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.
Seven ways to adapt the building envelope
Given the conclusions drawn from the studies, there are a number of ways to adapt the building envelope in a dynamic way to the shifting climate patterns—some that are passive and others that are active. Key design concepts and suggestions further enhance the adaptability of the building envelope to climate change, particularly with respect to the energy/carbon performance and thermal comfort.
1. Improve solar performance
Summertime thermal discomfort and overheating can be attributed in large part to direct solar-heat gains through windows. To effectively manage the carbon emissions associated with air conditioning an office building and the thermal comfort level, one of the key strategies is to limit these solar gains. There are a number of ways to do so:
- Make a stronger case to architects to reduce the glazing percentage on elevations with high sun exposure (e.g., south, east, and west in the northern hemisphere). Many state and local energy codes are already limiting their prescriptive window-to-wall ratios to 0.30 or 0.40, which is a good starting point. Too often, the building industry gets around this by using whole-building energy simulations to trade off, for example, lower façade performance with more efficient mechanical equipment. For existing buildings, vision glazing can be replaced with insulated spandrel panels. The spandrel panels can be placed strategically, such as at the lower 30 in. of floor-to-ceiling glass in an office environment, so that it doesn’t block views or beneficial daylighting. The added insulation will also improve the wall’s overall thermal performance.
- Add solar-control films to existing glazing: A quick and cost-effective fix for existing windows that may be single-glazed and/or uncoated is to add a thin film that provides solar control. Aesthetically, this is not the most desirable option, but it could be used as a temporary solution for glazed units nearing their end-of-life.
- Apply solar-control glass coatings: Pretty much a prerequisite for all warm climates these days, solar-control glass coatings can be provided by glass suppliers and manufacturers to significantly cut down the total solar energy that is transmitted through transparent glazing. Note that this is different from a purely low-emissivity (low-e) coating. While a solar-control coating uses layers of metal or metal oxides to reflect and block direct solar irradiation and, therefore, reduce solar heat gain, a low-e coating reduces just the reradiated longwave infrared component of the heat gain on the glazing. In practice, the terminology can be a bit muddled, with both types of coatings commonly referred to in the industry as “low-e.”
- Install external shading devices: Either fixed or operable shading devices are always more effective on the exterior than on the interior side of a building. This is because external shading devices block out direct solar radiation before it is transmitted through vision glass rather than after, so this creates less solar load on the HVAC equipment and improved thermal comfort conditions. Compared to fixed shading, operable shades have the added benefit of allowing building occupants to control the effective SHGC throughout the seasons, but their moving parts also mean more potential maintenance and reliability issues.
- Use electrochromic glass: The most technologically advanced of the options listed, electrochromic glass actively uses a small amount of electricity to control the level of window tint. SHGC values can be controlled between a range of values (0.09 to 0.46). Some building energy codes and standards have already incorporated clauses on how to model the performance of electrochromic glazing, so it should be seen as a viable solution that can be used now rather than some time in the indefinite future.
2. Optimize thermal performance
The results of the study showed that, for a 75% glazed, south-facing office bay, the thermal performance (i.e., U-factor) had a relatively low impact on overall carbon emissions or thermal comfort. However, for other glazing percentages, orientations, and building types, thermal insulation could factor more significantly. A few options to improve the thermal performance are shown below.
- Use continuous insulation where possible on walls and roofs, and account for the thermal bridging of window framing and metal or wood studs in walls when designing HVAC equipment.
- Replace uncoated glazing with insulated low-e glazing.
- Consider using vacuum insulated panels. An inch of this product can provide about an R-25 performance, when the same thickness of rigid foam (e.g., polyisocyanurate) insulation can only provide about an R-7 or R-8 performance. Because of its effectiveness per inch of thickness, it is a great idea for existing buildings that may have limited space to play with. Unfortunately, these units are still relatively expensive, but market demands could easily change this in the near future.
3. Add thermal mass
Nowadays, with certain architectural aesthetics leaning toward exposed concrete interiors, it is a good opportunity to use thermal mass to the building’s functional benefit in moderating internal temperatures. Thermal mass has a very important impact on the thermal performance of buildings as it helps temper the diurnal temperature swings dynamically, which in turn reduces overheating and could help achieve an acceptable level of internal comfort and reduced energy use. For the mechanically ventilated Las Vegas office models, the engineering team added thermal mass, and doing so reduced the overall carbon emissions by up to 28% for the current weather data and by 17% for the 2080s weather data. Adding thermal mass also shifted the optimal SHGC values for these cases from 0.23 to 0.30 (current) and from 0.21 to 0.28 (2080s).
If lightweight buildings are being designed, another way to introduce thermal mass is to use phase change materials (PCMs), which provide a similar effect but can be easily implemented into drywall, for example. To be effective, a PCM needs to have a high heat of fusion. PCMs use a building’s excess heat to melt until the PCM is in a liquid state, and when the internal air temperature drops, the PCM releases the stored latent heat back to the space and resolidifies. PCMs can also be used to upgrade existing buildings because they are relatively thin, easy to install, and cost-effective.
