435 Indio Way, Sunnyvale, Calif.

A lot of 1970s-era office buildings are cold concrete boxes that seem to repel light. One of these buildings in Sunnyvale, Calif., was particularly dark, derelict, and impossible to rent. To make this building rentable, the developer became intrigued by the idea that a net zero energy renovation could be a sustainable and profitable option for this building.

The design team retrofitted the existing uninsulated office building at 435 Indio Way to be a net zero energy (NZE) building, creating a new and replicable model for profitable net zero retrofits. The building has notably also gone from Class C- to Class B+ in real estate terms and leased out in record time. The developer now has a savvy business case for future net zero retrofits.
The developer worked with designers on the net zero design concepts for the building envelope and interiors and on all mechanical, electrical, and plumbing. The designers also did the daylighting modeling and the energy calculations to size the PV, and designed all the mechanical systems and wrote the sequencing as well as the building monitoring system.

The renovation of 435 Indio Way successfully transformed a 1970s single-story concrete tilt-up building into a net zero energy office that leased in record time.

RMW architecture & interiors Photography: © 2014 Bruce Damonte

Energy Efficiency
Designing a net zero office building requires aggressive, but realistic energy-efficiency goals. The project focused on being as energy efficient as possible within the developer budget, primarily through upgrading the envelope and greatly reducing the mechanical loads. The building is 100% daylit and 100% naturally ventilated. Roof-mounted photovoltaic and solar thermal systems were implemented to offset the predicted energy use.

The project had a target EUI lower than the code baseline of 40 kBtu/ft²·yr, determined using EnergyPlus 7.2. Due to the building’s 50% occupancy, the year one data shows a measured EUI of 13.5 kBtu/ft²·yr, which is still lower than the EUI recalculated by the energy model at 50% occupancy and 50% plug loads. This difference is likely due to the energy modeler’s underestimation of the concrete floor’s heat capacity (the ability to absorb heat) and the tenant’s attention to energy-efficient use of the building.

The 435 Indio Way building has two rooftop packaged unit heat pumps providing heating and cooling when needed. These units were significantly downsized due to the passive strategies applied to the building envelope. With careful attention to our calculations, we were able to comfortably get the system to two 11 ton heat pump units. This enabled us to offset the total building energy use by using just the roof for PV, as well as leaving room for skylights, HVAC equipment, and solar thermal.

The natural ventilation system is fully automated, making it possible to turn off the rooftop units when the temperature outside is optimal. The automated operable windows and skylights also allow for a night flush sequence, pre-charging the thermal mass of the building on cooling days. This strategy coupled with the 5 5/8 in. of exterior polystyrene, dynamic glazing, and ceiling fans allowed us to significantly reduce the overall building tonnage and system runtime.

When natural ventilation is insufficient for indoor cooling or unemployed during the heating season, the rooftop packaged units will deliver ventilation based on CO₂ sensors through a dedicated outdoor air system. Thanks to the building’s well-insulated envelope, the dedicated outdoor air rooftop packaged units are able to provide adequate space conditioning without extra air beyond the ventilation requirement.

The daylight design was maximized by implementing a custom skylight design that was carefully modeled to provide 100% of the daylight requirements from overhead. Electrochromic glazing was used in place of the existing windows to lower the peak cooling demands on the HVAC system. The design firm has done multiple plug load studies on similar building types. We used this knowledge to help inform a realistic plug load allowance in our model.

Forty-three custom skylights designed by the design firm make the building 100% daylit.

RMW architecture & interiors Photography: © 2014 Bruce Damonte

Because it was a spec building renovation, there was no way of knowing who would eventually occupy the building. Energy use from plug loads had to be estimated based on the design firm’s anecdotal knowledge. The developer created a performance-based lease for future tenants with “carrot-and-stick” clauses for good energy-saving behavior.

In the first year, the PV generated 164,000 kWh based on metered data, and the partially occupied building used 125,000 kWh, resulting in the building being 30% net positive. The PV generation is lower than the predicted 266,000 kWh/yr as it was found that one of the inverters failed to report production data.

After it is fully occupied, the building is expected to be 7 kBtu/ft²·yr net positive with full PV generation. Besides PV panels, the building uses a solar thermal system with electric backup heating for its domestic hot water, leading to little to no energy consumption.

Indoor Air Quality And Thermal Comfort
Occupant comfort is a top priority in this building, and indoor air quality plays a major role. Operable windows allow natural ventilation and enable occupant thermal comfort control. Naturally ventilated spaces are designed to comply with Section 5.1 of ASHRAE Standard 62.1-2007. A 100% outdoor air ventilation system provides fan-based ventilation on days when windows are closed. CO₂ sensors are installed to modulate ventilation rates, reducing energy consumption while maintaining indoor air quality. MERV 13 filters on the air-handling units ensure that clean air, free of pollutants and particulates, enters the building. In addition to providing increased filtration, we are providing 30% more ventilation than required by Standard 62.1-2007. The airflow rates were calculated with a system ventilation efficiency of 1.0 for 100% outdoor air, and a zone air-distribution effectiveness of 1.0 for ceiling supply. Sensors for CO₂ and VOC content in building air are installed to monitor indoor air quality.

Ceiling fans have also been installed, which help occupants control comfort levels. The use of ceiling fans in warmer seasons helps reduce the perceived temperature in occupied areas.
The building is designed to meet both the analytical and graphical methods of ASHRAE Standard 55-2010. The passive building strategies meet the expanded thermal comfort range in the analytical method by using increased air movement and high mass radiant surfaces. Advanced insulation and dynamic glazing help prevent radiant asymmetry to a high degree.

