When approaching energy-reduction strategies to attain net zero energy building (NZEB) designation on new commercial construction projects, the lowest hanging fruit is often the most overlooked.
Choosing high-performance windows with appropriate U values, Solar Heat Gain Coefficient (SHGC) and Visual Transmittance (VT) can dramatically decrease a structure’s energy use and operational costs, but these benefits could be lost if window selection occurs in a bubble. Window systems represent an integral part of the thermal envelope, which directly impacts mechanical systems. High-performance windows won’t do much good if placed in a low-performance fenestration.
Failure to address window systems from a holistic vantage with other structural and mechanical systems early in the design process can result in poor structural performance and additional costs. Commissioning is also critical, but building envelope performance receives little attention from most commissioning agents. Energy benchmarking and transparency programs are revealing that many commercial structures in the United States are underperforming.
Even following green building guidelines such as USGBC’s Leadership in Energy and Environmental Design (LEED) program will not guarantee energy-saving success. A report from USGBC’s New Building Institute cited that 25% of structures conducting post-occupancy reporting indicated a level of energy use that significantly exceeded their expectations. In order to ensure that the many benefits of high-performance windows don’t get lost en route to designing a NZEB, address the following issues as a team early in design phase.
Water Intrusion Management
The performance of a window system is dependent on its connection to adjacent envelope elements. While windows have a high resistance to water vapor, framing designs and construction techniques can leave them prone to water intrusion. Uncontrolled water intrusion destroys interior finishes, deteriorates structural framing and decreases indoor air quality (IAQ).
The most common leaky culprit is improperly caulked window framing elements, which undermine the window’s rainwater channels. Structural intersections where windows abut other structural elements create opportunities for water intrusion and thermal bridging, and the shop drawings from window and curtain wall manufacturers fail to address the necessary details.
To avoid these issues, verify proper installation techniques and design a glazing system with a water intrusion management plan. Window design documents must clearly delineate rainwater management and channeling requirements and should include designs for drainage, internal drain paths, integrated weep systems, provisions for sub sills and flashing, and gaskets & sealants.
Anyone standing next to a window on a sunny day understands that heat radiates readily through windows and glazing systems. Designs for NZEB must take this radiative heat into account, balancing it with interior heating and cooling loads and daylighting strategies. They must also include true thermal breaks in the installation of window frames, the absence of which can lead to condensation and maintenance issues caused by thermal bridging.
Thermal & Visual Comfort
Many commercial buildings in use today suffer from drafts caused by poor thermal performance, and visual discomfort from glaring glazing systems. This is largely due to the fact that design teams often view windows and glazing systems as a mere facade. Because of this, relative decisions are often based on aesthetics and first costs and rarely reflect the attention to long-term performance necessary to achieve a NZEB.
Air movement through window systems directly impacts thermal comfort. In addition to poor product design and/or quality control, this air movement can result from missing design details during construction. Maintaining the airtight integrity of insulation where the glazing system connects to the building envelope can increase the thermal performance of window systems. Meticulous window details in the design documents – especially at intersections and connections – are critical to guide contractors during construction.
Daylighting Strategies & Visible Transmittance (VT)
Designing for daylighting is an effective strategy to reduce energy use while enhancing thermal and visual comfort. However, improperly designed daylighting can create discomfort with glare requiring costly window treatments. Office buildings are often initially designed with floor-to-ceiling windows that end up heavily tinted in the final design. Some teams reach final design with window systems lacking shading to maximize daylight and end up causing glare. To realize the best daylighting outcome and achieve a NZEB, design teams must achieve an optimal balance between visible transmittance and visual comfort.
HVAC and Lighting
It’s almost impossible to achieve a NZEB without addressing HVAC and lighting systems. The performance of window and glazing systems directly impacts the design of a structure’s HVAC and lighting systems, presenting an opportunity for energy savings. Integrating efficient window systems with daylighting strategies incorporating climate zones and structural orientations can lead to significant energy reduction and cost savings. In addition to reducing perimeter lighting needs, a well-designed building envelope and glazing system can reduce the size and energy draw of HVAC systems.
High-performance windows and glazing systems reduce a building’s peak heating and cooling loads. These loads determine the size of the structure’s heat pump, furnace, air conditioner, fans, and ductwork. Reducing the burden on these systems reduces their size, resulting in significant initial and operational cost savings. A smaller HVAC system running at high levels around the clock is much better for air quality and energy consumption than a larger system hitting peaks and valleys.
Collaboration and Commissioning
Teams interested in designing net zero commercial facilities should assume a holistic multi-disciplinary approach to the building envelope system. Windows can help achieve a NZEB in many ways, including the generation of on-site renewable energy via photovoltaic technology. High-performance windows are a critical component of any energy-saving design, but lose effectiveness if not considered an integral part of this system.
Design teams should define acceptance criteria for the building envelope system in early charrettes, and resist the temptation to over-focus on costly window aesthetics in lieu of energy-saving envelope strategies. Acceptance criteria should include daylight harvesting with glare control, energy-efficient windows, waterproofing, and durable cladding. Ideally, these criteria would contribute to a window system commissioning process. Collaboration and commissioning will help ensure that your structure achieves net zero energy use and performs as a NZEB long after the commissioning agent has gone.