Integrating a high-performance building

This case study shows how design teams can integrate building envelope with HVAC, lighting, and shading.

By Scott Bowman, PE, LEED AP, BD+C, KJWW Engineering Consultants, Des Moines, Io March 12, 2014

Learning objectives

  1. Learn how early analysis and integrated design can lead to high levels of building performance that cannot be achieved without the cooperation and collaboration of the entire design team.
  2. Gain insight into the interdependency on multiple building systems in an integrated project delivery mode, particularly with regard to daylighting systems.
  3. Understand high-performance envelope systems and how tuning the envelope to the particular needs of an elevation’s exposure can be beneficial.

The U.S. Green Building Council LEED Platinum Iowa Utilities Board and Office of Consumer Advocate (IUB/OCA) building in Des Moines, Iowa, is a model for the maximum integration of building envelope, HVAC, lighting, and shading. The 44,640-sq-ft facility illustrates how a high-performance building can be achieved on a modest budget ($9.5 million, or $213/gross sq ft) using off-the-shelf tools, intense system integration, and an energy design philosophy of “Use Less. Use Efficiently. Make On-Site.”

The integration began with the owner and design team targeting exemplary energy efficiency, and the agreement that all design decisions would be measured against their effect on the energy performance. The earliest goal of the project was to meet an energy use intensity (EUI) of 28.0 kBtu/sq ft/year, equivalent to 60% energy savings beyond the energy code baseline of ASHRAE Standard 90.1-2004. Every decision―from the envelope to HVAC to lighting―was made with this goal in mind. The team didn’t use solar to make up for inefficiencies; it designed the building for ultra-high performance first and then added the benefits of on-site energy production.

Building envelope

The envelope of the IUB/OCA uses several strategies to mitigate heating and cooling loads, capture passive energy and daylighting, and allow for natural ventilation. These strategies, in turn, reduce the building energy needs and lay the foundation of the building’s ability to use less.

Envelope design began with building orientation. Running the two-wing structure along an east-west axis with a shallow north-south depth provided proper solar orientation. This orientation, along with an optimized footprint depth, allowed the building to take advantage of the more appropriate and controllable north and south daylight and natural ventilation opportunities.

The envelope has a window-to-wall ratio of 39% and employs high-performance glass, specifically tuned to each elevation’s exposures. At the south elevation, where fenestration is protected by the daylight-harvesting sunscreen, glazing with a higher solar heat gain coefficient (SHGC) of 0.62 and a visible light transmittance of 74% was employed. The higher SHGC at the south allows for the envelope to maximize passive heat gain in the winter months when the sunscreen allows for direct light penetration. Visible light transmittance is maximized at this area to work in concert with the same sunscreen and provide ample daylight from the southern source. At the west and east elevations, a lower SHGC of 0.38 and a visible light transmittance of 44% were used to minimize heat gain and uncontrolled daylight. These windows were placed for key views at circulation terminations and minimized; they have additional fritting to minimize glare and provide more shading.

Operable windows are located within 15 ft of 53% of the interior space and are integrated with the BAS, which identifies favorable exterior conditions and sends an e-mail to occupants when windows can be opened. (The system shuts down a zone’s heat pumps when windows are open.) Similarly, the system notifies occupants when they should close windows.

The architects and engineers also were obsessively detailed about the envelope to eliminate thermal bridging. In the Midwestern climate of hot/cold extremes, white precast concrete (with continuous insulation and non-thermally conductive ties) provides a simple yet high-performance envelope, eliminating traditional thermal bridging at roof interfaces, foundation walls, and wall openings. Continuous insulation wraps uninterrupted from the roof into the thermal wythe of the wall panel and then down and around the foundation system and across the underside of the slab on grade. Particular attention was paid to the interface of the ground floor slab into the vertical wall construction, one area proven to be a significant heat sink in other high-performance buildings.

Lighting and shading

Ultimately, 95% of regularly occupied spaces have daylight and views due to the optimized glazing at the northern and southern elevations. West and east elevations received minimal openings as they are the least effective at daylighting and deliver excessive heat gain and glare. These and other strategies reduced the overall lighting energy use in the building by 70% over the code baseline. Key components of the design include the use of light tubes, daylight-responsive dimming, and daylight-harvesting sunscreens.

Articulation at each façade was determined by sun exposure; louvered sunscreens, with horizontal blades and vertical fabric panels at the south elevation of each wing, reflect daylight during all seasons, block unwanted summertime heat gain, and allow passive winter heating. The parabolic profile reflects high elevation summer sun off of the curved portion and low winter sun angles primarily off of the flat portion of the louvers. The sunscreens, combined with an optimal building footprint depth, allow daylight to penetrate deeply into the building during all seasons. Zinc-clad office enclosures on the north elevations take advantage of diffused northern light. Solid west and east elevations define the mass of each wing with glazing strategically located to frame views.

Open office workstations on the south further maximized daylighting. Even the workstation furniture―based on daylight modeling―was optimized. At the inner-most point of the building, daylight modeling demonstrated that the selected partitions (36-in. solid and 16-in. glass) allowed for the required foot-candles at the work surface without artificial lighting for 70% of the time. The owner’s existing 64-in.-tall solid partitions only allowed for this kind of performance for approximately 30% of the time.

