Dynamic and adaptive building envelopes
While designing the appropriate HVAC system is essential for new and existing buildings, there are several other aspects that play into the building’s energy efficiency. Designing the building envelope correctly is key to an efficient HVAC system.
- Determine the appropriate building envelope design for each individual building.
- Make use of modeling tools to design the building envelope.
- Choose to adapt the building envelope in a dynamic way to the shifting climate patterns.
To the general population, it may seem far-fetched to imagine a building envelope that morphs and adapts to its changing environment. To a professional in the building and construction industry (e.g., anyone ranging from a building owner, developer, architect, engineer, façade designer, contractor, or supplier/manufacturer), the terms “resilience” and ”adaptation” have become the buzzwords that indicate a shift in the way we think about building design.
While most engineers are not a fan of buzzwords, because they tend to detract from the real motivation and intent behind the words themselves, many do believe in the intent and need for resilience as well as the mitigation and adaptation measures that will brace us for the environmental changes that we are facing. This is especially true considering some of the conclusions drawn from the Fifth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC). For example, the IPCC has stated with high confidence that “impacts from recent climate-related extremes, such as heat waves, droughts, and floods, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability” and “adaptation and mitigation choices in the near term will affect the risks of climate change throughout the 21st century.”
When teaching at the Southern California Institute of Architecture (SCI-Arc) on building envelope performance, one of the very first concepts taught to these architects of the future is: “There is no one-size-fits-all solution to building envelope design.”
This means that a design that works brilliantly in one climate could be completely inappropriate in another, so the context is extremely important. It is not just the climate that needs to be assessed; one also needs to fully understand the building typology, local code restrictions, and client goals, among other considerations such as local traditions and cultural norms of the building occupants. A successful and beautiful architectural design is one that is context-appropriate. But what if this context is changing over the design life of a building? How do we design a building envelope to adapt to a morphing and ever more challenging climate?
To address these questions, building envelope physicist Annalisa Simonella and I modeled the effect that future climate conditions in a given location (London) would have on the carbon emissions and thermal comfort of a typical open-plan office at the perimeter of a building. We simulated the office’s heating, cooling, and lighting energy use with different façade U-factors, solar-heat-gain coefficients (SHGC), and ventilation options, and we applied weather data that had been morphed to represent likely future carbon emissions scenarios. We then converted the energy use to a total annual carbon emissions value using published greenhouse gas conversion factors. Thermal comfort was determined using overheating criteria, where dry-resultant (i.e., operative) temperatures above 79 F were considered uncomfortable. In the models, we assumed the perimeter office module would have the following constant attributes:
- Geometry: 30 (W) x20 (D) x13 (H) ft
- Orientation: south-facing
- Glazing percentage: 75% on a south-facing exterior wall (all other surfaces are modeled as adiabatic).
Note: Energy codes typically limit glazing percentages to around 40%, but this is not necessarily a mandatory limit, generally only a prescriptive one. If a whole-building energy simulation approach is taken to show energy compliance, architects generally want a much higher glazing percentage, and 75% glazing is a far more realistic representation of current architectural trends. The difference in façade performance is usually made up through efficiencies in other areas, such as mechanical equipment.
- Infiltration: 0.15 air changes/hour (ACH)
- Daylight control
- 6 occupants
- Lighting load: 0.8 W/sq ft
- Equipment load: 1.5 W/sq ft
- Setpoint temperatures:
Heating: 72 F
Cooling: 75 F.
The variable parameters tested were:
- Thermal transmittance (U-factors): 0.3, 0.4, and 0.5 Btu/h/sq ft/F
- Solar performance, SHGC: 0.12, 0.28, 0.44, 0.60 (and their corresponding visible light transmittances, VLT: 0.15, 0.50, 0.70, 0.78)
- Ventilation system: mechanical versus mixed-mode operation
Mixed-mode operation was modeled such that natural ventilation would only occur during occupied and economizing hours
- Weather data: existing versus 2080s scenario.
