The emerging prevalence of mass timber structures and modular MEP and façade systems point to the goals most projects share: design and build a beautiful and sustainable building, on time, and on budget. This case study of mid-rise housing that utilizes mass timber and modular MEP / façade systems illustrates the reduced carbon footprint and related cost and schedule savings of this type of design when compared to a traditional concrete structure.
By focusing on materials, fabrication, and installation, the case study is a road map for how collaborative modular design can reduce operational and embodied carbon by 60%, cost by 10%, and construction time by 15%. Collaborative delivery of this kind requires close coordination with fabricators and contractors facilitated by shared BIM models and data. Watch the related panel discussion for how owners can influence collaborative delivery adoption and the best practices that can be implemented at project kick-off and extended through the construction to post-occupancy use.
Mass timber structural and modular MEP systems in building design offer opportunities for decarbonization and sustainable development in a way that has not been possible until now. The adoption of new codes with updated standards for tall wood buildings make it possible to now build up to 18 stories in mass timber, opening up new design possibilities for a range of building types. These projects can be built faster, more cost effectively, and with a higher decarbonization potential than traditional concrete and steel structures.
Multifamily and student housing are building types that are particularly well-suited to harness all the benefits of building with mass timber and modular MEP, given the repetitive and modular nature of the residential units. The housing crisis will continue despite the fluctuations in urban density during the past couple of years. As new codes are adopted, the potential is finally unlocked for mass timber and modular MEP systems to be a part of the solution to the housing crisis, with a focus on the critical objective of decarbonizing new development.
This case study examines how a mass timber mid-rise housing project with decentralized MEP and modular façade systems will perform as it relates embodied and operational carbon, construction cost, and construction time. Connecting all components of the case study design (mass timber structure, decentralized MEP, modular façade) is the concept of shared data sets. Project teams that closely collaborate with fabricators and contractors from day one can extend the value of data by sharing model data and components to advance the design, complete analysis, and set the stage for a streamlined fabrication and assembly process in the field.
California is now enforcing new code provisions that pave the way for taller, bigger buildings made of large format engineered wood, commonly known as mass timber. The new code provisions delineate clear requirements for three new Type IV construction sub-types, all with specific limitations for height and material exposure as shown in the diagram below.
Mass timber offers several benefits over traditional construction methods ranging from the renewability of the materials and the carbon-sequestering potential to the faster and more streamlined construction process. Below is a summary of the benefits of mass timber.
The program and spaces for multifamily housing are particularly synergistic with mass timber structural frames in that the dimensional modules for the dwelling units align with ideal structural grid spacing and material widths within a mass timber system. When examining ideal square footages and layouts for dwelling units, it is often the case that a 12’-0” structural grid aligns with the typical partition and demising of wall locations, allowing the columns to remain clear of the open space within the units. When interior columns are needed, they can fall in an unobtrusive location in the units and can be exposed as an intentional architectural feature.
The structure for mass timber buildings is prefabricated off-site and shipped to the project site for installation / assembly. Shop fabrication for the mass timber columns and slabs is often done with a multi-axis CNC router, which allows for close quality control over dimensional tolerances and can achieve tolerances as low as 1/16” (compared to 1” or more for concrete or steel).
The same principles apply for modular MEP components which are also prefabricated and maintain the tight tolerances of manufactured components. This prefabricated synergy between building systems allows the building to be thought of as a kit of parts that is assembled on site instead of being custom built in place. This approach does require close coordination between the design team, fabricators, and contractors but this front end effort is rewarded with increased speed of construction in the field and greater quality control over the final results.
The Carbon Story
The carbon cycle illustrates an inventory of the location of carbon dioxide (CO2) within our planet’s carbon pools. The impact of removing CO2 from the atmosphere relates to exchange and time. Forests sequester and store carbon, cycling it from atmosphere to biosphere to soil carbon and back. Land use change and poor management practices break this cycle. Best management practices and market demand can support it. Longevity of use, opportunity for reuse, and being mindful about end-of-life are essential factors for maintaining and increasing storage of carbon in timber products. The longer a unit of carbon is stored, the better. Some sources call for each “offset” unit to be stored for at least 100 years to be considered a complete negative emission.
