The Boston design-build firm Placetailor is unusual in a number of ways. For one thing, the architects who run the company don't spend all day at the drawing board—they also run the jobs and get their hands dirty on site doing the physical work of construction. Placetailor has spent the last five years developing small infill sites in the Boston neighborhoods of Roxbury and Jamaica Plain. Each small site poses its own challenge in terms of location, context, and site conditions, and as the company's name implies, the designers and builders tailor their solutions to the site.
JLC spent several days on site in the past few weeks with Placetailor, following the action as the company constructed an insulated stemwall and slab foundation for a three-family apartment building on a busy Jamaica Plain street (see slideshow).
The plan, created by another architect, calls for all the building's loads to be carried on the exterior walls. The existing surface soils on the site, which consisted mostly of soft soil and fill, wouldn't readily support a load-bearing insulated slab; and in any case, the wall system includes some point loads that would be complicated to support with a slab. So the design calls for footings placed well below existing grade in good bearing soil. A foundation stemwall rests on that footing to support the bearing walls of the frame, and a non-load-bearing insulated slab sits within the stemwall (and will also be polished to serve as the finished floor for the first-floor dwelling unit).
The original plans called for a conventionally-formed concrete stemwall, with rigid foam insulation to be installed on the outside after forms were stripped and before the wall was backfilled. But concrete contractors are scarce in the busy Boston economy (especially inside the city), and prices for small jobs can be steep. After reviewing a few bids, Placetailor decided to revise the plan, and to form and pour the stemwalls using insulated concrete forms (ICFs). While the insulated forms added cost, the decision meant that Placetailor could form and pour the foundation with their own labor and on their own schedule. And the ICF system also raised the R value of the foundation system. This meant that the designers could reduce the required insulation underneath the interior slab, while still preserving the same energy performance, thus recouping a significant portion of the up-charge for materials incurred by the use of the ICF system.
When Placetailor started to work on the stemwalls, one advantage to their chosen strategy became evident: It's far easier to carry lightweight foam forms down into a hole in the ground on a tight site and set them up than it is to accomplish the same task using heavy plywood forms. And as project manager, Steve Daly, pointed out, "The really nice thing is we don't have to haul them back out again or clean them or store them."
During the slab pour, Placetailor demonstrated another interesting method: The crew constructed a 26-foot-long concrete screed out of wood I-joists, and used it to strike and level the entire floor slab dead flat. Pieces of angle iron, screwed to wood members that were fastened to the plastic ribs in the ICF forms, served as guides for the long screed. It wasn't easy—the crew members tasked with running the long screed were sweating on an 85-degree morning—but it was fast, accurate, and simple.
Delays in the arrival of the second and third loads of concrete required for the floor slab meant that bull-floating and power-troweling of the first section of slab had started while the third section of slab was still not placed. While most of the crew stood idle for a short time while waiting for the next truck, it turned out to be a good thing that there was plenty of labor on hand during the periods when multiple stages of placement and finish had to be carried out at once.