By Andrew Steingiser

A paradigm shift has jarred the conventional building design process in Massachusetts. In the last year and a half, design professionals have been adjusting since the Department of Environmental Resources created the updated Stretch Code and Specialized Opt-In Code. The new codes target the operational carbon emissions of the built environment and requires Passive House certification for all new multifamily residential buildings over 12,000 square feet. As a result, the early-phase, whole-building design and building science approach familiar to Passive House practitioners is now becoming the common language among the entire AEC community and building owners.

Leggat McCall Properties, a Boston-based development firm with an extensive portfolio, responded to the ongoing housing shortage and need for climate change mitigation by committing to Passive House certification for all 15 buildings at the Bunker Hill Housing development, prior to the code requiring it. The project’s mixed-rate housing, containing 2,699 units, is replacing lower density, aging public housing on the site.

RDH was engaged as the building enclosure consultant and Phius Certified Verifier as part of the design team with Stantec. The project proved to be unconventional in many ways.

Creating an Economy of Scale

Leggat McCall sought to create an economy of scale across the development by producing a kit of parts that could be mass customized for all 15 buildings, which vary from four to ten stories. Economy of scale means more housing for more people. To develop this kit of parts, the design team started with the first building in the development, Building M, as a test case.

Nick Nigro, Senior Project Manager at Leggat McCall, stated, “The goal was to drive productivity as far as possible through an intelligent, value-packed set of components that can be put together in an infinite array of configurations, sizes, buildings, et cetera.”

Developing a Kit of Parts

The design team went though many iterations of components for the kit of parts. Ultimately, the team landed on cross-laminated timber (CLT) as a floor system, a precast concrete core, and an exterior wall panel system. This approach was intended to minimize on-site labor to install and finish the prefabricated components of  each building. In Building M, the 7-ply CLT structural floor panels span 62 feet from exterior wall to exterior wall, without any intermediate columns or bearing walls. These floor panels bear upon a panelized, Passive House exterior wall system that is fully clad with pre-installed windows (see Figure 1). That’s a lot to ask of a building enclosure system.

We designed a rainscreen wall system on cold-formed metal framing with three inches of exterior mineral wool and a thermally broken clip-and-rail system supporting the cladding. Various cladding types such as thin brick, metal panel, and fiber cement board were applied to the base backup wall system. Panels were “wet sealed” with two lines of silicone sealant. Maintaining consistent sealant joint dimensions between panels was key to ensuring continuity in the air and water control layers. Pre-installed cladding added challenges to accessing joints that were out of tolerance.

The Wall Panel Design Evolves

The initial panel was designed to be site clad, but the team ultimately decided to use factory-installed cladding to reduce on-site labor costs. The ideal design process for a project like this would involve an early design assist from the fabricator. On larger commercial buildings, curtainwall fabricators create 3-D digital twin models to facilitate coordination with work by other trades. This process and skillset are not typically employed by trades that address the scale and type of Building M.

Given the combination of novel project goals, an evolving panel design, and the fixed project timeline, Leggat McCall deployed a second team of RDH’s façade engineering group, affectionately dubbed “RDH 2,” led by a team with prefabrication experience. The RDH 2 team worked closely with the fabricator to provide guidance and assist with performance mock-up (PMU) testing, fabrication, QA/QC, and installation. This team aimed to ensure the installed panels met the project’s performance goals.

Performance Mockup Testing Saves the Day

As enclosure consultants, RDH’s main concern is durability. One of the most difficult details in a panelized system is the typical panel joint, especially four-way panel joints. The four-way intersection of air and water control layers needs to be carefully detailed in 3-D.

Many enclosure details seem straightforward enough on paper, or even in the 3-D environment of a digital twin. Reality can be difficult. Therefore, RDH cannot over-emphasize the importance of PMU testing. Performance mock-up testing proved critical for the Building M project.

Several air and water tests were performed on the PMU, including a partial “whole-building” airtightness test of the façade sample with a sealed back chamber. This testing informed the detailing prior to fabrication of the project panels.

Team Coordination Encourages Troubleshooting

Informed by the PMU testing, the RDH 2 team developed a graphically oriented shop manual for fabrication. Using this manual, the fabricator, contractor, owner, and design teams pressed on together to fabricate the panels. On February 2, 2024, the first panel of the first building was flown over Charlestown.

Over the next 10 months, the wall panels and CLT floors were fabricated and installed. The kit of parts took shape on-site, achieving the desired on-site efficiency. The design, construction, fabrication, and owner teams remained engaged throughout the process. Collaborative troubleshooting of the enclosure system occurred on-site through many field observations, field reports, and ongoing owner/architect/contractor meetings. Coordination meetings with the design team and all trades installing the control layers were conducted on an ongoing basis. Ongoing coordination is a key step not only for a Passive House building, but especially for a panelized building.

Planning for Moisture Management

 A panelized wall system complements mass timber buildings because the building can be enclosed rapidly, a whole floor at a time, which limits potential moisture exposure time during construction. Moisture management planning is a critical part of mass timber building construction, and RDH provides consulting services in this area. We outlined our process in the guide Moisture Risk Management Strategies for Mass Timber Buildings (Version 2), with an updated Version 3 to be published soon.

In conjunction with the specific building enclosure design, the team developed a moisture management plan for Building M, which could also be applied across the other buildings in the development. The rapid enclosure of walls left most of the bulk water management on the floors above to the contractor. This work required active water management after weather events, with moisture monitoring performed on the CLT throughout the construction phase.

Success with Airtightness Testing

RDH’s Phius Certified Verifier engaged with the project during the design phases and coordinated with the contractor to develop the testing plan. By August 20, 2024, Building M was airtight and the first mid-construction whole-building airtightness test was performed. It passed the first time. Then it passed a second interim test. It also passed the final test at 0.05 CFM50/ft² (Phius limit 0.06 CFM50/ft²).

This success was a testament to project collaboration among all parties involved. From design through construction, the project had a high degree of continuity among all of the key personnel.

Early design phase planning can also optimize the project delivery. The Bunker Hill developer intends to extend this process beyond Bunker Hill to many more projects. According to Adelaide Grady, Senior Vice President and Partner at Leggat McCall Properties:

We want to use this kit of parts for any multifamily building anywhere. It’s not going to be perfectly customized for a site, but it can be mostly customized for a site in a rational way. We’re trying to drive towards a way of achieving productivity and efficiency so we’re getting to lower or stabilized construction costs without making these things lower quality, and without hurting the labor side of things. It makes the labor easier and less expensive so we can produce a whole lot more housing and put more people to work.

This systematic approach to building component prefabrication and installation can serve as a means of further cost optimization for any development seeking an economy of scale. As we continue to face escalating construction costs and housing shortages nationally, we can apply the lessons learned from early phase coordination work on this project to optimize both cost and operational carbon emissions.