Claire Halpin and Sarah Kramer
The Collected Papers of the Continental Cordwood Conference and Cordwood Building (second edition)
Why Wood Masonry?
The Arcus Center’s design and construction were directly informed by the history and process of building with wood masonry. From the building’s overall form to the construction sequence to the wall composition, the design sought to expand the potential of this traditional “hand” method by merging it with newer technologies. Using this technique at an institutional scale presented interesting logistical and technical challenges that allowed us to explore new and different ways to incorporate the benefits of this method.
Remaining true to the communal spirit of this heritage technique, the entire process of design and construction—including the decision to use wood masonry, the procurement of the materials, and the specifications of construction and detailing—was approached as a collaboration among the client, Kalamazoo College; consultants, and team of architects.
The building’s form was shaped by its use, context, and material. The purpose of the Arcus Center is to bring together students, faculty, visiting scholars, social justice leaders, and members of the public to engage in conversation and activities aimed at creating a more just world. It provides a safe, welcoming space in which to have the sometimes difficult discussions surrounding issues of injustice, to share personal stories, and to organize action.
Located at the apex of the Kalamazoo College campus, on a hilltop site that incorporates a major incline, the Arcus Center engages each of its distinct contexts—the campus, a residential community, and an old-growth grove—through transparent glass facades. The arcing masonry walls simultaneously embrace three distinct landscape conditions.
The wings of the plan intersect at an informal meeting space: a crossroads for convening. The presence of a hearth and kitchen for sharing food at the center of the building creates the potential for frequent informal meetings and casual, chance encounters. Smaller meeting rooms and individual workspaces are nestled into the area bordering the main assembly spaces.
The building’s visually open, day-lit interior, made possible by the large glass facades and clerestory, is designed to support conversation and community by allowing for different configurations and ways of gathering that begin to break down psychological and cultural barriers between people and facilitate understanding.
While we envisioned using glass for its transparency, we also wanted the design to resonate with the vernacular of the local architecture and relate to the Georgian brick language of the campus. Yet, while Kalamazoo College has a strong history of social justice leadership that dates back to its founding, much of its campus is comprised of buildings whose style evokes colonial- and plantation-era attitudes.
Looking to identify more socially conscious and environmentally sustainable alternatives, we explored local building traditions. Cordwood masonry appealed to us as a sustainable, more democratic, socially and environmentally friendly method of construction. As a collaborative building process that allows people with a wide range of abilities and strengths to participate in construction, cordwood masonry embodies the values the Arcus Center was founded on.
Its aesthetic also appealed to us: the individuality of each log, with its unique shape, size, color, and growth pattern, could be seen as reflecting the diverse population the Center serves. We were pleased to discover that wood masonry sequesters more carbon than is released in building it, responding to today’s need to reduce carbon pollution—one of many environmental issues embraced by social justice movements. Further solidifying our decision to use this method, we discovered cordwood structures in Michigan dating back more than a hundred years, and we learned that Kalamazoo is located within the growth range of Northern White Cedar, one of the best species of wood for this use.
Learning from the Experts
Once we resolved to use wood for the facade and began seriously looking into the technique of cordwood masonry, we sought out the experts. To learn more about the process and how we might go about scaling it up for commercial construction, we turned to Rob and Jaki Roy of the Earthwood Building School. Rob and Jaki and the collective knowledge of the continental cordwood community have been essential to the design, execution, and successful application of this technique within a commercial setting.
With the broad base of knowledge that the Roys offered, we could turn our focus to the design challenge of combining this traditional, hand-built method with commercial construction technologies, codes, and regulations. We were also challenged by some initial skepticism from the client, as well as contractors and engineers who were unfamiliar with alternative building methods. Here, too, the Roys were instrumental in securing confidence in the project, hosting a workshop in northern Michigan for a group of Studio Gang architects, as well as representatives from the client, contractor, and Kalamazoo community. Imparting their wisdom and good will, the Roys instilled in the entire team the confidence and enthusiasm necessary to forge ahead. They also introduced us to technological advances innovated within the cordwood community. With this sturdy foundation, we sought to build on these innovations and explore opportunities to use digital tools to play with the surface geometry.
The Arcus Center’s cantilevered wing navigates the incline of the site and engages the campus context.
Playing by the Rules
While the social justice mission and site topography led to the tri-axial organization of the building, we further defined the scheme by establishing rules for the integration of the hand-crafted wood masonry and contemporary steel construction.
