For the Multifamily Sector, Product Innovations Boost Design and Construction Success

It seems fitting that wider adoption of thin-shell concrete, a technology popular worldwide but somehow unfavored in the U.S., has resulted from the exigencies of the multifamily construction boom.

“The load-bearing, tunnel-form systems with modular aluminum formwork, one of the most common techniques across the globe, is now being used more commonly in the U.S. multifamily sector,” says Gensler’s Brooks Howell, AIA, a Principal and leader of varied residential works. “It’s a game changer for housing in many markets if you can find a qualified formwork subcontractor, so it’s clear why the rest of the world sees it as a solution.”

The big reasons are speed and cost, says Howell, who has served as a special advisor on development and permitting for two city of Houston mayors. “A post-tensioned concrete tower is double the cost as compared to thin shells, per rentable square foot,” he says. “It simplifies high-rise construction with a dramatic reduction in cost.” Yet identifying that reliable specialty provider is critical, as is engaging an experienced structural engineer. The calculation of structural loads is relatively complex for these structures, according to the American Concrete Institute, which has published code requirements for concrete thin shells. Because of their surfacelike geometry, for example, determining precise buckling loads is tricky.

New projects like Ten Oaks, a 12-story thin-shell concrete design by Gensler, located about 30 miles from Houston on a site adjacent to a four-story stick-framed project, offers the developer-owner Resia several benefits. “Our design provides for lots of repetition,” says Howell, “with only three unit types and one window type, one kitchen, and one bathroom vanity, for example. All this simplifies millwork and other building products as well as shipping and installation.”

Another fast-turn technique shaping the multifamily market is adaptive reuse, says Tallal Bhutta, Founder and CEO of general contractor and design-build firm BDB Construction. New projects delivered by BDB include the recent conversions of two high-rise hotels in Midtown Manhattan in record time: a former Hilton Doubletree, now an apartment building, and the iconic Marriott East Hotel, just converted into student housing. “These conversions took only seven months from the closing of building sale to the owners obtaining temporary certificates of occupancy, or TCOs, which allow the start of physical occupancy,” says Bhutta.

For conversion projects like these and for office-to-residential adaptations, adding daylight and air are key challenges. “Fenestration replacement often includes newer requirements for operable access to fresh air ventilation, while upgrading the energy-savings performance attributes well beyond the original installations,” says LEED-accredited architect Sean M. Stadler, FAIA, Managing Principal of WDG Architecture.

Stadler adds that composite windows are becoming popular also: “These windows provide superior structural quality for mid- to high-rise concrete multifamily projects with much better thermal properties while being lower in cost than quality aluminum windows,” he says. Many of these products offer tilt-turn window operability, which can make a terrace door serve additionally as a ventilation opening.

Windows to the World

A new generation of fenestration techniques combines energy efficiency and long-term performance with the market-pleasing trend toward more indoor-outdoor experiences. For wood framed or wood-podium projects, Stadler adds, the market continues to have a preference for vinyl window systems because their cost advantage makes it difficult for products like composite windows to compete. “The thermal performance of vinyl windows is far greater than that of aluminum windows, but they can restrain design opportunities because, for example, there can still be limited color options,” he says.

In fact, thermally broken aluminum windows and doors are critical to boost their performance in multifamily projects, among others. While they may add to first costs, these high-performance products incorporate a reinforced polyamide bar between the inner and exterior aluminum profiles of the window units, creating an insulated barrier within the window frame, according to the U.S. Department of Energy. Studies show that thermal breaks improve sound isolation up to 80% and slow the conduction of heat and cold by a thousand times as compared to standard aluminum without breaks.

Planning ahead is essential for these favored products and systems, as many are long-lead-time items. According to architecture firm Cooper Carry, the best strategy for fenestration is bringing in the “glazing installer earlier as a design-assist trade.” For the firm’s project in Atlanta, the project team specified glass imported from Colombia, a jurisdiction requiring unusually extensive research into specifications and performance. Leveraging an existing relationship and combined pricing with a project in Nashville, Cooper Carry achieved an increase in glazing at budget, thanks to the economies of scale. “Early project meetings with the glass installer and a façades and waterproofing consultant reviewed limitations and details, along with pricing, in order to manage escalation,” according to the firm’s architects.

Echoing those ideas, Norr’s George Sorich, the firm’s Vice President, Residential, says fenestration specs and installation methods are improving in part due to these improved delivery methods. “One process change is encouraging our general contractor partners to keep most of the enclosure work completed by one subcontractor,” he explains. “Also, we expect to see more and more vertical photovoltaic panels utilized as part of the exterior envelope enclosures as we move closer and closer to net zero,” citing new products and studies confirming their efficacy.

