A Firm Specializing in Public Work Takes on a Private Prefab Home to Test Methods

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Single-family homes are not typically on the docket for Workshop Architecture. Rather, the Toronto-based firm specializes in public work and social housing with an emphasis on equity and sustainability. But this private home presented an opportunity to test out prefabrication construction methods that could later be applied to larger scale public work.

Dubbed the Unfinished House for its restrained approach to aesthetics, material reduction, and budget, the home uses Passive House-standard prefabricated wall and roof panels. While the 1,400-square-foot pilot project leveraged off-site construction methods, the architects stress that it was custom designed for the site. Here, the studio explains why they made an exception for the Unfinished House and what’s next for the firm. 

Beyond the traditional benefits, why was prefab the preferred delivery method for the Unfinished House? 

This house is custom designed for its site. We used panelized prefabrication. With this method, instead of being framed on-site, the house comes flat-packed on a truck, a jigsaw puzzle of 8- to 16-foot-wide wall and roof pieces that have the structure, insulation and vapor barrier, sheathing, windows, and exterior doors all preassembled.

We were interested in collaborating with the manufacturer during the design stage to work out everything in more detail than you usually can on a building site. Simple Life Homes, the prefab panel manufacturer, uses a 3-D BIM (Building Information Model) and we also work this way. During the design stage we went back and forth between architect and manufacturer with the 3-D model.

All the framing elements were modeled, which allowed us to consider both their placement aesthetically and also how to achieve air barrier continuity at panel joints (where panels meet) in three dimensions. Plumbing and electrical services were grouped into areas to be concealed, complete with in-slab conduit stubs, to avoid passing through areas of visual-grade exposed framing.

You describe the project as a "pilot for panelized prefabricated construction." Tell us more about what that means.

Through this project we wanted to test the ability for the prefab manufacturer to apply their product and systems to a home with more geometric complexity than the homes they were typically producing. We were also testing the ability to achieve a bespoke aesthetic vision rather than typical builder-grade construction of drywall, finishes, and trims to hide the gaps in joins.

We used this small private house as a pilot to see how the process of developing a design in discussion with the prefab manufacturer could unfold in order for us to then be able to recommend it to our public sector clients. With a private client there is more flexibility in how to procure contractor services. After having completed the house project, we were in a better position to know what may work (or not) with the more stringent construction tender processes typically used for public work.

Can you provide some context around pricing, if that’s a value add for your clients?

Cost of the prefabricated envelope (walls and roof) is comparable to traditional construction, maybe cheaper, but cost was not the driving factor. Only the envelope of the house was prefabricated, all the site servicing, plumbing, electrical, exterior cladding, and finishing (albeit limited in this project to some tile work and cabinets) was completed on-site and would be same cost whether the envelope was built on site or not. 

What qualities make the prefab design of the Unfinished House stand apart from others?

The name "Unfinished House" refers to an aesthetic attitude, an approach to material reduction, and budget restraint that leaves parts of the design incomplete. Self-finishing materials were selected to reduce resources and reveal the building’s construction. Tile and plywood are the only added "finishes." They cover areas with wiring and plumbing.

As the insulation in a high-efficiency envelope, like the prefab panels used in this house, is outboard (on the exterior side of the vapor barrier, which in this house is the plywood sheathing), we saw an opportunity for showing the wood structure on the interior. In turn, we could eliminate the need for an extra layer of finishes, like another sheet of plywood or drywall—the latter we didn’t want for aesthetic reasons, because of issues with indoor air quality, and because it constitutes about 15 percent of all construction debris and leaches toxic chemicals from landfills into groundwater. For us, the project was a test to first understand the constraints of off-site fabrication, and then to push these limits. For example, by using visual-grade wood for the framing and sheathing (a first for the panel manufacturer) we sought to expand the already impressive waste reduction and material economy potential presented by the prefabricated process.

Do you have any other prefab projects underway?

After testing a high-performance envelope prefabricated panel system in the Unfinished House, we are now using them to overclad an existing 16-unit seniors residence for the Toronto Community Housing Corporation to meet EnerPHit, which is the Passive House standard for renovations.

By using a panel system that goes overtop the existing walls and roof, residents are able to stay in place during construction, which is currently underway. On-site construction time, dust, and noise will be minimized, and with the thick, continuous insulation and air barrier tightness the building’s energy use will be reduced by 90 percent. We perform thermographic scans of heat loss and blower door tests before and after to compare performance and to see that we are achieving real energy savings and thermal comfort results.

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