Mass Timber Buildings

PMX 15, Part 2: How to manufacture high-design, low-carbon timber buildings

In the second installment of our PMX 15 series, we describe how we preserved production efficiency and sustainability without sacrificing a building’s beauty.

By Mark Bauernhuber, Lily Huang, and Kristin Slavin

A rendering of a PMX 15 kitchen design

A rendering of a potential interior kitchen design for the PMX 15 mass timber model. (Sidewalk Labs)

This post was co-written by Mark Bauernhuber and Kristin Slavin. It’s the second of two posts exploring our PMX 15 design. We recommend starting with the series introduction here.

In the first PMX 15 exploration, we talked about overcoming the challenges of designing timber buildings in a seismic region. But for factory-made mass timber buildings to have a transformative impact on cities, they must be able to tackle place-specific code challenges without undermining the goals of carbon footprint, attractive design, and production efficiency.

Here’s how our team worked to retain that balance for PMX 15.

A more efficient “kit of parts”

As mentioned in the series introduction, we took PMX 15 to the next level of modeling, designing out the building completely, such that it’s ready for regulatory approvals. That extra level of thinking taught us a number of important things about our kit of parts, including the precise details of every part (things like exact size, fire rating, acoustics) and how the part gets built (things like where the screws go and its factory production line).

A quick refresher on our kit of parts: it’s the basic set of building components that can be made in a factory and assembled quickly at a construction site. For PMX, our kit includes three core components made in our factory (floor cassettes, structural beams and columns, and facade panels) paired with other “fit-out” items procured through partners (kitchen pods, bathroom pods, and elevators). Think of it like the set of basic LEGO pieces that, when put together in different ways, can generate countless imaginative structural designs.

For PMX 15, we increased the efficiency of the kit of parts in some key ways. The biggest involved the design advances for our floor cassette, which is the primary component that divides floors in a building. For PMX 35, we had more than 350 unique floor cassette types throughout the building design. But for PMX 15, we reduced that number of unique cassette types down to just six.

To do that, we actually changed the building layout itself. In a typical building, the vertical risers (things like plumbing and electrical ducts) are often housed in residential units, cutting through floor panels in oddly shaped ways. We moved the risers into corridors so they no longer penetrate unit floors, enabling us to keep the cassettes standard. That left us with just three types of cassettes for floor geometry (left, right, center) and three more for the roof panels — efficiency gains that lead to dramatic savings around project speed and cost.

Additionally, the PMX 15 team made sure the “fit out” parts of our kit were also designed for efficient shipping, including kitchen and bathroom pods manufactured off-site by partner suppliers. A big challenge with modular kitchens is they often come as boxes, which means you’re shipping the empty air in the middle of the box. PMX 15 kitchens were designed with linear modular sections, each two-feet wide, that could be combined to create a variety of different setups, including L- and U-shaped kitchens.

This modularity meant we could pack numerous kitchen units onto a single truck, with minimal empty space. Combined with the ability to “flat-pack” timber beams, columns, and wall panels inside a standard truck, we estimate that PMX 15 would require 85 percent fewer site deliveries than a typical construction approach for a comparable building. That not only makes for faster completion, it also reduces shipping emissions, which brings us to sustainability.

Far less energy use and embodied carbon

The efficiency of PMX 15’s kit of parts creates some important sustainability gains when it comes to “operational” carbon, which is the carbon emitted during the everyday use of a building through things like heating and cooling systems and lights. We estimate an 80 percent reduction in energy use for PMX 15 relative to the industry standards set by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

The key to these gains is the ability to manufacture thermally efficient (meaning air- and water-tight) facades that meet high-performance “Passive House” building standards. That’s a lot of technical terms, but it basically just means the building exterior doesn’t have any holes in it. When it comes to energy, a building is like a boat: if there’s a hole in it, you’re in trouble. Tiny gaps and construction imperfections in a building’s facade can cause a tremendous amount of energy loss. As hot or cold air escapes through these holes, air-conditioning or heating systems must work even harder to keep interior spaces comfortable.