To simplify and speed façade construction, Gensler’s Howell cites the significant benefits of working with window wall systems, which can be installed between slabs from the inside of the structure, obviating the need for scaffolding or lift equipment. WDG’s Stadler concurs, pointing to modular window-wall systems that also incorporate opaque wall elements, which he believes increase quality control due to their fabrication in shop environments, reducing trade coordination in the field as well as incidental material waste.

These pre-assembled exterior wall assemblies, many adopted in Canada, are mainly seen to speed field installation with shop-level quality control. “These assemblies may include an exterior wall weather barrier, exterior finishes, and sometimes windows,” says Stadler. “Some are panelized systems with cold-formed metal framing structure with metal façades, lightweight precast, or EIFS, exterior insulation and finish systems.”

Similar strategies have led to a critical eye for other exterior metal-and-glass systems, adds Norr’s Sorich. “In multifamily projects we see today, it’s increasingly rare to see the use of traditional curtain walls, due to cost. But many hybrid systems—which are still essentially window walls—will have metal panels or spandrel glass glazed into the system for fire rating or interior layout reasons,” says Sorich, whose firm is active on multifamily projects in Nashville, Philadelphia, Calgary, Cleveland, and Chicago.

Accelerating delivery of multifamily housing developments 

Slashing project schedules is a recurring theme in practically every market. “A faster building enclosure means faster fit-out and faster occupancy,” says Alexander Briseno, AIA, NCARB, LEED AP, a Principal and Studio Design Leader for the commercial and mixed-use sectors at HKS. “With multifamily products, speed to market is one factor that ties into a client’s proforma; this translates into an expedited income stream for the client.” It’s also a driver of high-rise design solutions employing more prefabrication, he adds: “This system of construction can scale depending on local market conditions, but to not implement it at all does a disservice to our clients.”

Experienced building teams say that the more penetrations a building has, the more opportunities arise for moisture infiltration. This explains why prefabricated façades have so many proponents. The stick-built enclosure systems popular in metro areas such as Atlanta, for example, tend to be more susceptible to moisture infiltration than those in Washington, D.C., say, where unitized systems are the norm. HKS adds that prefabricated façades can be fully enclosed and tested in a factory and have only four lines of potential infiltration around a 10-foot, 8-inch x 28-foot panel, which spans about 300 sf of enclosure area. “In contrast, a stick-built system relies on the accuracy and quality control of every brick anchor and mullion jamb,” according to the firm.

Examples of prefabrication range from basics such as unitized window walls and façade panelization to modularized façades with pre-glazing—and extend to true modular construction. “While the lower end of these systems doesn’t require additional planning, pre-glazed and module systems necessarily affect façade design,” says Briseno. “Prefabrication systems must be paired with rigorous planning grids and expertly located material transitions.” According to the façade engineering firm Simpson, Gumpertz & Heger, structures with less repetition, unique wall conditions or geometries, smaller surface areas, or large open sites may not benefit from prefabricated panelized systems, and instead “may be more suitable for field-fabricated (i.e., stick-built) methods.”

“Technologies can certainly absorb more dramatic design configurations, but at a cost that generally pencils with only the most luxurious product types,” says Briseno, who has served in AIA and the Urban Land Institute, as well as on architectural review boards, nonprofit groups, and academic positions. “While I appreciate the process and beauty of a gridded façade system, the speed, efficiency, and simplicity are what appeal to many of our clients.”

“Time is everything,” adds BDB’s Bhutta. “On the construction site, the building team’s goal is exceeding the owner’s expectations related to schedule—and budget, of course. For the best possible results, it’s essential to involve the contractor in the design/schematic phase to avoid delays later on, and provide for constructability review and budgeting involving the full building team.”

In addition, says Bhutta, whose projects are concentrated in the Northeast, project managers and tradespeople should be trained to predict unforeseen conditions and to reduce uncertainty using field experience and relevant knowledge. All of this allows for more consistent schedule acceleration and “critical path innovations,” which describes the creative application of expertise for better phasing, sequencing and resolution.

Transparent and Opaque walls systems for multifamily housing

For both fenestration and opaque wall systems, variety is the spice of life, say designers from coast to coast. “Patterns are more varied and the cost of vinyl colors has decreased, so we’re finding more readily available colors lets us explore different styles,” says Jeff Mulcrone, AIA, LEED AP, a Partner and Director of Design with BSB Design. He notes that many teams aim high during conceptual design so that premium-level fenestration and façade system specs will remain robust even after value engineering.

“This is common in mixed use, especially,” says Mulcrone. “We use a premium commercial product on the storefronts and club spaces but shift to more typical vinyl windows for residential levels above.” To improve the appearance of these solutions, his team relies on clever design strategies to blur the distinction between floors—and quality levels.

