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International Klein Blue, the ultramarine hue favored by mid-century artist Yves Klein, begins as a notoriously fickle pigment that’s hard to work with. Without getting too into the nitty-gritty, it needs a binder to become paint, and with the wrong kind, it dulls—and quickly. But if you get it just right, it’s the kind of color that is naturally enveloping, striking, and grounding all at once. 

Its development was happening around the same time that I.M. Pei and Associates’ Society Hill Towers were being built in the Philadelphia neighborhood of the same name. The concrete trio, constructed in the early 1960s, was a central figure in the urban renewal taking place in Washington Square East at the time. But while the towers often get all the attention, even today, it’s the 37 brick-clad townhomes that anchor the buildings to the rest of the historic area. Each unit rises three stories with a centrally located staircase and is punctuated by arched doorways and clerestory windows, plus a private courtyard and small steel balconies. 

When an empty-nest couple approached Brooklyn-based design firm TBo about helping them renovate one, principals Bretaigne Walliser and Thom Dalmas jumped at the chance.

The owners, who were relocating from nearby Wilmington, Delaware, wanted a home that would feel bright and welcoming and intimate enough for when it was just the two of them, but could flex to accommodate their grown children and grandchildren. Structurally, the townhouse was sound, but the internal systems needed updating and the finishes and layout required a complete overhaul. Original Formica, small ceramic tile, and vinyl were everywhere, deteriorating and worse for wear. The kitchen was narrow, and the bathrooms smaller than what’s preferable these days. One choice Pei made that Walliser wanted to retain, though, was increased privacy as you head upstairs. “The primary idea that he had was that there was social space below on the first two floors, and then it would be increasingly more intimate on the upper floor,” she explains.

Hai Lounge Chair, Hem; Norr Mälarstrand Rug, Nordic Knots; Chairs, Vintage Clifford Pascoe.
Wood-Panel Refrigerator, Fisher Paykel; Cooktop, Gaggenau; Tube Candleholder, Hay.
Wall Oven and Microwave, Gaggenau; Custom Concealed Pantries.

Working together with local Hivemind Construction, TBo recast the space in 2023 in a way that honored Pei’s initial vision while adding contemporary, earthy touches that drew on the homeowners’ time living in northern Europe and interest in art. 

First, they removed walls to create easier circulation around the staircase and integrated cabinets with flush doors, which the owners use for pantry goods, cookbooks, wineglasses, and more. After refinishing the original oak flooring (it was, says Walliser, “a crazy orange”), the team took cues from Pei’s choice of material to inform the custom white oak millwork by Loubier Design. Given the smaller scale of mid-century dwellings, this allowed the designer to maximize storage that kept things tidy and navigable, but not too hidden, in the galley kitchen. 

Apex Lamp, Hay; Trefoil Table, Form & Refine.
Minta Faucet, Grohe; Tint Glasses, Hay.

Across the way, Walliser reveals that they tried and tried to cast the concrete countertops and sink in that elusive Yves Klein Blue. “It can’t withstand any heat,” she says, acknowledging its limited alkali resistance. “So when you’re casting concrete, there’s a lot of heat generated from the chemical reaction. If it kills that blue, the pigment just dies and turns gray.” After giving it a few shots, they ended up with cobalt blue, a much stronger and more stable pigment. Then they proceeded to paint the curved base in Benjamin Moore’s Watertown to create one cohesive wow-factor piece. (Bonus: The end caps are cabinets that actually open.) The couple passes a good portion of their time seated nearby, where there are views of their garden through the patio doors.

Custom Blackened Steel Railings.

“One of the challenges we faced was what to do with the stair railing,” the homeowner says. “We had to replace the railing to code, and [had] tried any number of designs when Thom came up with this beautiful, simple metal railing that fits perfectly.” It’s a subtle statement, employing metal and geometry like Pei did, that doesn’t abandon functionality—it’s easy for them to grab as they descend the stairs.

Linen Bedspread, Quince; Custom White Oak Millwork Closets.
Vintage Stool.

Up those stairs, the primary bedroom is complemented by custom oak wardrobes and vintage light fixtures, like a Murano pendant lamp and Swedish table lamps. The space merges with a small library, where an oak dk3 Royal System hangs opposite an original brick fireplace. Dutch maps, art books, and other vintage ephemera mingle with a Flos kelvin lamp and Mies van der Rohe MR rattan side chair. They are both connected to a primary bath, which is in the darker core of the home. 

Glo-Ball Lights, Flos; Sinks, Duravit; Faucets, Graff; Wall Tile, Ann Sacks; Custom White Oak Vanities.

Pei’s use of transom-style windows inspired Walliser to employ the technique to bring more light in. “That was a way of borrowing from his language that he had established in the houses and sort of manifesting it in a new way for the owners,” she says. For the bathroom floors, the designers took a page from Bauhaus artist Anni Albers; the cement tile placement is inspired by her work. “We studied her sketches and weavings, along with traditional and modern quilting, and created a series of ‘woven’ tile schemes,” Walliser explains. The idea is mirrored in the other two bathrooms, with patchwork tile in combinations of pink and green and variations on blue. 

Pond Mirror, Ferm Living; Door Handle, Emtek; Cement Floor Tile, Mosaic House.

From the Scandinavian furnishings and oak wood to Albers textiles, TBo’s thoughtful renovation kept true to the building’s modernist roots without staying stuck in the past. “It’s just very airy; the scale is wonderful,” Walliser says of the new interiors. And that stunning blue makes the homeowners happy every time they see it.