Think Wood: Hi Ruth, Bobby. To get us started can you explain a bit about Passive House, for those who might be less familiar?

Ruth Mandl: Passivhaus is a building standard that was developed in Germany in the early 1990’s. It focuses on high-performance construction through the use of extensive insulation, energy-efficient windows, and the use of an air-tight membrane that wraps the entire structure, reducing the impacts of hot and cold weather. Passive House design can offer huge energy savings and ultimately cut harmful greenhouse gas emissions. When done right, all of these factors can reduce the operational energy consumption of a building by 80 to 90%. 


TW: How did you get started with Passive House design?

RM: Our own house was actually our first Passive House project and now serves as a great test case. Working with an existing building from 1889, it was important to us to keep and reuse as much of the original moldings and millwork as possible, while updating the building to the Passive House standard. We removed all of the wood finishings and stored them in a tent in our backyard. Once the high-performance thermal envelope was completed, we reinstalled most of these materials; what we couldn’t reuse, we donated for reuse. The project demonstrated to us that it is possible to achieve a high-performance Passive House renovation while retaining salvaged materials.


TW: How does CO Adaptive approach Passive House design in renovation and adaptive reuse projects?

Bobby Johnston: Depending on the context and condition of the building, our approach is to carefully consider what we can retain from the original construction while updating the thermal envelope to meet high efficiency standards. The goal is always to update the building for airtightness without demolishing the interiors. We are starting to focus much more on how we can approach adaptive reuse and renovations with minimal waste, opting to focus on deconstruction rather than demolition.  It’s a trade-off between minimizing a building’s embodied carbon by retaining as much of the existing structure as possible and the operational benefits that you achieve through Passive House design.


TW: How do Passive House structures perform in different climates?

RM: Given the ever-hotter weather due to climate change, buildings in the U.S. need to mitigate heat as much as cold temperatures, particularly in southern climates and during summer months. Passive House was originally developed for Germany—a relatively cold climate—but it also works really well in a hot climate, as long as you’re preventing the sun in the summer from penetrating the interior. The key is in getting your building envelope to work harder than your active systems—in turn reducing your reliance on HVAC systems.


TW: What role can prefabricated timber systems play in Passive House design?

RM: Along with minimizing demolition in renovations, we are starting to look more rigorously at prefabrication and modularity in our adaptive reuse and renovation projects—and how we can make use of demountable timber panels, ultimately prefabricating portions of a Passive House envelope for retrofit applications. Thermally, wood is superior to steel and timber and offers added thermal mass. All of our Passive House projects use wood as the primary material. Wood provides more versatility and given its natural renewability, it lends itself to our goals of regenerative, circular design. There’s more modularity and prefabrication in new builds but we haven’t seen a lot in the retrofit market, especially in an urban context. I think that’s really what is needed. So we’re working on a prototype here at the office, doing just that.


TW: Is Passive House part of a broader vision for your architectural practice?

RM: Very much so—we’re constantly evolving what sustainability means to our practice, but we know at the core we want to think as systemically and regeneratively as we possibly can. Cutting embodied and operational energy is definitely a part of that. In urban environments, these principles are best applied in adapting historic building stock, reducing demolition when it can be avoided, and respecting and optimizing the materials that we’re using and reusing. To do this, it makes sense to use low carbon building materials—like timber—and design for disassembly with adaptability in mind, so that things can be taken apart, reused, and become part of a circular design loop.

BJ: Our passion is for retrofitting historic and aging buildings but we found that there can be a significant cost to doing that. We’d like to find ways to curb those costs and make Passive House design more affordable and accessible. Prefabricated timber systems, modularity, and systemized assemblies are part of that vision. I think our cities—in particular those with larger, older building stock—need solutions that can be easily implemented. We want to retrofit existing buildings to higher performance while doing it in a way that is affordable and not completely disruptive. At present, there’s a gap in the market and our goal, as a practice, is to fill that gap. 



Ruth and Bobby live in Brooklyn, New York, where they completed a self-commissioned renovation of their own brownstone, transforming a beautiful, century-old building into a highly resilient home to take their family into the future, while respecting and celebrating the past. Their firm’s name CO Adaptive takes inspiration from the same biological term used to describe the process by which a bee adapts to a flower, just as the flower adapts to the bee.


Learn about
CO Adaptive's approach to low-carbon construction.

To stay within 1.5°C warming, greenhouse gas emissions need to decline 45% below 2010 levels by 2030 and reach net zero emissions by 2050. The built environment accounts for 40% of GHG emissions, so buildings—and the climate impact of their construction and operation—are an essential part of reducing carbon emissions. Meanwhile, our need for buildings is not going away. About 60% of buildings that will exist by 2050 haven’t been built yet. This means constructing a city the size of Stockholm or Milan every week until 2050, or a city the size of Singapore or New York every month.

The good news is that cutting carbon in cities could mean a U.S. $20 trillion boost to global gross domestic product (GDP). In addition, design firms with green building expertise stand to gain from a market set to grow nearly 15% by the end of 2027.

Our sector has a critical role to play. Discover how cities are leading the way and forging new approaches with the help of the AEC sector and how your design team can play a role in cutting your city’s carbon footprint.

What does net-zero carbon mean? Is it different from carbon neutral?


Net zero refers to a state in which the greenhouse gases going into the atmosphere are balanced by removal out of the atmosphere. The term net zero is important because—for CO2 at least—this is the state at which global warming stops.


Carbon neutral means that any CO2 released into the atmosphere from an activity or project is balanced by an equivalent amount being removed.


Climate positive (also known as net negative) means that an activity’s GHG removals exceed its emissions.


High Impact Climate Solutions in Cities

Cities are major contributors to climate change—which means they can also be a key solution to it.

Over half of the world’s population lives in cities. According to UN Habitat, cities consume 78%  of the world’s energy and produce more than 60% of greenhouse gas emissions, yet they cover less than 2% of the Earth’s surface. 