Along with daylighting and shading, appropriate luminaires, lamps, and controls were selected to maximize energy reduction of lighting while providing the greatest visual acuity and comfort. Integration of these strategies in the IUB/OCA resulted in a lighting power density (LPD) of just 0.62 W/sq ft. This is 40% below a similarly designed building according to ASHRAE/IES Standard 90.1 lighting allowances using the building area method for office buildings (which allows 1.0 W/sq ft).

Light fixtures in the open office, private offices, conference rooms, and lobby have 0 to 10 V dimming ballasts and controls to take full advantage of the sunscreens. Light fixtures specified throughout the project typically use single lamp T5 or T5HO. Fixtures are grouped into zones based on their proximity to exterior glazing or skylights. Daylight sensors are positioned around the building perimeter and mounted to the ceiling and/or the bottom of linear pendant fixtures. Each sensor provides daylight information to the lighting controls for automatic adjustment. Lighting levels for each daylight zone are programmable from the lighting control software.

Occupancy sensors are used throughout the building, even in the open office, where at night and on weekends lighting is only allowed if staff is working.

HVAC systems

The envelope’s ability to capture passive energy, harvest daylight, reduce solar heat gain, eliminate energy-wasting thermal bridging, and provide for natural ventilation―along with the high-performance lighting to reduce lighting use during peak operation―paved the way for a smaller, less costly, and more energy-efficient HVAC system.

The IUB/OCA―designed to maintain a heating temperature of 72 F and a cooling setpoint of 74 F with a maximum of 50% relative humidity―employs a high-efficiency geothermal heating and cooling system with a geothermal field tied to dual-stage water-to-air heat pumps with electronically commutated motors (ECM), and variable speed pumping. A total energy recovery unit (both sensible and latent) provides energy savings by capturing the heating or cooling from the exhaust air and pre-tempering fresh ventilation air.

This dedicated outdoor air system feeds into the back of individual heat pumps in the building, providing good indoor air quality for the users. The ventilation air supplied to the building is monitored and trended. CO2 sensors are employed in densely occupied spaces, and a demand control sequence is used to move ventilation air to the rest of the building versus an almost empty room.

As a result of the integration of high-performance envelope, shading and lighting, the load consumed by heating and cooling is drastically reduced.

Figure 4 shows the distribution of the energy use for a code-compliant baseline for the building and program. Notice that the heating and cooling are the largest loads, and together are over half the energy use. (This is for a packaged variable air volume, or VAV, system with direct exchange, or DX, heating and cooling, and is defined by ASHRAE 90.1-2004.) Figure 4 also shows the distribution of the energy use as designed for the IUB/OCA. Notice the largest load is now plug load, larger than heating and cooling combined.

Notice too that the fan/pump energy also increased as a percentage of the overall use. This is why ECM motors were specified for the heat pump fans, because they are extremely efficient and adjustable. This left the hydronic pumping system to be optimized. Because typical energy modeling techniques do not reflect the pumping use well, a spreadsheet was used to evaluate different methods of pumping. Several systems configurations were evaluated, from central pumping with variable frequency drives (VFDs) to fully distributed pumping at each piece of equipment.

Because several of the loads needed a constant flow when on, the optimum system was a combination of central and distributed pumping. The heat pumps were installed with automatic isolation valves so that when off, the flow through the unit is closed, and the flow of the overall system was reduced by the VFD 

on the central pumps. Then a couple of larger loads, such as the energy recovery ventilation (ERV) and a water-to-water heat pump, were provided with smaller distributed pumps that only activate when the unit is on. This analysis was confirmed in the measurement and verification process, as this portion of the building load was using less energy than the energy model predicted.

Integration equals efficiency

The IUB/OCA recently achieved an Energy Star score of 100, and over the first two years of operation has performed extremely well, with an EUI of 21.2 kBtu/sq ft/year without renewable energy, and 16.7 with renewable photovoltaics (PV). This exceeds the baseline by more than 77% and the LEED energy model by 27%. The building is outperforming efficiency targets, and the PV is providing more on-site energy than expected.

Energy modeling was completed by The Weidt Group, Des Moines, Iowa, which is also subcontracted through the Iowa Energy Center to conduct the highly detailed measurement and verification (M&V), which has shown significant results. This process included the refinement of the design energy model to include actual building schedules, actual plug load energy use, and real matched weather data, providing the design team with a view into the decision-making process for the project.

Preliminary results from the most recent year reviewed (from March 2012 to April 2013) show the energy model is predicting energy use to within 1% of actual. Having this data informed the design team on the energy modeling tools used. For example, the model slightly over-predicted savings from daylighting and under-predicted savings from fans and pumps. Decision making was not compromised, but the ability of current software still does limit some predictions. This was demonstrated dramatically with the plug load of the building, which was much less than expected due to occupancy controls at workstations and diligent management by the users to use only what is needed. (Plug load prediction and control need to be addressed by the industry, and are mostly outside the control of building design teams.)

The maximum integration of building envelope, HVAC, lighting, and shading allowed the design team to not only reach but surpass the energy performance goal of 28 kBtu/sq ft/year.The IUB/OCA’s affordable, optimized, and integrated design strategies provide a model for other facilities anywhere in the country.

Watch this video for more about the project.

Scott Bowman is principal/corporate sustainability leader at KJWW Engineering Consultants and has over 30 years of experience in high-performance building system design and overall project management. His specialties are in direct digital controls, energy efficiency, sustainable and green design, and systems commissioning. Carey Nagle serves as a leader in the design and management of high-performance and sustainable projects in his role as a project manager/project architect at BNIM. His broad range of project experience consists of several office buildings, higher education projects, theaters, and museums.