4. Design green walls/roofs and living facades
Not only can vegetated walls filter the surrounding urban air, making it cleaner for building occupants who rely on natural ventilation, green walls/roofs and living façades also can help absorb carbon dioxide as they photosynthesize, which helps reduce the overall embodied carbon of a building. Green walls and roofs can also help lower the building’s cooling loads by adding to the overall insulating performance of the building envelope as it creates a better thermal buffer. If the vegetation is deciduous, then it could be designed in a way to work harmoniously with the building’s solar shading needs over the seasons (see Figure 6).
5. Use natural ventilation
We are not always lucky in a commercial-building environment to know what sorts of noises, air pollution, precipitation, and humidity levels we would be inviting in along with the outside air, but if the external conditions allow the possibility of natural ventilation, operable windows are preferred. They can remain locked until they need to be put in use at a later time for either a mixed-mode or naturally ventilated building operation. Fixed windows on existing buildings also can be replaced with operable ones.
Personal control of operable windows in an open-office environment may not always be practical or recommended because building occupants may not be operating the windows in the most energy-efficient manner, and the perception of thermal comfort varies from individual to individual. Operable windows in open-plan offices should be controlled by an automated building-management system that can sense when outside air temperatures are at economizing conditions and can also shut the windows during unoccupied hours, and indoor temperature sensors need to be placed in appropriate locations. Even with an automated system, however, there can still be complaints of flying papers and thermal discomfort from individuals.
Operable window design is usually trickier than designing fully sealed windows and curtain walling because operable windows are more susceptible to air and water leakage, and there are more moving parts for maintenance. Therefore, the detailing of operable windows needs to be well executed, and window manufacturers need to offer reliable products with good warranties. Natural ventilation for the University of Sheffield’s Jessop West Building (Figure 1) was designed so that the outside air would first go through an acoustic baffle below the windows to filter out the traffic noise, then enter the cellular office spaces. Building occupants also have individual, operable windows for personal control of their thermal comfort if traffic noise is not too bad.
Another design consideration for natural ventilation is to have an open-ceiling plan and well-designed HVAC ductwork to prevent the obstruction of airflow, especially for low-height ceilings like the 13-ft height modeled.
6. Enhance natural daylighting
A few quick fixes can easily improve natural daylighting in existing buildings dramatically.
- Add light shelves to bounce natural daylight further back into a deep space.
- Use venetian blinds where you can adjust the slats at the top differently from the slats at the bottom. This allows the building occupant to let in more daylight at the top section, while the bottom section serves the function of a glare blind where the light hits the desk and occupant directly.
- Use automatic shading controls so that blinds are not inadvertently left down throughout the day, even when there is no direct glare.
- The best approach to take for daylighting new buildings is to properly design the façade layout so that there are more clerestory windows at the higher level, and the lower 30 in. of an exterior façade can be an opaque spandrel zone because, as mentioned earlier, it does not bring in any useful daylight at desk height.
7. Educate building occupants on adaptive comfort
Even with the design changes considered above, more extreme weather such as hotter summers and colder winters will require a shift in building occupants’ behavior and their response to the environment. For the buildings to use less energy despite these more extreme conditions, it may be necessary to relax the design setpoint temperatures, so that they float upward in the summer or downward in the winter. This contrasts with the predicted mean vote method where design setpoint temperatures are fixed for heating and cooling seasons, respectively.
Building occupants will have to adapt their thermal comfort expectations accordingly, but for adaptive comfort to work, the following criteria must be met:
- Building occupants are allowed a more flexible dress code.
- Occupants will have access to operable windows that they can control. (For office buildings, this is more applicable to cellular offices, because opening windows in an open office would affect a group of people rather than just the individual who is opening the window.)
- Shading from direct solar radiation is provided to occupants.
- Occupants can increase the local air movement (e.g., through the use of desk fans, ceiling fans, or ventilation).
- Occupants can ideally move about the office and change their workstation location.
In addition to these individual suggestions, we could combine some of the ideas into more holistic solutions. For example, a more integrated solution could be to place the PCMs behind spandrel panels that are operable to allow cross-ventilation. Simple yet effective concepts taken from the local architectural vernacular should also be applied more widely.
For example, a design professional had been to Haiti a few years ago and recalled the locals using loosely woven, wetted vetiver mats serving as both shading devices and as elements providing evaporative cooling, an effective way to improve thermal comfort in dry climates. A further benefit to this mat is that vetiver contains oils that prevent rotting. This is also true of citronella and lemongrass.
Another consideration is to account for the embodied carbon associated with designing the building envelope. What materials are being used? From where are they sourced? As buildings become optimized for their context/climate and HVAC equipment becomes more efficient, the carbon emissions associated with extracting raw materials, manufacturing these materials into useful products, and transporting the products to site all start to play a much bigger role in climate change mitigation.
We all know what happens to species that are unable to adapt to their environment. The ones that thrive are essentially those who have the flexibility to survive and handle a wide range of environmental conditions and can respond well to threats. Let us learn from nature’s success stories and start designing more flexible buildings that can weather the storm, both literally and figuratively.
Irene C. Pau is senior façade engineer/building physicist at Arup. She has given talks on high-performance building envelopes for the American Institute of Architects Committee on the Environment (COTE), presented findings of her thermal comfort and energy efficiency research at the ASHRAE High Performance Buildings Conference, and is a frequent guest lecturer at the Southern California Institute of Architecture (SCI-Arc).