We can greatly reduce our per capita carbon footprint by changing how we plan, build, manage, and power cities and towns. The ideal low carbon and resilient city is well-designed, compact, walkable, and has good public transportation.

Photo credit: Adam Blank

A number of organizations are dedicated to this premise. A partnership between the Cities Alliance, the World Bank, UN-HABITAT and the United Nations Environment Programme (UNEP) works globally to help cities address climate challenges through sustainable urban development. Another organization, called C40, is a network of mayors of nearly 100 cities around the world whose mission is to halve the emissions of its member cities within a decade while improving equity and building resilience. C40 cities earn their place through action—with membership based on performance requirements, not member fees.

Led by the city of Oslo, C40’s Clean Construction Forum helps cities working to achieve zero embodied emissions from buildings and infrastructure by 2050. They focus on reducing emissions from construction materials and machinery by:

  • Understanding the methods and data needed to establish city wide targets; 
  • Collaborating on available standards and tools to assess the environmental impact of materials and construction sites, and;
  • Using cities’ collective power to develop a market for low emission construction materials and construction equipment.

Cutting emissions will require a change in business as usual. According to UN News, “The extraction and manufacturing of materials for buildings such as steel and concrete and construction processes produce carbon dioxide[,] so using low carbon infrastructure will also slash emissions.” 

In addition to reduced emissions, cities built from bio-based materials such as timber that store carbon during their service lives can serve as constructed carbon sinks. They could increase the existing carbon pool of urban areas (1–12 GtC) by 25 to 170%.”


Climate Ready Boston

C40 member Boston is an example of one city cutting carbon and making progress on a number of fronts. New England’s largest city stands to be impacted disproportionately by rising seas due to global warming and launched Climate Ready Boston more than seven years ago to help the region plan for the impacts of climate change with an update in 2019 to boost its focus on curbing carbon.

A cornerstone of the plan is adopting a zero net carbon standard for new construction by 2030 and pursuing strategies to reduce building emissions over the next five years such as a zero net carbon standard for new municipal buildings and a carbon emissions performance standard to decarbonize existing large buildings. Along with these measures, Boston is also promoting policies that support walkable neighborhoods and enable residents to live car-free.

And so far, the city is seeing results: citywide GHG emissions are 17% lower than they were in 2005, and emissions from city government operations have been reduced by almost 25% in the same period. An embodied carbon technical advisory group formed in 2021 is exploring the use of more carbon sequestering materials and the city is setting stricter standards for city-funded low carbon affordable housing, inspiring solutions like six new mass timber affordable housing projects. 

To incentivize low-carbon construction, Boston launched the Boston Mass Timber Accelerator through grants from USDA Forest Service, Softwood Lumber Board, and ClimateWorks Foundation. The initiative provides design teams with technical assistance and funding grants to expand the use of low carbon mass timber products. This March, seven projects received funding ranging from a seven-story commercial office building to an eight-story, 215-unit affordable senior, and assisted living facility—along with other housing projects that provide equitable access to affordable accommodations while reducing their carbon footprint.

Photo credit: Jacob Licht
The project will focus on the lightweight structural benefits of mass timber for building additions—commercial office building addition and seven stories of new construction.

Boston is also home to a number of mass timber projects under construction including 11 E Lenox St, a 7-story, 34-unit multifamily project in the city’s Roxbury neighborhood.

Designed by Monte French Design Studio, it combines the thermal benefits of mass timber construction and Passive House design, reducing operational energy use by more than 80%. 

Slated for completion later this year, 11 E Lenox’s wood structural system will store 844 tons of CO2 throughout the building lifecycle and offset 327 tons of CO2 when compared to conventional steel or concrete alternatives.

Photo credit: Monte French Design Studio
The project team will assess cost effective implementation of mass timber at Mary Ellen McCormack Redevelopment in a high-rise multi-family residential building—302 units of mixed-income affordable housing in a nine-story building.

Portland’s Climate Action Plan

Over 3,000 miles west of Boston, the city of Portland and Multnomah County, OR is also making headway in the fight against climate change. The city’s Climate Action Plan authored in 2015, aims to reduce carbon emissions 80% below 1990 levels by 2050 with an interim goal of a 40% reduction by 2030.

In 2017, city leaders specified 2030 energy efficiency goals for the built environment, which include achieving zero net carbon emissions in all new buildings and homes.

The City’s June 2020 Climate Emergency Declaration committed to a “climate justice and equity-focused approach that centers Black, Indigenous, other communities of color and youth from those communities in the next chapter of climate action planning and implementation.” In addition to multiple initiatives in support of this approach, the City also amended carbon reduction targets to at least 50% by 2030 and net-zero carbon emissions before 2050. 

Portland has both a Sustainable Procurement Policy and Green Building Policy for City-owned facilities. The Sustainable Procurement Policy outlines best practices for purchasing activities including developing and applying a shadow price for carbon to inform decision-making on capital projects and purchases of goods and services, utilizing sustainably sourced wood for City-owned building and landscape projects, and specifying low-carbon services. Shadow pricing is a method of investment or decision analysis that adds a hypothetical surcharge to market prices for goods or services that involve significant carbon emissions in their supply chain.

Portland’s plan identified over 247 actions to be completed or significantly underway by the end of 2020. Nearly all the actions in the 2015 Climate Action Plan are underway, with 77% of actions complete. The region cut total local carbon emissions to 19% below 1990 levels, and per person, emissions were cut by 42% as of 2018.

Photo credit: Unsplash

Portland is also home to a rapidly growing stock of timber buildings. The state’s rich supply of local, sustainably harvested timber helped make it home to the first certified U.S. producer of mass timber, which opened in Riddle, OR in 2015.

