Abstract
LEED v5 represents a fundamental shift in the world's leading green building rating system- from prescriptive, component-based requirements to a performance-driven framework centered on deep decarbonization, indoor air quality (IAQ), and occupant well-being. While LEED v5 refrains from mandating specific technologies, its new prerequisites and credit structures strongly favor building systems that inherently align with these performance goals.
We posit that Underfloor Air Distribution (UFAD) is one such system. Through analysis of key LEED v5 credits related to operational carbon, peak thermal loads, ventilation effectiveness, and occupant satisfaction, UFAD’s core principles—buoyancy-driven airflow, reduced static pressure, and individualized control—provide a strategic and technically sound pathway to achieving strong results under the new rating system.
Introduction: The LEED v5 Paradigm Shift
The U.S. Green Building Council's (USGBC) release of LEED v5 marks a significant evolution of the world’s most widely used green building rating system. The update formalizes a commitment to tangible, measurable outcomes, organizing the system around three overarching impact areas: Decarbonization, Quality of Life, and Ecological Conservation & Restoration. Key changes include:
- Emphasis on Operational Carbon: A shift from pure Energy Use Intensity (EUI) metrics to absolute, verifiable carbon emissions, with new credits rewarding electrification and grid-interactive building design.
- Performance-Based IEQ: The IEQ category now prioritizes measured performance over prescriptive design, requiring on-site verification of ventilation and continuous monitoring of indoor air contaminants.
- Integrated Design: New prerequisites for a Climate Resilience Assessment and a Social Equity Assessment require project teams to consider the holistic impact of design choices from the earliest stages.
This new version demands that all building systems, particularly HVAC, prove their value not just in energy savings, but in their contribution to decarbonization, health, and resilience. As such, design teams must now seek out technologies that offer synergistic benefits across multiple performance metrics. UFAD is one of the few HVAC strategies uniquely aligned with LEED v5’s performance-based framework.
Technical Principles of Underfloor Air Distribution (UFAD)
UFAD is an HVAC system that utilizes a pressurized plenum created by a raised access floor to deliver conditioned air at low velocity directly into the occupied zone. Unlike traditional overhead systems that rely on a mixing process, UFAD leverages the principle of thermal stratification and buoyancy-driven airflow.
- Thermal Stratification: Cooler, denser supply air is introduced at floor level. As it absorbs heat from occupants, lighting, and equipment, it becomes less dense and rises by natural convection. This process creates a stable thermal gradient, with conditioned, clean air in the occupied zone (approximately 0 to 1.8 m) and warmer, contaminated air in the return zone above.
- Buoyancy-Driven Contaminant Removal: This stratified airflow is highly effective at removing contaminants. Pollutants exhaled by occupants or emitted from equipment are immediately carried upward by their own thermal plume and exhausted at ceiling level, preventing recirculation within the breathing zone, unlike overhead systems.
- Reduced Static Pressure: The underfloor plenum functions as a large, low-resistance air distribution path, allowing UFAD systems to operate at significantly lower supply static pressures than ducted overhead systems. When combined with appropriate diffuser selection and plenum sealing, this can translate into meaningful reductions in fan energy, which is a major contributor to HVAC electricity use.
These technical characteristics position UFAD as a strong candidate for LEED v5 projects seeking verified performance across energy, IAQ, and occupant experience metrics.
Strategic Alignment of UFAD with LEED v5 Credits
A detailed examination reveals that UFAD's functional attributes align with and directly contribute to the achievement of multiple, high-value credits across the LEED v5 rating system.
Energy & Atmosphere (EA) Credits: Decarbonization and Peak Load Reduction
- Operational Carbon Projection & Decarbonization Plan: LEED v5 requires projects to model and report a 25-year carbon trajectory, awarding points for electrification and decarbonization strategies. UFAD contributes in two important ways. First, reduced fan energy consumption (often 20-30% lower than traditional VAV systems, per industry
- benchmarks) directly lowers operational electricity use and the associated carbon footprint. Recent field evaluations further confirm UFAD’s efficient cooling performance in stratified spaces while maintaining neutral thermal comfort [1]. Second, the higher supply air temperatures required by UFAD [2] can improve the Coefficient of Performance (COP) of chillers and heat pumps, making all-electric HVAC systems more efficient and practical to implement.
- Reduce Peak Thermal Loads: This credit rewards strategies that flatten electrical demand curves and reduce peak loads. UFAD's higher supply air temperatures and reduced airflow help create more efficient cooling load profiles in non-uniform environments, helping to trim coincident peak demand during afternoon hours, as demonstrated in comparative studies of UFAD performance [3]. These characteristics make UFAD well-suited for grid-interactive building design, a major focus of LEED v5.
Materials & Resources (MR) Credits
In alignment with the decarbonization goals of LEED v5, the point allocation for the Materials and Resources credits has increased, underscoring the growing importance of tracking and reducing embodied carbon in building materials and construction practices. A trusted raised access floor manufacturer has 3rd party verified environmental documentation including Environmental Product Declarations (EPD) that support the whole building Life Cycle Assessment and can contribute to multiple credits in the category.