Whole Life Carbon
A significant portion of carbon emissions can be attributed to the construction of a building. For a typical building, embodied carbon makes up 40-60% of a building’s total carbon footprint over the first 30 years. This is further magnified with high performance or ZNE buildings that have low to zero operational carbon emissions and embodied carbon can make up to 100% of the building’s carbon footprint. Our current methods of design and construction do not adequately account for the embodied carbon impacts that are released prior to a building’s operation. There is an urgent need to address our embodied carbon impacts where the savings can be significant and realized immediately. The near-term reduction of carbon is most valuable to meet our long-term climate targets and avoid further climate disaster.
Sustainable Tools and Model Data
Embodied Carbon in Construction Calculator (EC3 Tool)
In 2019, Perkins&Will joined as pilot partners to the Embodied Carbon in Construction Calculator (EC3) tool. Developed by Building Transparency in conjunction with the Carbon Leadership Forum and C-Change Labs, EC3 took on an ambitious and long-elusive goal to compare and reduce embodied carbon emissions from construction materials through supply chain transparency and optimization. This free, open-access tool for architects, engineers, owners, construction companies, building material suppliers, and policy makers continues to transform how the AEC industry procures low-carbon construction materials.
SPEED and tallyCAT
Whole life carbon reduction, materials transparency, and human health and wellness are at the heart of our approach to whole-systems regenerative design. That's why we developed SPEED, an in-house energy modeling tool for early design. We are also co-creating a new open-access tool called tallyCAT. These tools are at the forefront of research-informed practice. They help us to reduce operational and embodied carbon through the design process, deliver better buildings, and transform our industry.
The case study design results in a 65% reduction in embodied carbon.
Our approach to energy efficiency and operational carbon reductions for the case study focuses on key strategies that:
Reduce energy demand through climate responsive passive and active strategies.
Electrify and decarbonize. Eliminate all fossil-fuel consuming equipment and appliances to provide higher efficiency and improved indoor air quality. Supply electricity through clean renewable energy sources either on-site or off-site.
Involve decentralized, modularized, and distributed systems to allow for low-cost off-site preassembly, reduced piping lengths and penetrations, high efficiency energy recovery, and individual occupant control over energy consumption.
Transition to low-GWP refrigerants.
Early-stage energy analysis was used to benchmark the building design, inform design decisions, and balance cost and long-term benefits over the entire building life-cycle.
The case study design achieves a 32.5% reduction in operational carbon when situated in San Francisco, CA and a 46.7% when situated in Atlanta, GA.
The case study’s MEP basis of design focuses on the concepts of decarbonization and modularity. The goal is to provide a high performing building that is low carbon, affordable, and comfortable for residents. Two climate scenarios have been analyzed as part of the study with the first climate location being the actual case study site in San Francisco, CA. The design for this climate scenario provides heating only. The second climate location for analysis is in Atlanta, GA which has more pronounced temperature and humidity fluctuations when compared to San Francisco. The design for this climate scenario provides heating and cooling.
The basis of design for both climate scenarios utilize system solutions that require no centralized ducting when compared to a typical MEP design for a multifamily housing building. The reduction or elimination of ducting means less embodied carbon for the construction of the building itself, and this massive reduction in physical material also results in time and cost savings for the project.
The distributed nature of modular MEP systems allows for the elimination of heating and cooling distribution systems and the significant energy losses of those systems. Modular systems that are preinstalled in a factory provide lower construction costs. Distributed systems are inherently more reliable and easier to maintain. When central systems fail, full building services are lost. Smaller distributed systems are easier and faster to replace.
Heating-Only Option: Electric Baseboards with ERV
In milder climates such as the Bay Area, residential buildings are designed with a robust building envelope and heating-only systems. This design strategy reduces the energy loads of the building by eliminating active cooling systems. Heat is provided via an electric baseboard heater in each bedroom and living room, with remote wall programmable thermostats. To contribute to the modular design, the heat recovery ventilator (HRV) is used for ventilation and exhaust. Each unit is actively ventilated at all times, which guarantees the delivery of dry, fresh air for the benefit of the occupants.
