure like to,” Infelt concludes.