The first of these rules relates to the way the building engages each of its three contexts with a transparent facade. The arcing walls that connect these facades, and simultaneously embrace three distinct landscape conditions, are hand-built with wood masonry. The masonry walls are set atop a continuous steel sill, rather than a wood frame, and terminated at each end by a clean steel edge. The steel structure allows for long, unbroken areas of wood masonry walls. Each wall incorporates an opening, for entry or light, and is treated in a specific way to manipulate its surface geometry. Typically, to create an opening, the geometry of each arcing wall is sliced and then “pushed” or “pulled” as is appropriate to shape the opening. The concave curvature of the walls compresses the masonry, which we learned was an added benefit when it comes to preventing cracking of the mortar.
Each wing of the building and each arced wall is similar but modified to its particular use and context. One wing meets the site at grade, a second descends along with the grade toward the grove adjacent to the site, and a third cantilevers out toward the college campus. Due to the site’s natural change in elevation (approximately 12’) and the emphasis on universal accessibility, this cantilevered end of the building projects out above the ground level, allowing the floor inside the building to remain level and for circulation to occur underneath.
The cantilever had several interesting aspects to its construction. Thornton Tomasetti, the structural engineer of record on the project, recommended that the wood masonry begin at the farthest end of the cantilever, symmetrically on each side, in order to load its most extreme end first and equally. This would ensure that any deflection—while unlikely—would be greatest at this initial loading and lessen as the masonry work progressed back, preventing cracking. The same principle was also applied to the masonry sequencing at lintels above openings: masonry was begun at the center of the lintel and then outward toward either end.
Thermal and Structural Constraints
Throughout the project, we researched and considered numerous types of wood masonry wall construction: through-wall masonry with a sawdust insulation cavity (as taught at Earthwood Building School workshops), the double-wall technique that has been described in previous papers and in "Cordwood Building: the State of the Art," and others. We studied the length of the logs, the composition of the mortar, created mockups, modeled different combinations, and tested wall assemblies for their thermal performance in the energy model our engineering team developed.
Continuing in the vein of the traditional cordwood method, we used locally sourced white cedar. Wood was purchased more than a year before construction began. It was sorted, cut to length, and allowed to dry under protected racks. Prior to the masonry work, it was then re-distributed into bins with a representative “random” mix and brought to the job site.
The possibility of using through-wall masonry exposed on the interior was given much consideration during the design process but ultimately was not used for several reasons, including the following: the current energy code, which requires a minimum continuous layer of insulation that would not have been achievable with sawdust insulation to the accepted standards of the governing code authority. Also, the design of the mechanical system depended on an airtight building enclosure for positive pressurization, which would have been challenging to provide with through-wall masonry due to the potential of air gaps at the log surfaces.
Structurally, the cantilevers and facade geometries could not have been achieved with through-wall masonry alone, and to satisfy the structural code requirements for a commercial building, a steel structure and metal stud wall were used in concert with the masonry.
Daylighting requirements also contributed to the decision not to use through-wall masonry, as light-colored walls were necessary to reflect ambient daylight back into the spaces. The final design is what we determined was the most appropriate combination for this project. It is in some ways based on a traditional masonry cavity wall that is common in other types of construction, except instead of stone or brick, white cedar logs are used.
The typical wall section consists of 11" wood masonry on the exterior, a 1 1⁄2" cavity, a continuous 1 1⁄2" layer of rigid foam insulation with a layer that also serves as a vapor barrier, a stud wall with spray foam insulation that also serves as an air barrier to the system, and then the interior finish (which varies throughout the building). This exterior wall assembly provides a total insulation value of approximately R-30. The wood masonry wall is continuous along each façade, broken only when the wall is terminated by the steel frames at each end. This is made possible by the masonry ties that connect the wood masonry intermittently to the backup stud wall.
Behind the masonry, the 1 1⁄2" air space allows any moisture penetrating the wood layer to drain and weep out of the system. Behind the airspace, the insulation, made up of 4 x 8" sheets of rigid foam, creates a continuous unbroken insulation layer around the building. This satisfies one of the energy code requirements that stipulates a minimum level of continuous insulation (in traditional stud walls, the effectiveness of insulation placed only between studs is reduced by the thermal bridging that occurs at stud locations). The insulation/sheathing layer is a product that includes a reflective film layer on its face that also acts as the weatherproof and vapor barrier in this system: contrary to traditional wall assemblies in cold-weather areas where vapor barriers are often located on the warm/interior side of the insulation, the dew-point analysis for this air-tight system places the vapor barrier on the sheathing layer.
The sheathing is attached to the stud wall that makes up the exterior wall of the building. Spray foam insulation in between the studs adds more insulation and forms a sealed air barrier that makes the entire wall assembly extremely energy efficient. The wall composition also aided sequencing during construction: by first erecting the steel frame, followed by the stud wall, and then insulated sheathing, interior work was able to occur while the masonry work progressed at its own pace on the outside. This is based on strategies we learned from other cordwood builders who, as common practice, construct the frame of the building and roof first, to allow the methodical masonry work to continue under its shelter.
