Increasing construction expenses and pressure to decarbonize with innovative climate solutions are challenging traditional approaches in design. For Austin Achieve, a tuition-free, open-enrollment public charter school, HKS put innovative ideas into action to balance sustainability and the needs of a diverse community.
During the design process for the Austin Achieve-Pflugerville Campus in Pflugerville, Texas, which opens this school year, our team challenged restrictive energy code requirements by taking a flexible approach. We exchanged passive design elements of the building enclosure—such as insulation—with more cost-effective active-solar-building technologies, including photovoltaics. We sought to reduce the embodied and operational carbon footprint of the project, in addition to providing energy and cost savings.
At the time of building permitting, requirements advanced from the 2018 International Energy Conservation Code—2018 IECC—to 2021 IECC. The new code addresses energy efficiency on several fronts, including cost, energy usage, use of natural resources, and the impact of energy usage on the environment.
Because of the code change and required performance improvements, the team needed to find more efficiencies in the building design. During design development, we conducted building energy modeling and generated response curves to understand the interactive effects of building-enclosure design and mechanical and lighting demands. We found that the building envelope would have less of an influence on the overall building performance than expected, and that by removing insulation, we’d be able to generate cost savings that could be invested in more active technologies.
In close collaboration with Austin Achieve and the Pflugerville Building Department, the team chose to pursue code compliance based on Total Building Performance, per 2021 IECC, a tradeoff method using building-energy modeling where the annual energy cost of the proposed design must be less than or equal to 85 percent of the annual energy cost of the standard reference design.
The modified envelope analysis and design that reduced roof and wall insulation from prescriptive requirements resulted in projected labor and material cost savings of approximately $170,000 and achieved cost savings of 31.8 percent and energy savings of 34.5 percent when compared with the standard reference design. The savings generated the idea to invest in a 100-kW photovoltaic (PV) array that, once in operation, could begin to decarbonize the charter school.
Integrating a 100-kW PV array presented a challenge, as the design had already progressed into the construction documents phase. Ideally, the array would be mounted on the roof, but the project’s packaged rooftop units made it impossible to arrange it in a way that avoided shading and provided a safe rooftop environment for maintenance. The team considered a second option to position the array on the ground, west of the project site and near the property line, visible from the surface parking lot.
Because of various logistical and financial constraints, especially current inflation rates, the client and team were unable to install the PV array on the project. But the analysis showed us that a PV array would generate up to 144,000 kWh of renewable energy per year, reduce carbon emission by 14 percent annually, and reduce operational costs by 13 percent. Furthermore, the reduction in insulation would reduce the carbon intensity of the building enclosure more than 47 percent.
The modified-envelope-approach design analysis did not go to waste. Austin Achieve asked the design team to evaluate the area needed to achieve net zero energy for potential planning of a middle school and high school. We found that PV arrays supportive of net zero would require seven times the area of the initial budget diversion for the Pflugerville Campus project—a helpful exercise for Austin Achieve so it can plan how to invest its resources for the future sustainability of its growing charter school network.Solar technology is often an afterthought to drive high performance with little planning or maintenance. This project speaks to the future of solar and its dependence on energy modeling early in design.
Tommy Zakrzewski holds a PhD in civil engineering and is a principal and director of building engineering physics at HKS.