Indoor Environmental Quality (IEQ) Credits: Verified Health and Comfort
- Verification of Ventilation & Filtration: This prerequisite requires compliance with ASHRAE 62.1-2022 and encourages the use of the Indoor Air Quality Procedure (IAQP), a performance-based compliance path. The IAQP requires a designer to demonstrate that contaminants are controlled to acceptable levels. This is where UFAD excels. According to ASHRAE 62.1-2019, UFAD systems can be assigned a Zone Air Distribution Effectiveness (Ez) of 1.2 to 1.5 for floor supply/ceiling return layouts [4]. This is a significant advantage over typical overhead mixing systems which are assigned 0.8 to 1.0. A higher Ez or ventilation effectiveness allows for a reduction in the required outdoor airflow, which in turn reduces energy consumption for cooling, heating, and dehumidification while still meeting or exceeding indoor air quality targets. The buoyancy-driven contaminant removal of UFAD is an evidence-based method for satisfying the IAQP.
- Continuous IAQ Monitoring: This credit rewards real-time monitoring of contaminants such as CO2 and TVOCs. UFAD’s stratified airflow allows sensors to be placed directly in the breathing zone—the most critical area for IAQ measurement [5][IW1] . The stable, low contaminant levels in the occupied zone simplify compliance and provide reliable data.
- Occupant Satisfaction Survey: A new credit rewards demonstrable occupant satisfaction. UFAD's personalized diffusers, which give occupants greater control over temperature and airflow, have been consistently shown to improve thermal comfort ratings and reduce comfort-related complaints [6]. This direct correlation with occupant satisfaction makes UFAD a powerful tool for achieving the new performance-based IEQ targets.
Integrative Process, Planning and Assessments (IP) Credits
- Assessment for Climate Resilience: The use of raised-access floors for UFAD makes the system inherently adaptable. The easy reconfigurability of the floor diffusers and the accessible plenum allow for simple layout changes as office spaces evolve, minimizing costly and resource-intensive demolition and construction [7]. This inherent flexibility aligns with LEED v5's new emphasis on resilience and long-term building performance.
Implementation and Commissioning Considerations
While UFAD offers significant advantages, successful integration within the LEED v5 framework requires careful planning and execution. The project team must ensure:
- Plenum Integrity: The underfloor plenum must be properly sealed and commissioned to prevent air leakage. Any unintended leakage can compromise system performance and reduce energy savings.
- Holistic Commissioning: As with any high-performance system, a thorough commissioning plan is essential to verify that the system is operating as designed and that the expected energy savings and IAQ benefits are being realized. The new LEED v5 commissioning prerequisites make this process even more important.
- Measurement and Documentation: Performance claims must be supported by modeling, testing, and monitored data aligned with LEED v5 requirements.
Conclusion
LEED v5 challenges the building industry to move beyond prescriptive calculations and deliver truly high-performance buildings that address the climate crisis and public health. This shift to a performance-centric version fundamentally changes the calculus for selecting building systems.
Underfloor Air Distribution is not a required component in LEED v5, but its intrinsic functional principles provide a powerful and direct solution to the new rating system’s most demanding requirements. From reducing operational carbon by lowering fan energy and improving heat pump efficiency, to providing verifiable ventilation and enhancing occupant well-being, UFAD acts as a strategic lever that drives performance across multiple categories. In a world where every kilowatt-hour and every breath of air is a metric to be measured, UFAD’s ability to deliver tangible, quantified benefits makes it an ideal technology for projects aiming to achieve LEED Platinum. In LEED v5, success is defined by measurable outcomes—not prescribed systems. UFAD stands out as a rare HVAC strategy that delivers across carbon, air quality, and occupant experience simultaneously. For project teams targeting high-performance outcomes, it is not just a viable option—it is a strategic advantage.
References
[1] Wu, Y.-C., Hsu, H.-C., Wang, H.-Y., & Pan, C.-Y. (2025). “Field Evaluation of Thermal Comfort and Cooling Performance of Underfloor Air Distribution Systems in Stratified Spaces.” Buildings, 15, 3241. https://doi.org/10.3390/buildings15173241.
[2] Webster, T., Bauman, F., Shi, M., & Reese, J. (2002). “Thermal stratification performance of underfloor air distribution (UFAD) systems.” UC Berkeley: Center for the Built Environment. Retrieved from https://escholarship.org/uc/item/6vv4g4d7
[3] Li, H., Cui, Q., Kong, X., & Fan, M. (2023). “Comparative Study on the Thermal Characteristics Under Different Air Distribution Strategies Oriented to Non-Uniform Environment: An Indoor Experiment.” Building and Environment, 246, 110982. https://doi.org/10.1016/j.buildenv.2023.110982
[4] ASHRAE Standard 62.1-2022. Ventilation for Acceptable Indoor Air Quality. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[5] Fisk, W. J. (2020). "Strategies to reduce indoor transmission of SARS-CoV-2 and other viruses." Building and Environment, 179, 106972.
[6] Fisk, W. J., Faulkner, D., Sullivan, D.P., Chao, C., Wan, M.P., Zagreus, L., & Webster, T. (2004). “Performance of Underfloor Air Distribution: Results of a Field Study.” Center for the Built Environment, University of California, Berkeley. https://escholarship.org/uc/item/9088399m
[7] Steven Winter Associates, Inc. (2020). "Embodied Carbon of Raised Access Floors." A study prepared for the Access Floor Industry Association.