Embodied Energy: Minimizing Waste with Off-Site Construction
May 01, 2015
A version of this article appears in the May/June 2015 issue of Home Energy Magazine.
Matthew Wadlund is adding infill housing stock to the Oceanview neighborhood of Berkeley, California, where he is replacing a single-family house with four single-family, three-story detached houses. The wall panels in Matthew's Oceanview Orchard homes are constructed off-site and craned into place, resulting in maximum material efficiency and near-zero waste. Tom White, Home Energy's publisher, spoke with Matthew about his decision to prefabricate wall panels off-site.
Tom White: How did off-site prefab meet your cost, time, and project goals?
Matthew Wadlund: Possibly the most attractive aspect of off-site prefab is simultaneous construction. Traditional construction is linear; generally, one thing needs to be done before the next can start. With off-site construction, the process becomes parallel; concrete work happens at the same time as framing. Another important aspect is accuracy. With computer numerical control-type framing, the entire framing package starts with a shop drawing. Mistakes are discovered in the design studio, not in the field. Parallel construction and accuracy equal savings and on-site waste is reduced with prefab, which is smart.
TW: Did the wall panels include seismic shear walls?
MW: All walls were fabricated with plywood sheathing attached—it’s needed to keep the wall true and intact when lifting off the truck into place. Designated shear walls were constructed off-site as such. Shear walls that were wider than a truck was able to deliver came in sections with overlaps and were stitched together on-site.
TW: Did you consider modular prefabrication versus panelization?
MW: Yes, our research indicated that modular prefab tends to be more prescriptive: One can purchase only plan A, B, or C. On a tight urban site like ours, those modular plans have difficulty maximizing the site and fitting the constraints.
TW: Were panel deliveries made just in time for assembly on-site?
MW:Yes, panels were fabricated off-site three days before erection on-site. For our project size and location, it worked wonderfully to deliver all the first-floor walls in one 18-wheel semi load, then three days later, with the floors framed, all the second-floor walls, and so on.
TW: How did you seal gaps and air leaks in the building envelope?
MW: Think of an Ikea cabinet: All the materials arrive flat and labeled with assembly instructions. The plywood overlaps from panel to panel for continuity and shear. When assembly is complete, there are no gaps.
TW: Were the roof and floors fabricated off-site?
MW: Originally, that was the plan. During whole-house shop drawing review, the general contractor determined that for this floor structure—collector beams, Parallam Strand Lumber rims, and I-joists—on-site floor framing would be nearly as fast and efficient as prefab. The curved roof is the signature design of this project; it’s where we parted from prefab and dedicated the extra attention needed.
TW: Were bottom- and top-plate penetrations predrilled?
Learn more about the Oceanview project.
MW: In seismically active areas like ours, anchoring is critical. Stud locations are preset in prefab. That means anchor bolts and hold-downs, as well as in-slab plumbing and electrical, all have to be accurately preset in the foundation for the panel install to go smoothly. Walls are moved into place and checked, and penetrations are drilled on-site. Walls are then secured into place. Likewise with floor-to-floor anchoring.
TW: What barriers do you see to off-site prefab gaining more acceptance among residential architects?
MW: Prefab off-site only becomes economically viable when there are three or more of a nearly identical element, so single-family custom residential will be slow to adopt prefab. But with all the benefits, it’s surprising that more multifamily buildings are not presently framed off-site.
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