Gensler’s Howell agrees that vinyl windows are often the first choice, and he cautions building teams to ensure the window assembly’s design pressure is adequate to resist moisture penetration, especially where severe weather is likely. Typical window details include a weep at the window sill, for example, a location where water intrusion is possible during storms with stronger winds if design pressures are underestimated. “Take care not to use inferior windows,” he says. “People are rethinking that now. We’re seeing issues on built projects in Kansas, for example, where seasonal 80-mph winds are routinely causing moisture problems.”

Solutions for detailing wall openings include nail-fin windows installed over peel-and-stick flashing behind the wall, as well as sub-sill systems with appropriate seals that can achieve superior weathering performance. While these proprietary manufacturer designs may be slightly more complicated to install in some U.S. regions—and slightly pricier—subsills allow for excellent drainage and, when fully sealed with purpose made end-caps or aluminum angle end-stops, make water ingress much less likely. Subsills can also serve as a base for the framing and can make installation easier where the structure may be uneven or out of tolerance, according to one supplier.

The jury is still out on how much transparency is ideal with modern façades. While some experts say they favor less glass to control heat gain, others say they use more glass, or higher window-wall ratios (WWRs), to boost natural lighting and indoor-outdoor connectivity.

“Façades are embracing more glass and less stucco,” says Hande Obuz, Principal and Senior Architect based in Stantec’s Miami office. Aside from glass curtain walls, she says, “We are using 2.5-inch-thick limestone cladding on a twin 29-story luxury condo tower, with a smattering of composite aluminum panels and column-beam covers of glass fiber-reinforced concrete,” or GFRC.

Obuz adds that specifications for fenestration and installation methods are improving recently, mostly because designers are required by code to specify product-approved hurricane-resistant tested assemblies. “These must meet the structural engineer’s established cladding design pressures and glass that provide minimum SHGC—solar heat-gain coefficient—and U-value established by overall building energy calculations,” she explains, noting that the U-values provide a measure of the insulating characteristics of façade assembly or insulating glass unit (IGU)—essentially, how much heat flow or heat loss occurs through the enclosure due to the difference between indoor and outdoor temperatures. “If drawings and specs for windows and doors need to be corrected during the construction administration phase, it typically entails added cost, possibly time delays, and even design changes.”

In some cases, project teams are seeing larger individual glass panels and IGUs used in ways to enhance drama and building performance, says Alexander Zilberman, AIA, NCARB, Founder and Principal of AZA, a firm specializing in high-end multifamily and luxury retail. “This is especially true for storefronts and primary façades in mixed-use residential buildings,” he explains. “For Aston Martin’s new Q New York, at the base of 450 Park Avenue in New York City, we worked to create a unique window installation of epic proportion, dubbed the Champagne Frame, which was a retrofit IGU measuring 22x11 feet—a size we learned is one of the largest glass openings installed in that city.” Inside, passersby see the Aston Martin display vehicle models set amid walnut floors, millwork, and a bespoke chandelier of 3,000 hand-blown, mirror-coated glass globes.

At every price level, however, other building teams are instead seeking budget choices with better pricing. BSB Design’s Mulcrone explains that color costs are still prohibitive for many brands, and that many design teams are hoping that color will expand to make the market more competitive. “We’d also love to see some innovation in window mounting flanges that would allow us to create some depth with window design,” he adds. “Adjusting the flange even an inch or two so the window is no longer flush with the façade would create a recessed look that really enhances the elevation’s appearance and function.”

Another Look at EIFS for multifamily construction

On the topic of opaque façades, building teams also see different tendencies. For example while Obuz and others see less stucco being used, for example, in Miami and other southeastern U.S. markets, Gensler’s Howell sees a revival of wet-applied exteriors. “Exterior insulation and finish systems, EIFS products, seem to be making a comeback in a number of markets,” he says. “With the drainage cavity behind the façade’s finish, these systems allow water that gets behind EIFS or stucco to escape rather than build up.”

For Cooper Carry’s multifamily studio, EIFS has been employed for a number of projects, including The Foundry. “We used a precast skin solution and it cut two or three months off the install time,” according to the firm. “Anything prefabricated—for example, wall panels that are factory made with cold-formed steel and a panelized or EIFS finish—seems to pay off well in terms of installation and construction schedules.”

For their variety of finish and material options, WDG’s Stadler likes open-joint rainscreen façade panels. “These are relatively lightweight and require less maintenance,” he explains. “These panel materials range from high-density fiber cement and high-pressure, exterior grade laminates to synthetic stone and large-format porcelain.”

Building teams should focus on trying to maintain a simple, straightforward drainage plane that allows gravity to draw moisture out of the wall, says Rob Muller, Senior Design Leader and Managing Partner with the firm BKV. “Using ventilated rainscreen technology, we establish a straight wall with a fluid-applied air and water barrier and then use varying material depths and sub-girt systems to create an enclosure with a flat drainage plane and a façade with depth and texture.”

