How engineers can create better glass curtain wall design

When consulted early, a qualified MEP engineer can help a design team make better informed and more insightful decisions about glazing, thickness, height, and other factors in glass curtain wall design.

By Anthony M. Montalto, PE, LEED AP; Jaros, Baum & Bolles (JB&B), New York City October 16, 2018

Learning Objectives

  • Ascertain the importance of getting critical team members involved early in the design process.
  • Learn ways to ensure optimal glass curtain wall design, particularly in health care settings.
  • Know how to best use thermal-performance standards in the design process.

When an architect creates a concept for a health care facility, the building’s HVAC system is usually not the first consideration. Along with thoughts about aesthetics, flexibility, and functionality, architectural designers of health care facilities should draw some inspiration from the project’s HVAC operation.

An example of where this holds true is with glass curtain wall façades, particularly within the relatively stringent code requirements, real-world constraints, and desired environment of a health care facility. A mechanical, electrical, and plumbing (MEP) engineer with a good understanding of the relevant building codes and governing authorities can save an architect or owner considerable worry, time, and money while helping to deliver a better-performing building.

Custom curtain walls are growing more popular as a design option and remain a subject about which many design and construction professionals lack sufficient knowledge. Team members also may have conflicting priorities. For example, a curtain wall designer may want minimal sightlines while the product providers may stress minimal movement. For this reason, it is critical to get all key stakeholders together in the same room early in the process.

“During preconstruction and the selection of the building envelope, we involve MEP engineers in a number of ways,” said Robert Gullickson, PE, vice president and senior mechanical/electrical engineer at Turner New York. “First, there is the issue of whether we require heating at the base where the occupants are or if we can just do it overhead. Also, if the glass is not thermally efficient, the MEP team has to investigate ways to recover this inefficiency for compliance with the energy code via energy recovery. And full-height glass scenarios cause significant difficulties in providing perimeter heat, with hot-water and fire safety issues.”

A qualified MEP engineer can help a design team make better-informed and more insightful decisions about glazing, thickness, height, and other factors in glass curtain wall design, ultimately improving the patients’ and medical professionals’ experience.

Typical vs. atypical approach

The standard procedure for some designers of glass curtain wall buildings is to maximize the perimeter glass by specifying it from floor to ceiling in as transparent a form as possible. The result is a space with the benefits of daylighting and the recuperative power of letting the sunshine in.

An often-quoted study by The Center for Health Design concluded that, “Adequate and appropriate exposure to light is critical for the health and well-being of patients as well as staff in health care settings. A combination of daylight and electric light can meet these needs. Natural light should be incorporated into lighting design in health care settings not only because it is beneficial to patients and staff, but also because it is light delivered at no cost and in a form that most people prefer.”

The study also noted that by controlling the body’s circadian system, light helps to reduce depression among patients, decrease lengths of stay, improve sleep and circadian rhythm, lessen agitation among dementia patients, ease pain, and improve adjustment to night-shift work among staff. Additionally, exposure to light is critical for vitamin D metabolism in the human body.

What is sometimes overlooked, however, is that highly transparent vision glass requires more energy for cooling and heating. This can necessitate more ductwork and, consequently, reduce the room volume. The benefit of floor-to-ceiling glass is often diminished when furniture and other room elements obscure significant parts of it.

MEP engineers also can quickly recognize when certain thermal effects may not be optimal or when a curtain wall design may not meet code. They can calculate the energy model and how the building is going to perform, taking into account more than just the visual effect.

Increasingly stringent standards

Codes, standards, and energy guidelines have become more stringent with respect to building energy consumption, requiring increasingly higher standards for building performance. These standards evaluate all energy-consumption devices and the building envelope while minimizing the amount of heating and cooling required for these envelopes.

ASHRAE Standard 90.1-2016: Energy Standard for Buildings Except Low-Rise Residential Buildings identifies a prescriptive path for energy code compliance for building envelopes. The accompanying table lists minimum R-values for opaque surfaces as well as maximum U-values and solar heat-gain coefficients (SHGC) for fenestrations (vision glass).

These thermal-performance standards are a good starting point for building design as it relates to the envelope. However, keep in mind there are alternative ways to comply with local energy codes, and the prescriptive path can be the most stringent.

Engineers also can use the energy-cost budget method, as outlined in ASHRAE 90.1, via an energy model to determine overall building energy usage as compared with the ASHRAE baseline case. This method offers trade-offs between very high-performing systems and systems that may not be as efficient, allowing the designer to look at total building energy consumption when developing a building envelope strategy.

It also is worth noting that stricter regulations for building envelope commissioning may be coming soon. While ASTM E2813-12: Standard Practice for Building Enclosure Commissioning and National Institute of Building Sciences (NIBS) Guideline 3-2012: Building Enclosure Commissioning Process BECx provide parameters for building enclosure commissioning, the regulations are still in their infancy. With the U.S. Green Building Council‘s statement in January 2018 that buildings account for almost 40% of carbon dioxide emissions in the United States, it is inevitable that government agencies and the building industry will strive to make these complex structures more energy-efficient. This includes the curtain wall, a major factor in the equation.

Understanding the options

The greatest value an MEP engineer adds to the early design process is to ensure that the designer knows all the available options and to guide the process toward decisions that best satisfy the needs of the facility and the owner. This often means striking the right balance of aesthetics, function, energy efficiency, and cost (both capital and operational).

For example, in New York City a shading device, such as an awning, louver, or even a deciduous tree, can block some of the solar radiation in summer months when the sun angle is higher but allow more sunlight in winter months when the sun angle is lower. This is the type of critical detail that might be overlooked without an MEP engineer’s insight.

Other variables include interior glass height, thickness (single-, double-, or triple-pane), glazing, shading coefficient, and other interior and exterior effects.

The referenced case study on the thermal-performance effects of changes in vision glass illustrates how a few simple changes to the vision glass and curtain wall can impact other energy-related and space factors in a patient room. By reducing the glass height from 8.5 to 7.5 ft, along with changes to the glazing (from a U-value of 0.40 to 0.33) and a decrease in shading coefficient (from 0.40 to 0.26), the peak cubic feet per minute (cfm) of air moved in a patient room dips 28%, the air-change rate falls from 20 to 14.5/hour, and the size of ductwork and risers drops substantially. These relatively modest changes also result in 82 fewer metric tons of cooling capacity needed in the chiller plant.

Reducing the height of the interior glass is often a minor sacrifice. While no one wants to return to the days of closed-off rooms and punch windows, designers can still enhance connectivity to the outdoors without specifying a wall of glass. A 3-ft sill at floor level draws relatively little away from the desired effects of sunlight and view. But this barely perceptible modification can make a major difference in energy efficiency, ductwork size, and short- and long-term costs.

All this points to the fact that there is a clear advantage to seeking the early input of an MEP engineer on projects where engineering expertise plays a major role. Architects can retain the integrity of the design while ensuring better energy efficiency, greater cost-effectiveness, and more satisfied end users.