Heating and Cooling Option: All-In-One Heating, Cooling, Ventilation, Hot Water
In climates that require cooling, the case study uses a modular box that provides heating, cooling, ventilation, and domestic hot water within one unit. This would be placed in a closet in each living space. The unit recovers waste heat from cooling to heat the hot water. When providing ventilation, heat is recovered in the winter from the exhaust air. The unit would use CO2 as a refrigerant, significantly lowering climate impacts of typical refrigerants. The unit would run at significantly higher COPs than typical heat pumps available today.
Cost and Time
When measured against a traditional concrete structure, the case study saves 10% in construction cost.
Considerations for Cost Analysis
Lighter structure results in foundations savings
Current market value of CLT is extremely competitive
CLT walls, while expensive, offset interior construction
Savings associated with vapor membranes
Savings in schedule means less supervision, general conditions, and general requirements
When measured against a traditional concrete structure, the case study saves 15% in construction time.
Considerations for Schedule Analysis
CLT productivity results in significant savings in superstructure schedule
Overall schedule savings between 80% and 90% of concrete
Warehousing of materials may be required to ensure just-in-time delivery
Front-loading of materials procurement and design-assist procurement will alter traditional cash-flow curves
Shared Data Flow
The project lifecycle can be streamlined and compressed through early integration of fabricator and contractor involvement, including bidding and shop drawing review. If more data is exchanged and shared earlier in the process, less review time is required from all parties, ensuring faster and better outcomes in the field.
If models and component data are shared early in the design process, then the design team can work with elements that are based on true built geometries and material limitations, informing the design direction in a way that limits schedule and cost risk later in the project. In addition, if the shop drawing review process starts sooner that means that fabrication can start sooner which has an inherent schedule benefit for the project. For housing projects in particular, moving up a move-in date in time has a substantial associated cost value based on financing schedules for rents. For campus housing, meeting move-in dates is critical to project success.
Working across Platforms
One of the current challenges in ensuring and extending the life and value of data on a project is the need to change platforms multiple times across the design and fabrication process. Working in a similar digital space is possible via transferring IFC and other file types between platforms, but there is a huge opportunity window in this process for a software developer to provide a combined platform or a series of directly coordinated and connected platforms. The fewer platform changes there are in the process and the more the design and fabrication digital workflow space is combined or made to look similar, the higher the value of the shared data and its ability to optimize and achieve project outcomes and the goals of the owner team.
Streamlining and improving the approach to shared data can also have benefits beyond fabrication of discreet systems or components. It can also encourage a holistic approach to design and coordination at an early stage. A primary goal and benefit of working with shared data is to encourage multiple connected systems to merge is a controlled and considered way.
For example, fabrication for mass timber components must take into account embedments and penetrations for other building systems, like MEP, in order to accommodate voids for these systems in the prefabrication process. If the right data is shared in the right way, then all of the building’s systems will be coordinated with one another going into prefabrication, thereby allowing for a kit-of-parts assembly in the field with minimum need for on-site correction.
Shared data is also integral to the process of energy modeling and carbon analysis. As noted previously, building components can be tracked and measured with great detail using tools like EC3 and tallyCAT. Fabricators are integral to this analysis process as they are often able to provide environmental product declarations, chain of custody documentation, and transportation emissions data to the design team to plug into digital tools. This allows the team to analyze the data thoroughly, set goals, and create a road map for project performance.
The ultimate goal is a completed project that is decarbonized, cost effective, delivered on time, and beautifully providing a design solution for its owners and stakeholders. Recent innovations in mass timber, modular construction, building system technology, and shared data can help teams achieve this goal with a high degree of control over project outcomes.
Anders Carpenter is a senior associate with Perkins&Will in San Francisco where he helps lead the Higher Education studio. He is a licensed architect in California with 18 years of experience in all phases of project design, documentation, and delivery. Anders has experience with a variety of project types and sizes including universities, multifamily housing, museums, libraries, K-12 schools, and civic projects. He also has additional prior experience as a designer within the context of a multidisciplinary international design consultancy, with a focus on branded interior architecture, exhibit design, installations, and graphic design. Anders has a passion for working with project stakeholders to collaborate on meaningful design solutions, with a strong focus on sustainability and innovation.