Oregon was also the first state to adopt the 2021 International Building Code to allow tall mass timber buildings. Two mass timber projects in Portland’s Burnside Bridgehead neighborhood standout: Sideyard, a 20,000 square-foot commercial office and infill project that makes the most of a narrow site, and Fair-Haired Dumbbell, a mixed-use office and retail complex designed by FFA Architecture and Interiors that includes two canted six-story towers wrapped in hand-painted artwork. Adjacent to one another, each with their own distinct design, these projects reveal just how versatile mass timber can be while also boosting density and helping to revitalize urban areas with climate-conscious ingenuity. 

Portland is also seeing a rise in the design of nearly all-wood buildings, boosting carbon-capture by combining light-frame wood and mass timber construction—a recent example being the Cascada, a mixed-use project to be built in the city’s lively Alberta Arts District. This project includes hotel, coworking, restaurant, and wellness program space as well as leasable retail spaces.

Photo credit: KUDA Photography

Taking Action In Your City

The actions taken by cities like Boston and Portland can only happen through collaboration with forward-thinking architects, developers, and other building professionals looking to innovate in the fight against climate change.

Design teams can help their city through an increased focus on reducing embodied and operational carbon emissions and by building with bio-based materials, turning urban centers into carbon sinks—and literally converting the built environment from a carbon emitter to a long-term carbon storage solution.

As C40’s executive director emphasized in a past global competition for solutions, “[We need] inventive collaboration to combat the climate crisis—from the skills and creativity of architects, artists, environmentalists and entrepreneurs. The creation of new and exciting developments in cities not only reduces carbon emissions from construction, but also builds the resilient urban environments we need to cope with rising temperatures and more extreme climate events.”

Photo credit: Zixi Zhou

Most homebuilders are familiar with operational carbon, which refers to the greenhouse gas emissions from a home’s energy use after it’s built, but embodied carbon looks at all the energy it took to build the home in the first place. That includes the energy that went into manufacturing and transporting its materials, as well as during the act of its physical construction. Right now, that number is higher than most green-focused homebuilders probably realize.

For example, the average U.S. home produces 45 kg of CO2 per square meter annually from an operational perspective according to a 2020 University of Michigan study. But a recent Canadian report found embodied carbon accounted for 250 kg of CO2 per square meter in residential buildings, or more than five times as much.

That means even the most energy-efficient new homes built today have already contributed five years’ worth of greenhouse gas emissions to the environment before their new owners move in. Put another way, the embodied carbon of constructing the 1.6 million homes built in the United States last year put the same amount of greenhouse gases into the atmosphere as operating 22 million cars for an entire year.

Think Wood spoke to Aaron Smith to learn more about net zero carbon houses. As CEO of the Energy & Environmental Building Alliance (EEBA) as well as his own firm, GreenSmith Builders, Aaron is passionate about sustainable and low carbon construction.

Think Wood: Tell us about EEBA. What is your mission?

Aaron Smith: In 1982, a group of builders from the US, Canada, and Scandinavia came together and said, “Let’s try to build more energy efficient homes.” We’ve seen that transition to energy efficiency through building science and education, but today I would say the focus for EEBA’s builders is healthy, electric, resilient, decarbonized, and net zero.

We’ve heard a lot about net zero energy homes but EEBA also has a focus on embodied carbon?

AS: We’re talking about both embodied and operational carbon now. The primary thing we’re trying to do is get people educated about what decarbonization is, what an environmental product declaration is, what a lifecycle assessment is. When I look at two different materials, what does it mean to think about the embodied carbon in that material?

Why does lowering carbon emissions matter to builders?

AS: It matters for a few reasons. First, builders have a pride of craftsmanship and they extend that same care to thinking about the environmental impact of the materials they use because they’re thinking about the future. My grandfather was a builder and he always asked, “Are you happy with what you did? Because it’s going to be there for 100 years. 

And, we’re starting to see carbon codified across North America. California, Oregon, Washington, they’re headed in that direction. I’ve got a buddy in Texas who builds 1,000 homes a year. This year, his investment firm called him and said 20% of the product that you build for us now needs to be net zero. That’s not quite to decarbonization yet, but when finance starts telling production builders you need to do this, that’ll be a game changer.

What steps can builders take in the design phase to achieve low carbon homes?

AS: EEBA members asked us to research the best tools accessible to builders for calculating operational and embodied carbon. We counsel them to put their houses into a building information modeling (BIM) system like AutoCAD or Revit. You can import that into some whole-building life cycle analysis (WBLCA) tools and start to generate a carbon footprint for the home design. We’ve added kilograms per ton of CO2 for your home to EEBA’s intake form because we want to start benchmarking it, too. In the future, we’ll start to compare those benchmarks across the industry.

Can you tell us about your Gateway to Zero program?

AS: There is a path to zero for everyone, but with so many programs, guidelines, and standards, finding the one that’s right for you can be overwhelming. Though we know that “getting to zero” is not always a linear process, we’ve organized resources in categories from “base energy code” to “zero embodied carbon.” EEBA can help you become a more sustainable builder from wherever you’re starting. You may be at code today; you may then want to look at the Energy Star program. It’s free, and it’ll make you a better builder. And then you might want to move to one of the zero energy programs out there. We provide support throughout this entire journey.

Does building with wood provide any advantages when low carbon construction is the goal?

AS: Once you’re educated on how material selection impacts a home’s carbon footprint, I think it’s a little bit ‘back to the future’. You start to say, “Well, instead of putting the steel beam in, I could put a cross laminated timber beam in and have a carbon sink instead of a huge carbon cost to my home.” To reduce carbon, I think it’s going to be a return to wood in a lot of cases. Homes can literally be a carbon sink instead of emitting carbon.

A Trent University graduate student’s study compared the net carbon emissions of the same building using different materials in each modeled scenario.