By eliminating holes, thermally efficient facades unlock two key improvements:

  • More consistent interior temperatures. By preventing air from escaping or entering, a room in a building with thermally efficient facades retains a more consistent temperature — even when it’s very hot or very cold outside. That means a heating or cooling system doesn’t have to work as hard to keep people comfortable. (Of course, on a nice day, occupants can still open a window to enjoy the fresh air.)
  • More energy-efficient heating and cooling systems. Temperature consistency frees you from relying on energy-intensive AC units and fans that work harder to maintain a comfortable interior. Instead, you can use what’s called a “radiant” system that extends across an entire ceiling or floor. Radiant heating (and cooling) works by directly heating (or cooling) the surfaces in a space rather than just the air, a more efficient use of energy.

Thermally efficient facades alone account for roughly 20 percent of PMX’s energy reduction, but they lead to even greater energy savings by enabling a switch to radiant heating and cooling. Of course, those gains require that airtight facades are truly airtight. That’s where manufacturing can help. By creating the entire facade in a controlled factory setting, you can eliminate the air-loss risks that come with on-site construction, when a single nail in the wrong place can throw off an entire temperature balance.

A factory also enables you to ensure a facade meets air and water performance standards prior to shipping and assembly. In a typical construction approach, air- and water-tightness are tested once a building is complete, and if you discover a breach, there’s very little you can do to correct it short of pulling apart the facade — a very costly process. In a factory approach, we can test the facade hundreds of times before it arrives on site and have third parties certify the results, ensuring it will work as designed.

Operational carbon is a big part of a building’s carbon footprint, but it’s also essential to understand “embodied” carbon, which refers to the emissions created during the manufacturing and construction of a building itself. Embodied carbon is a major climate challenge; building construction accounts for roughly 20 percent of all global emissions, according to the Carbon Leadership Forum.

For PMX 15, the embodied carbon impact was nearly 34 percent less than a comparable concrete building, given the vastly greater amounts of energy needed to create those materials than to create timber. That’s the equivalent of removing 864 cars from the road for an entire year.

The embodied carbon savings get even more striking if you factor in “sequestration.” Sequestration refers to the carbon that trees naturally pull from the air, which remains trapped inside them for their entire lifetime. When you do that for PMX 15, the carbon footprint of its structure almost disappears:

As part of our digital suite, we’re developing a “lifecycle emissions” tool to map out the carbon impact of each individual component in our kit of parts, enabling a super-precise calculation for both the operational and embodied carbon impact of a building. More on that work in a future post.

Preserving high-quality design

Efficiency and sustainability are core keys for the PMX team, but they need not come at the expense of great building design. The long history of manufactured buildings often raises fears of becoming cookie-cutter, but our design team set out to show that the PMX system can accommodate a wide range of design flexibility. What we found is that our kit of parts enables the same floor plan flexibility as a traditionally constructed building does, and in many ways provides even greater flexibility.

One important decision to enable design flexibility was the creation of the “rainscreen” facade as a basic component in the PMX 15 kit of parts. A traditional facade system includes precast panels that are locked into shape from front to back: to change the finish you need to change the entire panel system. For a rainscreen facade, the engineering in the connections between panels and core of the panel (such as the weather resistive barriers) is fixed and can’t be changed. But the exterior layers (including the cladding materials and window size and shape) can be adjusted by designers without impacting performance — just like swapping out a smartphone cover doesn’t impact your camera or touchscreen.

As a result, we’ve found that the kit of parts can now recreate the majority of all mid-rise buildings in North America.

There are still certain types of buildings that wouldn’t be economical to create with our kit, such as the bespoke curvature that often defines a Frank Gehry building. But that approach is intentional, enabling a balance of architectural expression and construction efficiency. Using the PMX kit of parts, developers can explore a wide range of designs — or concentrate unique shapes in high-impact areas of a building — while preserving the gains in project speed, cost, and reliability that can have a significant social benefit for a whole city, and for the planet.

If you’re a developer or timber professional who wants to help us advance mass timber building design or offsite manufacturing, send us a note through our website or email factory@sidewalklabs.com.

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