We then compared the results between the simulations using the existing London 1980s design summer year (DSY) and those using morphed weather data for the 2080s decade. (At the time of modeling, the 1980s DSY data was the industry standard weather file to use for the United Kingdom. For cities in the United States, typical meteorological year (TMY3) weather data was used.)
From the plots of the weather data, we quickly noticed that the future 2080s climate scenario would pose more of a summertime overheating (i.e., thermal discomfort) risk to London’s buildings than the existing 1980s design weather data would. Therefore, associated increases in air conditioning energy consumption due to hotter summers would also be expected. Other predicted climate trends affecting HVAC design—in addition to higher summer dry-bulb temperatures—include higher solar irradiation due to predicted changes in cloud type and cover and higher coincident dry- and wet-bulb temperatures. On the other hand, warmer winters were also expected for London’s future climate, leading one to conclude that winter heating requirements would be correspondingly less onerous. These observations were all validated by the simulations.
The more interesting conclusion, however, was that we were able to identify the building envelope’s optimal performance for each of the different climate conditions. For the specific configuration of the open-plan office model, the optimal SHGC leading to the lowest carbon emissions was 0.24 for the existing climate conditions, whereas the optimal SHGC for the 2080s scenario was 0.19. (These optimal values were for a U-factor of 0.3 Btu/h/sq ft/F; see Figure 3.)
Surprisingly, “the lower the SHGC, the better” is not always the right answer. Looking at the overall annual simulation for the 1980s weather data, we noticed that the slightly higher SHGC of 0.24 helped heat the building with “free” solar energy in the wintertime, while still maintaining acceptable thermal comfort levels in the summertime. With a slightly higher SHGC, you also get a higher visible light transmittance (VLT), so less artificial lighting is needed for an office. Comparing results from the different climate scenarios side by side, we begin to see that what is “optimized” for the existing set of weather data would be suboptimal a few decades down the line, causing increased use of energy to run the air conditioning, for example. The U-factor, on the other hand, had only a minor impact on the energy and comfort results for the specific configuration considered, because buildings with lower glazing percentages will be more affected by differences in U-value.
Adding in a mixed-mode operation—allowing occupants to use natural ventilation or mechanical cooling—led to a much bigger shift in optimal SHGC values. For the London models, the optimal values became 0.46 and 0.35, respectively, for the 1980s and 2080s weather scenarios. This means that there is a lot more flexibility to the building envelope design if one allows the use of operable windows when the outdoor conditions are suitable.
The study was extended to capture a wider (more extreme) range of climates, so Miami, San Francisco, New York City, and Minneapolis—which represent ASHRAE Climate Zones 1, 3, 4, and 6, respectively—were all modeled. Furthermore, to test climates with different humidities, Atlanta (“moist” Climate Zone 3A) and Las Vegas (“dry” Climate Zone 3B) were modeled to compare against San Francisco (“marine” Climate Zone 3C). Results from the different cities’ simulations drew some valuable conclusions on how best to add resilience and adaptability into the building envelope design that is specific to each climate zone’s particular challenges.
Figure 5 summarizes the ideal SHGCs and U-factors for each city, both for current (TMY3) weather data and 2080s weather data, and using mechanical and mixed-mode ventilation.
The expected pattern of Miami (the hottest city considered) having the lowest optimal SHGC and Minneapolis (coldest city) having the highest optimal SHGC was observed based on modeled carbon emissions. Also, the optimal SHGC decreases over time with a warming climate trend. This is true unless we build in opportunities to open windows and operate in mixed-mode, which is what the arrow points to. Allowing for natural ventilation not only improves the thermal comfort for occupants, it also reduces the energy consumption and shifts the optimal SHGC values back up, even above the current values in five of the six cities considered. In moist climates, however, be careful not to let in high-humidity air with natural ventilation, as this could adversely affect thermal comfort. Similar to the London study, the changing climate has very little impact on the optimal U-factor for each U.S. city considered, assuming a 75% glazed, south-facing office.