The results were drastically different depending on the products used:

  • The highest emission scenario used clay tiles, steel joists, high-VOC carpets, steel framing, cement brick for cladding, and high embodied carbon concrete for the slab.
  • For the same structure built to the same level of performance (to local code), the net carbon emissions were lowered by 144 tonnes by using trusses and asphalt shingles, drywall and mineral wool, wood framing and drywall, fiber cement for the cladding, and an average concrete with mineral wool for the slab.

Keep reading to learn more about two U.S. homebuilders making affordable energy-efficient low-carbon home construction a reality.

Thrive Home Builders

Thrive Home Builders have made energy efficient and eco-friendly features central to the design and construction of their clients’ homes. This includes achieving environmental certifications like LEED®, Indoor airPLUS, Zero Energy Ready Homes, and Energy Star®. The Denver based homebuilder has a successful track record delivering zero operational energy homes and is now turning its attention to their homes’ embodied carbon impact.

Thrive is pioneering a new design process that evaluates the use of  carbon measurement tools and technologies like BIM to better understand the embodied carbon impact of materials and how it can curb emissions through design and offset measures. The firm has been recognized by the U.S. Department of Energy’s Housing Innovation Awards since 2017 for its commitment to the department’s Zero Energy Ready Home program (DOE ZERH).

“Thrive Home Builders delivers energy-efficient homes with a mission focused on healthy and local solutions. We’ve been making conscious choices in the design of our homes for 30 years to make them healthy for families, the environment, and the planet. We think the carbon conversation is a great tie-in to what we are doing already—and we believe it’s becoming clear for our buyers: you can’t be healthy yourself without a healthy home and a healthy planet.”

Bill Rectanus, Chief Operating Officer

Bettr Homes

With a mantra of “people, planet, purpose!” Bettr Homes is focused on the triple bottom line, while delivering sustainable build-to-rent homes designed to be high performance, healthy, and resilient. The firm exclusively builds net-zero energy homes that meet certification standards like Zero Energy Ready Home, EPA Indoor Air Plus, Energy Star, LEED and Resnet HERS Index.

Bettr Homes’ vertically integrated business model positions the company to work well with real estate investment trusts (REITs) to deliver affordable net-zero rental homes, expanding access to much needed entry-level environmentally-sustainable housing. By attracting ESG-focused investors that perceive long-term benefits of healthy, affordable, low carbon homes, Bettr is demonstrating that sustainable home construction is not only good for the environment, but for the bottom line as well.

“What we’ve created with Bettr Homes is a triple-bottom line enterprise and we’re working with investors to add affordable net-zero homes to their rental portfolio—in doing so we’re creating healthy communities with a focus on decarbonizing the rental housing business. Its success is showing us, when you manage to an ESG standard, you create real value.”

Corey Donahue, Co-Founder


Think Wood: What inspired you to take on such an ambitious project—the tallest timber tower in the world to date?

Tim Gokhman: Innovation has always been a focus for New Land. We have a pioneering history and a higher tolerance for risk because that’s something we’ve always done—whether it be taking a risk geographically, such as building in a less desirable, up-and-coming neighborhood, or being one of the first firms to use radiant hydronic floors in some of our multifamily projects. 

When I saw the renderings for River Beech Tower—a conceptual tall timber design by Perkins&Will and Thornton Tomasetti—it was a revelation that not only was it possible to build a highrise with wood, you could also expose that structure inside, making the visual experience for the occupant incomparable. Our strong experience in multifamily development and ability to put together a team of proven mass timber experts gave us the confidence to move forward with the project.


TW: How did you know the project would pencil out?

TG: When it came to penciling it out, we treated Ascent more or less like any conventional development project. If we were able to realize additional benefits through mass timber, we saw that as a bonus. In fact, the primary driver for this project wasn’t speed of construction, environmental benefits, or carbon savings—although we will realize all of those advantages. The initial motivation was the feeling you get when you walk into a mass timber building. A few years ago I didn’t know the term biophilia, and now it’s become a central tenet of this project. While we don’t yet have enough data on tall wood, and I can’t yet quantify that benefit entirely—whether through leasing rates or ROI—intuitively you can see the added value and differentiator is there for this project. We expect it to continue to pay off. 


TW: What was one of the biggest challenges in this project?

TG: While there was a lot to figure out in terms of the design and a fair bit of troubleshooting for some of the timber engineering solutions, by far our biggest challenge was insurance. Tall mass timber is a relatively new building type, so there just aren’t a lot of data points, which of course makes an insurance underwriter’s job very tough. There’s no precedent. When you’re pioneering a project like this, you’re working with a smaller pool of potential insurers, but in the end, we made it work. I think that over time, as more and more of these types of tall timber projects are completed, it will get easier and cheaper.


TW: What do you see as some of the most significant benefits of using mass timber?

TG: We were able to erect the building faster with mass timber and through the use of BIM and just-in-time deliveries than we would have been able to do using other materials. The amazing thing about mass timber is that you’re not really giving anything up. I often compare it to Tesla—as a car company, they didn’t have to convince you to give something up in order to do something that’s right for the environment because it wasn’t at the expense of aesthetics or performance. Similarly, mass timber checks all the boxes: it’s faster and more precise; it’s light-weight and easy to work with; it’s beautiful; it’s sustainable; and it delivers a superior experience across all categories. When you are looking to shift the paradigm like mass timber is, whether it’s fair or not, that’s the bar you have to set. You have to be as good, or better, than conventional construction.


TW: Do you see more tall timber projects in the future for New Land Enterprises?

TG: Absolutely—that’s the goal. We have just a few months left of work on Ascent, but already the team is taking in the lessons learned and contemplating where we might build the next one. We’ve invested a lot of time and R&D into this project, and we definitely see this as a model and case study to repeat. With our experience on Ascent, we will be able to streamline the process and realize even more of the benefits of tall mass timber construction on future projects.

Learn more about
Ascent in this project profile.

1) Is mass timber fire resistant?