The skylights are carefully designed to avoid creating glare and hot spots in the space. The temperatures of the walls and floor are closely monitored to take full advantage of night flush while maintaining indoor comfort quality. A full control sequence is implemented to optimize the passive system and make sure the active system is adequately engaged when necessary. The rooftop packaged units meet the graphical method of Standard 55-2010. The overall approach allows for flexibility at the tenant level for any user requirements and allows for occupant comfort control if requested.

Innovation
The building was designed to be net positive with passive system. The walls were externally insulated to a value of R-20 and the roof to R-40. All of the existing windows and exterior doors (except the north elevation) were replaced with dynamic glass, electrochromic glazing that tints automatically to reduce solar gain to as low as 3%.

In addition, the design team works with the architect to minimize the infiltration load by well detailing openings into the building.

A unique skylight design was developed by the design firm. The intent was to minimize the opening areas on the roof for an effective passive system, as a good glass can only insulate to the level of R-5 when compared to a R-40 roof, especially applying to the heating season. The skylights face south and are tilted toward the sun with a pyramid shape to collect the maximum quantity of daylight with the smallest aperture. A diffusing film is applied on the internal side of the glass to reduce glare. This design using low-e glass maintains the integrity of the high performance envelope.

The skylights are designed to provide 100% of necessary light to the space during daylight hours most of the year. To complement daylight variation, the indoor lighting design features individually dimmable LED fixtures in a task-ambient design that reduces fixture count and first cost.
The skylights are also operable as part of the natural ventilation scheme for the building, together with the operable vertical view glasses around the building. Cross-ventilation and stack effect were both fully used to maximize the effect of natural ventilation. The design includes motorized operators on all of the windows and skylights. These are controlled with the BMS based on external and internal temperatures. In a climate with diurnal swings of 40°F, the concept relies heavily on nighttime cooling of the thermal mass in the floor slab and existing tilt-up concrete walls, which were intentionally insulated on the exterior and left exposed on the interior. This strategy has greatly reduced cooling load and shifted peak closer to after hours.

To further promote natural ventilation, ceiling fans were installed with varying speeds, maintaining a comfortable environment. Due to the passive architectural design strategies, we were able to reduce the size and complexity of the HVAC systems for the building from a typical 350 ft²/ton to over 1,200 ft²/ton.

For the passive system to work well as intended, the design-build team collaborates with a master system integrator hired by the developer on the overall controls. An omni-controller was developed to function smoothly according to the defined sequence of operations for all the system components (sensors, window and skylight actuators, room fans, etc.) as well as the standard HVAC system controls. The windows and skylights are both equipped with actuators that can respond to signals for a range of operation.

To further reduce energy consumption, circuit metering was installed to provide feedback on a tenant’s plug load use. It turns out that the monitoring provided an alert to the tenant that a batch of equipment is using excessive amounts of energy compared to energy-efficient alternatives. The tenant realized that replacement of their old equipment, which was driving the high energy consumption, would be a cost-effective action and developed a plan to do so.

Operation and Maintenance
If the design of an energy-efficient building is done correctly, it often leads to a simple system to operate and maintain. In this case, off the shelf high efficient rooftop packaged units were used that are easily serviced and cost effective. The enhanced controls reduce the run-time on the equipment, reducing the maintenance costs.

The BMS tracks the mechanical and electrical systems, as well as the operation energy used within the building. An ongoing monitoring-based commissioning contract was put into place which categorizes energy distribution in the building, and trend analysis allows problems with the systems to be identified. This has allowed for further reductions in energy usage as well as data to help inform our future modeling and design. The data has also been used in several case studies.

Cost Effectiveness
The net zero approach is assessed to be more profitable than a code minimum approach, making a strong business case for the developer. Though the construction cost including the PV system is $50/ft² more than a code minimum building, due to utility and government rebates and earlier lease-up, the additional construction cost drops to $27/ft². The appeal to tenants of occupying a NZE building with higher indoor air quality greatly reduces the lease-up time to three months compared to 18 months of similar buildings.

The operating and maintenance cost is less because of the building’s simple and low-energy consuming systems. The reserve requirement is also lower, providing additional savings, because of the smaller HVAC system and the lower cost for tenant improvements upon turnover. The sustainability aspects of the building result in a higher rental income and a lower churn rate than market comparables, providing extra boost to cash flow. All of the above have contributed to a $239,654 increase in rental income. The difference between the NZE building and a code minimum building in net cash flow after debt service is $3.87/ft². The additional construction cost is expected to pay back within seven years.

The higher profitability has been confirmed by the experience of the first year of operation. The project has been such a success that the designers and the Indio team have implemented this strategy on two more buildings, with a fourth in design. The developer has branded the concept and expects to do many more of these important projects.

Environmental Impact
The 435 Indio Way office building positively impact the environment in several ways. Low-flow fixtures and dual flush toilets reduced water consumption by 40% over a code office building’s. A carbon study was performed to quantify the building’s carbon impacts over an assumed 20-year life-span, including both construction and operation aspects. Since the project is a retrofit, it has a lower construction carbon footprint, less energy was required for deconstruction, and the majority of the structure was reused from the existing building.

The building further impacts the environment in a positive way through all-electric systems that are 100% offset with rooftop PV panels, and water heated by a solar thermal system. With the PV systems, it achieves a net positive rate of 8 tons/yr. •

About the Authors
Shannon Allison is an associate principal and Alex Krickx, BEMP, LEED AP, is a senior building performance engineer at Integral Group in Oakland, Calif.