Variety is essential in cladding choices, adds BKV’s Muller, noting that his architect teams are employing “a broad range in color, texture, and cost: masonry, corrugated metal, composite metal, fiber cement, stone, simulated stone,” he says. “There’s a strong trend for durable materials with the look of natural wood,” which is seen in their project Waterford Bay in St. Paul, Minn.

“Façades and cladding in general are a balance between three key factors,” says Muller: “Creating a high-quality interior environment with abundant natural light, responding to expectations and zoning requirements of city planning officials—typically looking for higher-quality materials and a breakdown of the building massing—and finding economical materials and methods to allow projects to pencil.” He adds that the architect’s task is to find creative ways to integrate these disparate goals into a unified façade composition, pointing to the firm’s design for The Fynn in Chicago as an example.

Infill, Low-Rise, and Indoor-Outdoor design for multifamily housing 

To maximize project value and take advantage of current market conditions, say multifamily experts, more teams are building on hemmed in lots or varying project heights and areas.

“Urban infill projects can relate contextually to their surroundings, such as with a podium of brick or masonry and a hybrid window-wall system and slab covers above,” says Norr’s Sorich, who recently wrote a white paper on densification and urban regeneration. “These are easier to build on upper stories, requiring only one subcontractor, working from the interior.”

Low-rise is also trending, says HKS’s Briseno. “Low-rise projects are currently the only product that is easily penciling for many of our clients’ proformas due to volatility in costs and financing,” he explains. “Since much of our work is based on very large urban-scaled master plans, we’ve been utilizing creative phasing plans to implement placemaking elements and low-rise product in the initial offering, focusing on speed to market and placemaking, a marketing strategy that will help entice retail and tenant interest in later phases. We then position high-rise and commercial projects in a later phase, when either construction prices cool, or lease rates increase.”

Briseno and others also note that the balance of amenities and unit size has been shifting. While average unit sizes have been decreasing over the past several years while amenity space allocation increased, the trend has reversed due to volatility in the construction and financing industries, placing heavier constraints on budgets. According to HKS, this could risk the marketability of projects in the future unless a creative phasing strategy is employed.”

While amenity space shrinks, projects offering indoor-outdoor connectivity are on the rise, say many building teams. “We’re taking advantage of courtyard spaces to create a great exterior environment and a great view for inward-facing units,” says BKV’s Muller. “Introducing nooks throughout the building to use as work areas outside of the unit.”

Kelly Farrell, who Co-leads the global practice for Gensler, explains that connecting with the outdoors is a basic human need, and successful building teams are making the most of this natural amenity. “The design should give residents a place for their whole selves to retreat, not just outdoor spaces for entertaining,” she says, pointing to restorative gardens and other designed spaces where people can linger—longer—outside. “What really works best is creating a variety of spaces, not just a big pool and deck, but a varied outdoor program with places to sit down and read a book, small entertaining spaces, and parklike areas for families and other people.”

These outdoor spaces need to function 24 hours of the day, adds Gensler’s Farrell, and offer ways to connect to the comfort of other people or to just spend some time alone. “We need to enjoy these spaces and not feel crowded,” she says, adding that the health benefits are significant: “Fresh air does amazing things for our minds and bodies, and outdoor time and daylight are natural ways to help us get more sleep, which lowers our cortisol levels and allow us to relax.”

According to Cooper Carry, these benefits are leading more multifamily developers to go beyond a traditional fitness center. Instead they are leaning toward including planned spaces for yoga studios or flex rooms with indoor and outdoor connection, spa-like areas with wet and dry saunas, greenhouse spaces, community gardens, and teaching kitchens connected to outdoor dining areas, says the firm. Others are planning rooftop spaces with views, and unique amenities like bowling alleys as well as a plethora of electric vehicle chargers. “Co-working areas are large spaces, separate from the clubroom, with multiple different types of working stations from lounge seating areas to soundproof podcast and phone rooms to full size, technologically equipped conference rooms,” say the architects.

For exterior spaces on roof areas, frequent renovations are typical. Says WDG’s Stadler: “Rooftop amenities are also being upgraded in aging buildings, to include more refined finishes such as porcelain paver systems, premium oversized parasols, cabanas, outdoor kitchens, firepits, infinity-edge swimming pool water features, and enhanced speaker and lighting systems.”

Teams should program and design indoor-outdoor amenities with care, cautions Stantec’s Obuz. “Developers ask for the outdoor spaces as a selling point, but they are not appropriate for all climates. For example, in Miami, summers are long and uncomfortably hot and humid outside, says Obuz. “Multiple tenants entering and exiting the outdoor space can overwhelm the air-conditioning equipment, waste a lot of electricity, cause condensation on the diffuser grilles, and take hours to cool the space back to comfort level.

“Not to mention allowing mosquitos, flies, and cockroaches to enter,” adds Obuz.