Heavy timber and mass timber building elements char at a slow and predictable rate, providing for inherent fire resistance. During fire exposure, mass timber chars on the outside, which forms an insulating layer protecting interior wood from damage. During a fire resistance test of a 5-ply cross-laminated timber (CLT) panel wall, the panel was subjected to temperatures exceeding 1,800 degrees Fahrenheit. The assembly sustained loads for three hours and six minutes, far more than the two-hour fire resistance rating that building codes require. Additionally, when the code requires mass timber to be protected with gypsum wall board, it can achieve nearly damage-free performance during a contents-fire burnout event.


2) How is a structural member’s fire resistance measured?

Fire resistance ratings for mass timber building elements are commonly developed from empirical models contained in the National Design Specification® for Wood Construction and TR10 – Calculating the Fire Resistance of Wood Members and Assemblies. The models use char rate data collected during ASTM E119 fire resistance test of exposed mass timber. Since char rates are predictable, it has been possible to develop structural models that account for the loss of cross section due to charring and the ability of the member to support the applied load. Like all materials, structural failure eventually occurs. For steel, failure is due to weakening of the metal. For mass timber, failure occurs when the cross section is no longer adequate.


3) What’s important to know about fire resistance rated assemblies?

Fire resistance rating is performance criteria used in the building code to ensure the structure will not collapse when exposed to fire. The fire resistance rating time assigned to a building is based on the perceived risk due to building area, height, and occupancy. Buildings that represent a greater risk are required to have a greater fire resistance rating. Any material that can achieve the required fire resistance is permitted, but there are often limits to the allowable height and area of combustible construction.


4) Does a building’s construction type determine how mass timber systems can be used?

Buildings are classified according to height and area limitations so the construction type does have an effect on how mass timber systems can be used. In the 2021 International Building Code (IBC), mass timber construction is classified as Type IV and has four subcategories, A, B, C, and Heavy Timber (HT). The code currently allows:

  • Type IV-A – Maximum 18 stories, with gypsum wallboard on all mass timber elements.
  • Type IV-B – Maximum 12 stories, limited-area of exposed mass timber walls and ceilings allowed.
  • Type IV-C – Maximum 9 stories, all exposed mass timber designed for a two-hour fire resistance.
  • Type IV-HT – Maximum 6 stories, previously the Heavy Timber construction type under the 2018 IBC.

Types III and V permit the use of light wood framing throughout much of the structure. However, it is possible to classify a building as Type III or Type V when constructed primarily of mass timber building elements due to the height and area of the building. For example, a mass timber building six stories in height could be classified as Type III-A or a four-story building as a Type IV.


5) How does mass timber’s fire performance compare to other structural materials?

Building codes require all building systems to perform to the same level of safety, regardless of material—and wood-frame construction is approved in the IBC and International Residential Code (IRC). 

For mass timber specifically, the result of the years-long ICC process to develop and approve safety requirements for each of the new types of construction (Type IV-A, Type IV-B, and Type IV-C), tall mass timber buildings have fire protection requirements more robust than those required for comparable noncombustible buildings.

Ken Bland is Vice President of Codes and Regulations for the American Wood Council. He is a former building official and has a BS in Architectural Engineering and an MS in Fire Protection Engineering. He is also a licensed engineer.

Think Wood: Hi Steven, we’re excited to chat with you. Let’s start with a bit about your background and how you came to be interested in mass timber.

Steven Paynter: I grew up visiting Norway and Scandinavia and appreciated how these regions were innovating with timber construction. Early on, I developed a love and affinity for wood as a building material and when I started my career in the UK, I had an opportunity to use mass timber in a number of public projects—secondary schools, universities, and a hospital—where I began to see its potential. Mass timber was generally limited to lower rise construction and some hybrid designs at the time, typically post and beam structures or as main features in public spaces and atriums. Then about 15 years ago, I relocated to the North American market where I’ve further explored mass timber and wood’s design possibilities. This started with the redevelopment of an early 20th century heavy timber building, and has continued from there, leading to my recent work on a prototype for a tall timber, net-zero high-rise.


TW: Can you tell us more about this tall timber prototype?

SP: The project is a collaboration with urban innovation company Sidewalk Labs and explores how factory-produced timber buildings can grow even taller by designing a model at 35 stories—a height that’s yet to be achieved in practice. We call this Proto-Model X 35—or PMX 35. It’s a mass timber prototype of a net-zero tower that we can use for testing, like a concept car in the auto industry. Having a prototype like this is a way for us to push boundaries through digital modeling.

PMX 35 is designed in a way that allows 100% of the above grade structure and about 90% of the facade system and MEP to be fabricated offsite. This was incredibly challenging to achieve and we repeatedly reran our modeling to reduce the number of unique parts in the building, to increase the standardization. As an example, we started with 3,500+ unique floor panels and by the time we finished, we were down to six. We also managed to greatly reduce the per piece install time by streamlining connection details. All in all, the project has required a lot of detailed analysis and patient problem solvingand it’s become an important and pivotal project for Gensler.


TW: PMX 35 models tall wood construction, which is a new frontier. From your perspective, what are the advantages of building with mass timber?

SP: The things that made mass timber a great material for centuries still apply today. Aesthetically, we are drawn to wood as a natural material. It’s lightweight and easy to work with—an advantage over other heavy, carbon-intensive materials. This makes mass timber well suited for prefabrication. It’s really through offsite construction with a factory mindset that we can achieve efficiencies that translate into cutting carbon and increasing affordability.

Mass timber offers the chance to create something beautiful and sustainable in a way that no other structural material does. When it comes to climate change, the construction and building industry is on the hook for nearly 50% of all global CO2. Now is the time for us to change that and be more proactive as an industry.


TW: Speaking of being proactive, how can mass timber help Gensler achieve the firm’s climate goals?

