Philadelphia's largest post-pandemic office-to-residential conversion just wrapped, and the fenestration package tells the real story. Here's how a 17-story 1950s tower absorbed 1,560 new architectural windows without losing its limestone bones—and what specifiers should take from it.
The Largest Post-Pandemic Conversion in Philly Just Delivered Its Window Package
Office-to-residential conversions have been the industry's favorite talking point for three years. Actual completed projects—with real fenestration schedules and real facade decisions—have been rarer. That's changing. 17 Market West, a 17-story tower at 1701 West Market Street in Philadelphia, has now been documented as an adaptive reuse case study, and the window package is instructive for anyone bidding conversion work in 2026 and 2027.
The project used 1,560 large, energy-efficient Crystal architectural-rated windows across a building that now houses 297 apartments over ground-floor retail and four floors of enclosed parking. The floor plates deliver 305,000 square feet of one- and two-bedroom units, sitting on 13 residential floors plus a penthouse level.
Why This Building Actually Worked as a Conversion
Most 1950s and 1960s office towers are conversion nightmares. Deep floor plates leave interior space too far from any window to be habitable, forcing expensive core-out demolition to punch light wells. 17 Market West is the exception: originally built in 1957, its smaller floor plate made the conversion feasible without gutting the middle of the building. That geometry is why it made it to market first among several permitted Philadelphia conversions—it was, as the developer put it, positioned perfectly.
The design team, working within the existing structure, cut limestone spandrels at the floor lines to introduce taller windows and add vertical expression to what had been a horizontal 1950s elevation. Portions of the original Indiana limestone were preserved to honor the mid-century heritage while a sleek metal channel system was added to draw the eye upward to a rooftop sky frame.
That's the practical challenge conversion architects keep running into: you have to make a postwar office building look like something people want to sleep in, without erasing whatever gave it character or triggering a historic review fight. Cutting spandrels to enlarge openings—rather than reframing entire bays—is emerging as the pragmatic middle path.
The Spec: What Went Into the 1,560 Openings
The window package used a repetitive symmetrical pattern throughout the tower. Each facade features four combination windows in a horizontal bay spanning a 21-foot-wide by 7.5-foot-tall opening between vertical columns. The east and west facades run 11 bays per floor; the north and south facades run four bays per floor.
Two Crystal models did the work:
- Series 8520 Project-Out Awning
- Series 8510 Fixed/Picture
All sashes were arranged in 7.5-foot-tall vertical combinations using common master frames and impost mullions, in either fixed-over-fixed or awning-over-fixed stacks. The top windows carried two horizontal exterior-applied grids, so that grids and horizontal mullions divide each tall combination into four equal portions—a detail that reads as mid-century from the street.
On the glazing side: 1-inch insulating glass units, fitted using a special glazing bead into optional 1-¼-inch glazing pocket frames. The larger pocket was required to accommodate the applied exterior ⅞-inch horizontal grids—a small but critical dimensional call-out for anyone specifying grid-detailed replacement windows in a historic-adjacent envelope. Vision panels used Vitro Solarban 70 low-E coated glass, with OpaciCoat 300 medium gray applied to spandrel units. Frames, sills, and visible accessories carried a custom Mystic Bronze powder coat finish tested to AAMA 2604.
What Spec Writers and Contractors Should Take From This
A few things are worth pulling out of this project for the next conversion RFP that lands on your desk:
- Combination units, not one-offs. Crystal used three-piece mullions to group combination units horizontally, plus subsills, receptors, and strap-anchor accessories. That's the pattern: preassembled combinations reduce installation labor on a 1,560-opening job where site-built assembly would blow up the schedule.
- Architectural rating matters at this scale. These aren't residential replacement windows scaled up. Architectural-rated windows with 1-inch IGUs and Solarban 70 give you the energy performance to satisfy Pennsylvania's adopted energy code while carrying wind-load and air/water performance appropriate to a 17-story building.
- The glazing pocket dimension is a design constraint. If exterior grids are on the elevation, the 1-¼-inch pocket becomes a hard requirement upstream of the glass specification. Missing that on early drawings triggers change orders.
- Spandrel glass is part of the fenestration package. OpaciCoat 300 in the same IGU carrier keeps the elevation reading consistently across vision and non-vision zones—easier than mixing curtain wall spandrel panels with punched window openings.
Adaptive reuse of 1950s and 1960s office stock is the single biggest opportunity in urban commercial glazing right now. 17 Market West shows what the finished spec sheet actually looks like—and where the pocket details, grid geometry, and combination-unit logistics are going to make or break the next conversion project.