Integrating a high-performance building
This case study shows how design teams can integrate building envelope with HVAC, lighting, and shading.
- 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.
- Gain insight into the interdependency on multiple building systems in an integrated project delivery mode, particularly with regard to daylighting systems.
- 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.
- Owner: State of Iowa
- Project Architect: BNIM, Des Moines, Iowa
- MEP Engineer: KJWW Engineering Consultants, Des Moines, Iowa
- Structural Engineer: Charles Saul Engineering Inc., Des Moines, Iowa
- Civil Engineer: Snyder & Associates, Ankeny, Iowa
- General Contractor: J.P. Cullen & Sons Inc., Janesville, Wis.
- Energy Modeling: The Weidt Group, Des Moines, Iowa
- Commissioning Agent: Engineering Economics Inc., Lenexa, Kansas
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.