SP: The Gensler Cities Climate Challenge (GC3) is intended to set a new standard for reducing all carbon emissions in the built environment by 2030. To meet this ambitious goal, we need to reduce the carbon footprint of building materials, something that mass timber can help us achieve. Timber buildings can weigh up to 20% less than a comparable concrete building and the foundation size is also reduced, ultimately contributing to a reduction in embodied energy. Recent studies of timber buildings have shown over a 20% reduction in embodied carbon, and we are in the process of conducting a similar analysis on the PMX building that we expect will reveal similar, if not better, results. I think mass timber is a key part of making building more sustainable, addressing global challenges, and achieving our climate goals, both as a firm and as a broader industry.


TW: You specialize in commercial office design. With the rise of remote working, what does the office of tomorrow look like?

SP: The rise in remote working is definitely having an impact. Today, post-pandemic remote work means office designs need to do more to engage; they need to provide an employee experience and you need to ultimately convey your company culture in two days a week, not five. One thing we are really seeing is that when clients are designing or redeveloping office space, they’re looking to provide something special, an experience that reinforces a positive culture. They’re also looking for flexibility and adaptability, as well as healthy, inviting spaces with good ventilation and air quality. Mass timber offers an opportunity to achieve many of those things. Aesthetically, it offers warmth and I believe even changes the mood of everyone in the space for the better.


TW: Where do you see mass timber design and construction headed in the future?

SP: For me, the goal is that we stop treating it as a special niche material and see it as a valid, sustainable material for a broad spectrum of buildings. As a firm, we’re starting to explore and conceptualize our first all mass timber neighborhoods, and to me that would be an amazing next step in the evolution of our work. That’s the scale and scope that we need to make that larger impact—and to cut carbon emissions substantially. In the next five years I would like to see mass timber become “the norm” in design and by 2030 it should be the first choice of structure in most of our projects below 12 stories. That’s where we need to get to in order to meet our GC3 goals. This has to be the future.

Explore the Studio Spotlight to learn more about how Gensler is innovating with mass timber.

PMX 35
Rendering: Michael Green Architecture and Gensler

Think Wood: Hello Brent. Let’s start with prefabrication. Skylab has a long history with prefab, including your partnership with Method Homes to develop a modular grid system called HOMB. Tell us about this long-term exploration.

BG: At Skylab, we’ve been practicing modular construction for more than 20 years, but people are just now starting to really talk about it. Our experience with prefab stems from our early work in residential and retail designs, and now we’ve deployed these concepts across multiple scales, from a million square feet to a 2,000 square-foot house. It’s how we design for efficiency, for maximizing repetition, and for minimizing waste. It also creates harmony in a space. Because the pieces come pre-assembled, you can pre-engineer and pre-design. It speeds up the time on site, and you can work with a standard contractor while building a more sophisticated product.


TW: Your recent project, Outpost, is redefining the concept of mixed-use construction by combining industrial and commercial spaces. Do you see this as an emerging trend and why do you feel this is important?

BG: We’re seeing this convergence of processes and activities that previously were separated or decentralized, as well as the emergence of industrial space being more visible and cleaner. With Outpost, we were transitioning the project site from a working industrial waterfront to a waterfront for both people and makers. We’re also working with Nike on their new world headquarters where they want to move designers and makers closer together. Today, clients are looking for an assemblage of social ideas and mix-used concepts where everyone is in the same house, making and enjoying things together. I think people are thirsty for the authenticity of that experience. They want to see how things are made and engage in the process, even if it’s just visual. There’s also a heavy recreation factor activating many of these industrial waterfront areas. So we ask ourselves, how do all of these users and uses mix together, and how can our buildings respond to and support those needs?


TW: You’ve referred to Outpost as a prototype for future waterfront projects. What makes this a model that can/should be replicated?

BG: For Outpost we tried, in a sense, to use a humble approach with conventional materials and conventional systems, but at an elevated level. We moved traditional street-level retail to a shared second floor experience where tenants and guests can mingle with makers. The mix of central office tenants and small businesses has created its own community, and the porch concept has served as a mixing space for people who live, work and visit the space. We created a place for people to really experience the waterfront and all it has to offer. As a palette, it’s pretty accessible for other locations and other developers to use a similar approach, especially for an all-wood structure.


TW: Tell us about Skylab’s evolution working with wood and mass timber construction.

BG: Our shift to build with and design with timber has been driven by sustainability, by cost and by the beauty of wood itself. Our goal [for Outpost] was to use as much wood as possible, including the structure, the window systems, the doors, etc. Local material and labor also come into play here. It’s an easier material for the trades to engage and put together, it creates a warm and sustainable space, and it allows for developers to easily plug and play different tenants with little to no tenant-improvement dollars. For us, it’s not always a hyper-purist approach, however. We have a number of hybrid projects where we are engaging with CLT and mass timber to find a balance. We are leveraging wood in its various different forms, and we always explore the right materials mix for each project.


TW: What’s on the horizon for Skylab? How do you see mass timber construction contributing to the next generation projects?

BG: We see mass timber as a key material and structural approach for a broader scale of projects including office and institutional but also small and medium scale residential and hospitality buildings where there is more potential for repetition. Skylab is continuing to look at sourcing these prefabricated structures closer to building sites regionally across the west coast as more mass timber factories are coming online. In several of our current custom residential projects we are exploring the potential of mass timber solutions that expose the structure. In one particular project, we are utilizing CLT in a modular industrial way with the whole primary structure being delivered to the site as a complete kit for assembly. With the unpredictable cost of materials and labor these projects are a way to demonstrate the viability of a mass timber approach regionally and also showcasing wood as the finish. 


Brent currently resides in NE Portland, Oregon with his daughter Amelia and wife, Laura and dog Mayson. He continues to experiment on his own house which was a structure rebuilt with local reclaimed Oregon timber and a range of wood charred and stained finishes.


Learn more about Brent's work on Outpost in this project profile.