Pushing the Limits with Digital Tools
As described above, the continuous masonry walls were an important part of the design. However, in some locations inside the building there was a need for natural light, beyond what was provided by the glass facades and clerestory. In order to allow light and views through the wall without introducing new “slices” or geometries, porthole windows were integrated into the wall.
The design for these windows was directly inspired by the bottle-end techniques frequently used in cordwood construction; however, due to the energy requirements of the building, it was important in this case that the windows be thermally broken and contain insulated glass, which would not have been the case with bottle ends. Also, with a total wall thickness of nearly 2', it would have been difficult to source bottles of an adequate length for the wall assembly.
There are a few exceptions to the typical 11" composition of the wall described above: these include the three “frame” ends of the building where through-wall masonry was used. These locations are exterior-to-exterior and did not have the same thermal and structural requirements as the rest of the building. We took advantage of these locations to use the logs all the way through the wall, expressing the materiality of the logs as much as possible, with thicknesses varying from 2' to approximately 6'. The logs were laid perpendicular to the tangent of the curve on the exterior wall. At the interior face of the through-wall masonry, at the frames, the log ends were cut at an angle, with the resulting wall appearing elongated or elliptical in elevation.
Forty-six windows were prefabricated in three different sizes (8", 11", and 14" diameter) and integrated into the facade. Parameters were given for their heights and general locations, but otherwise we allowed for their flexible placement so that the window installers and masons could collaborate during construction.
One of the most unique “slices” in the building’s wood masonry facade is the warped wall. The geometry of this wall—concave on one side, convex on the other— forms an eye-shaped window opening perpendicular to the wall. To create this slice, we modeled the wall in 3-D, then sliced the geometry at even intervals to create 2-D shapes that served as the basis for custom metal studs. The subcontractor used a digital file of the 2-D shapes to cut the stud profiles. Each stud was slightly different, with the curved profile straightening incrementally.
Placed precisely along the wall, the studs created the armature for the “warped” shape. With the addition of the sheathing insulation layer, it became a smooth surface upon which the wood masonry wall was then built. The logs were likewise cut at an angle to form the smooth surface of the wall.
Mixing It Up
The size mix that was developed for the masonry was based on measuring from an existing Michigan cordwood masonry wall with an appropriately even distribution of log sizes. The sizes were intended to be distributed randomly throughout the wall—a process that was streamlined during construction by preparing bins of logs with an even distribution of diameters, so the masons had the correct “random” mix readily available to them.
In addition to size mix specifications, a set of “rules” was outlined in the drawings, stipulating that the logs were to be randomly and evenly distributed by size, with a maximum gap at tangent points of the logs of 1”; that primary checks must be within 45 degrees of bottom center; and that only full logs were to be used (no splits).
Beyond these stipulations, the masons working on the project placed the logs according to their judgment and collaborated with carpenters to cut custom logs at an angle where needed at through-wall locations and warped wall, as well as with other trades such as window installers to integrate the porthole windows into the mix.
Size Mix Specifications
Wood sizes shall vary from the smallest available diameter (approximately 2") to the largest size that is practical to cut using the methods discussed and approved by owner, contractor, and architect (approximately 12" diameter). The quantities of each diameter shall be proportioned as listed below to allow for a random distribution within the masonry construction:
Where X = diameter of log: 2" ≤ X <4": 20‐30% 4" ≤ X <6": 25‐35% 6" ≤ X <8": 25‐35% 8" ≤ X <10": 10‐15% 10" ≤ X <12": 2‐4%
Not the End
Although we were presented with numerous challenges in scaling this method for a commercial building, we believe we achieved numerous successes and learned many lessons that can be passed along to the cordwood community and to the architecture and construction professions at large. Perhaps these lessons will inflect future cordwood innovations.
For us, the Arcus Center represents a new hybrid typology blending a social, educational, cultural, and civic program-type. We are proud that it also represents a new hybrid method of construction, blending this traditional hand method with commercial technology. We hope that it can inspire further use of this energy-saving, renewable, collaborative method in large-scale construction projects.
A Final Note
While the wood masonry tradition has been advanced, for the most part, by those searching for sensible, cost-effective, and energy- efficient building techniques for their homes, we suggest that the environmental and cost benefits of this technology could be leveraged at a larger scale in future public and institutional buildings like the Arcus Center. How can this be accomplished?
To promote the increased use of wood masonry for mechanically conditioned buildings, the techniques developed and lessons learned at the Arcus Center could be formalized into standard details, material and installation specifications, and assembly diagrams for reference and adaptation.
In addition, the method used for calculating carbon sequestration and assessing the environmental benefits of this material versus alternatives could result in its recognition by regulatory agencies and environmental responsibility advocates like the United States Green Building Council (USGBC), resulting in further benefit to building owners using wood masonry.