Think Wood: Thanks for joining us Dan. INTRO is set to be America’s tallest mass timber project, and is one of the early adopters of new IBC code provisions for tall wood buildings. What inspired you to innovate with mass timber construction?

Dan Whalen: The site for INTRO is one of the most high-profile corners adjacent to Cleveland’s West Side Market. Being next to this historic landmark, we felt like we had to create a project of similar standing – we wanted to be different and historic in our own right. So we began this project knowing that we were going to be different, but we didn’t know what that would be at first. Then we thought, ‘If our building is still here in 50 years, what are people going to say about it?’ We want people to look at INTRO and know that it’s not just another apartment building. Timber helped us bring that vision to fruition.


TW: What makes timber a differentiator?

DW: We were blown away by its aesthetic qualities. Mass timber started out as a design differentiator, and then it snowballed from there. As we studied the product and building systems more and more, it became advantageous across a number of channels — from the ease and speed of construction; the cleanliness of the construction site; and the renewability and sustainability of the material — the story just started to become so strong and powerful.


TW: You mention the increased speed of construction with mass timber. Are you seeing a cost savings as a result of this material choice?

DW: Cost savings depends on how well you manage your job. The savings and the benefit come from how streamlined the workflow is and how well you manage just-in-time delivery and alignment of the trades that follow. That’s where you really see the benefit, efficiency and cost savings. We are measuring this project from cradle to grave. We’re documenting everything on every front so we know track the assumptions we are making and see if we are missing the mark and where we can improve.


TW: What challenges and opportunities are you encountering as you build this first-of-its-kind mass timber project?

DW: We’re educating others about mass timber every single day, and it’s reassuring to see that people are excited. When people walk by the building, it feels unique. They don’t quite know what it is, but they know that it’s different from everything else around them. Visitors also can see the efficiency firsthand. They watch the building one week, and then the next week construction is drastically further along. We’re also educating local building and fire code officials about mass timber’s strength and fire resistance. Think about the tallest trees; they’re hundreds of feet tall, so why not be able to build buildings out of the same materials at the same height?


TW: What are you seeing in terms of public interest and tenant response to INTRO so far?

DW: It’s still a bit early in the process, but I will say that we’re the talk of the town and one of the most anticipated projects in quite some time. We’ve had extreme interest on the retail side, and the registration for apartments is 300-400 names long. The phone also hasn’t stopped ringing for the event venue. We’ve had about 200 inquiries over the past five weeks. And this is without much, if any, promotion. The only marketing we’ve done is the construction fencing wrap. We have an A+ location, and we further cemented ourselves to this fortress of investment by doubling down with mass timber design. By having this one-of-a-kind beautiful building on a prime corner, it’s almost irreplaceable and truly something that we can showcase, nationally and internationally.


TW: What’s next for Harbor Bay?

DW: We are continuously innovating even during the construction process. We are taking copious notes and learning on the fly so that we can innovate taller and faster down the road. We want to develop timber buildings almost exclusively moving forward. The cities that we are working with are all salivating at the idea of having large scale timber projects, and we want to be the foremost developer of timber, especially on the multi-family front. Great quality and design combined with great locations – that’s what we’re hanging our hat on.

Learn more about Dan's work on INTRO in this project profile.

How can I build the safest deck for my clients?

When building a deck, the most important safety check is to make sure you’re up to code. The DCA 6 prescriptive deck building code and the recently released International Residential Code (IRC) contain everything you’ll need to know, from joist size to ledger attachment. When building, consider future maintenance. What type of wood preservative treatment are you planning to use? Different treatments will have different corrosive impacts on fasteners. When you are looking at the durability of fasteners, note where you are using them. Are they ledger or lag screws or are merely securing the deck boards? When installing life-safety fasteners, think about how difficult it will be to inspect, replace or repair those fasteners. Weigh initial cost vs replacement cost. Also consider any manufacturer recommendations specific to the wood deck you are building.



What should I look for during annual inspections?

Once a deck is built, keep it in great shape with regular maintenance and annual wood deck safety checks. These, along with proper maintenance and periodic refinishing will extend the life of the deck. This is especially true if you live in parts of the U.S. with heavy snow and moisture. A well-maintained deck can last twice as long as a neglected deck, with a life expectancy of up to 20 years.

In addition to continued compliance with building codes, missing connections are the number one warning sign of an unsafe deck. Go underneath the deck and examine how it’s put together. That starts with the ledger board, which fastens the deck to the house. Then, make sure posts, beams, joists, deck boards, railings, fasteners and connectors are all stable and in good condition. Remove fasteners and inspect, looking for patterns of corrosion. (A white and frosty appearance shows that a fastener has reached the end of its life.) After assessing your client’s deck substructure, be sure to check the flashing – an L-shaped piece of metal that diverts water and is on top of the ledger board. If the flashing is degraded, has holes, or is missing, replace it or have a new one installed.



What is the most common inspection/installation error?

Deck or guardrail collapse is rarely if ever caused by occupant overloading: the number one issue is lack of deck maintenance, decay or erosion of fasteners. The most common installation errors are related to the ledger board and flashing. Faulty installation allows water to enter and collect around the house, resulting in rot and decay. When installing and inspecting, make sure that you have proper drainage between the deck and the house. Also ensure that you are using the correct fasteners for the job. Nails are not enough. A safe deck features the use of heavy-duty, metal hardware throughout the entire structure, spanning from the house to the posts in the ground.



What’s changing with the 2021 IRC?

The pre-engineered design tables have been completely overhauled in a number of ways. 

  • Snow loading. The structural components in a deck can now be prescriptively sized for more than just a 40-psf live load, with the addition of 50-, 60-, and 70-psf snow loads in all of the design tables.
  • Tributary area. Table R507.3.1 has been expanded to provide a smaller, 5-square-foot area to size from, bringing the minimum diameter down to as small as 8 inches.
  • Post sizing. Table R507.4 was greatly expanded, allowing for variables such as snow load, species, and tributary area of deck supported. The common 4×4 post can now, under certain circumstances, extend as high as 14 feet.
  • Beam design. Table R507.5 was modified to include single-ply beams in all listed species, including redwood and cedar. 
  • Joist span. Table R507.6 was reorganized to better present the variables of joist span and joist cantilever. In addition, the “one-fourth-the-backspan” rule for joist cantilevers is easier to use because you only have to check the value in the table and not the value in the table and ¼ the backspan. More importantly, the change allows for more customized design. The previous table assumed the largest backspan; now you can use the actual back span of your deck.

Another major change to IRC guidelines is the distinction between guards and handrails. Guards prevent falling from one elevation to another, whereas handrails provide stability on stairs, etc. The minimum live load table, R301.5, now separates these features, primarily so the loading direction of guards can be independently evaluated. Until the 2021 IRC, guards have had to resist 500-lb. loads in all directions, which has resulted in some robust connection details that wouldn’t be necessary for an inward load. Under the 2021 IRC, guards will no longer be required to resist forces pulling inward or upward.

Deck, Deck, Choose.

Here are some more great resources: 

Think Wood: Hi Patricia. Tell us more about the Wood Utilization + Design Institute. What prompted the creation of this initiative at Clemson?

Patricia Layton: Timber is a big economic engine for South Carolina. The forestry industry generates more than $21 billion in revenue for the state economy.1 At Clemson, we recognized this overabundance of local timber, but have lacked adequate markets for wood construction. Sawmills have gone out of business and there are limited outlets to sell timber. We created the Institute to work with foresters, architects, engineers, constructors and building industry stakeholders to identify market share opportunities for wood products, and to increase the amount of wood going into non-residential buildings. Studies show that when there is an active forestry market, landowners more actively manage their land because they can see the economic potential.


TW: What sparked the Institute’s interest in mass timber in particular?

PL: We knew there was an opportunity for growth as we saw mass timber products being developed in Europe and shipped to America. Mass timber is extremely efficient in its build and design. During a build, you can accomplish so much in a very limited period of time. In 2012, we received a grant from the U.S. Department of Agriculture to help develop southern yellow pine CLT, a first-of-its kind product in the U.S. We collaborated with North Carolina State and the U.S. Forest Service to test the product. After receiving positive results, we used this to build our first mass timber building on the Clemson campus in 2019.


TW: Tell us more about this building — the Andy Quattlebaum Outdoor Education Center.

PL: The center was created as a destination to bring students together for outdoor recreation and socializing. It’s our first mass timber building, and was the second in the country to use southern yellow pine CLT. This facility provides a strong foundation for future innovation at Clemson and beyond. It changed the way we build, the way we work and the way we help our students enjoy nature. Everyone has been blown away by what the building looks like and how it feels. You just know that feeling when you walk into a mass timber building: it’s warm and inviting and, once you walk in, you never want to be anywhere else.


TW: The Center has become a showpiece for Clemson. What has prompted outsiders’ interest?

PL: After we invested our time and effort into mass timber research, we wanted to stand up and be a leader in this space. The center became a showplace and an opportunity for people to see the possibilities of wood construction. In fact, the first mass timber building in Mississippi, and a private residence in Florida were built because they toured and were inspired by the Quattlebaum facility. Other key universities also are stepping up and saying climate-smart construction is the right thing to do, and they are interested in our project for the same reasons. Arkansas has one of the nation’s largest wooden dorms; Oregon has a brand-new four-story mass timber building, and others are being built because they’ve come to see all of these buildings. For stakeholders, the interest in mass timber links back to their local forest products industry and maintaining healthy, sustainable forests.


TW: Given that the building sits on a University campus, does it also serve as an educational resource?

PL: The center is absolutely an integrated piece of our continued work and education. We are able to have a building that supports our students from a recreational standpoint, while also serving as a resource for mass timber research, including master’s theses on topics like vibration and moisture. We want our architecture students to understand the sawmill. We want them to go to the forest and learn about how the trees are grown. Sustainability is a critical facet of our education. It’s an opportunity to help our broad student body — not just the forestry and architecture students — appreciate this building type. We’ve done a tremendous number of in-person tours and we’ve had the project architect from Cooper Carry conduct webinars online. We’ve also used the building as a fundraising vehicle, and we’ve been able to help employers find and hire our students.

Andy Quattlebaum Outdoor Education Center
Photo credit: Jonathan Hillyer
We want our architecture students to understand the sawmill. We want them to go to the forest and learn about how the trees are grown.
Patricia Layton
Wood Utilization + Design Institute, Clemson University

TW: What challenges does the industry face in the adoption of mass timber construction?

PL: The biggest challenges that the industry faces relate to the supply chain. We have lots of wood, but we don’t have an oversupply of sawmill manufacturing, but with new sawmills coming online, this will help. We’re in the early days of an industrial wood building revolution where structural components are manufactured offsite, shipped, and put into place on-site. The use of mass timber speeds up the construction of a building, allows us to reduce waste, and even to deal with labor shortages in some cases. Getting architects and developers to understand that wood is an acceptable building material for buildings is critical.


TW: What’s next for you and the Institute?

PL: The Institute is focused on innovation, advocacy and partnerships. We are working with trade associations like WoodWorks and the American Wood Council to advance mass timber initiatives. This includes a number of grants supporting research for projects like CLT noise barriers and wind-resistance testing, as well as templated building designs for the Department of Defense, and disaster relief structures for the Forest Products Lab. We’re also in the design stage of a new mass timber forestry building at Clemson. Mass timber as a competitive construction material that will revolutionize how we think about contemporary development, and I’m honored to be one of its many advocates.

Learn more about Patricia's work on the Andy Quattlebaum Outdoor Education Center in this project profile.

Andy Quattlebaum Outdoor Education Center
Photo credit: Jonathan Hillyer
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