Compact building design to reduce energy use

On a recent trip to Atlanta, I was reacquainted with the concept of the hotel atrium. The purpose of my visit was to represent Swegon at Living Futures 24, and the venue was the Omni hotel near the convention center. The Omni has a fantastic atrium, like many other hotels in Atlanta and elsewhere. Exchanging ideas about making buildings that are more friendly to their occupants and to the environment was my reason for attending the conference, the annual conference of the International Living Futures Institute (#ILFI). Between conversations and presentations, I wondered about the energy efficiency of a building with an atrium. How much energy is consumed ventilating and controlling the temperature, humidity and quality of air in a huge volume of indoor space that no person will ever occupy? Why consume so much land and building materials for relatively few occupants? According to a recent article (How the American Atrium Hotel Became a Global Icon - Bloomberg), hotels with atria were built in a different time, when grandeur attracted people to buildings and those people pay the bills. I enjoy being in a nice atrium, and I understand the motivation to build a building that is enjoyable for its occupants. But I wonder about the impact of atria on buildings. Building owners and developers seeking to construct buildings with lower initial investment and lower operating costs can benefit from buildings that are more compact.

Compact building benefits

Compact buildings use fewer construction materials, are easier to insulate, have fewer thermal bridging risks and are simpler to make airtight. If the design team intentionally seeks to take advantage of these compactness features, the resulting building can operate with very low energy use intensity (EUI). Buildings designed with Passive House concepts, for example, typically include compactness as a design goal. Betances Residences in New York is an example of a building design that took advantage of compactness to achieve superior energy performance. (Read about this project and earn continuing education credits at Continuing Education: Affordable Housing and Energy Performance | Architectural Record.

Compact building design

Compact buildings may be characterized by having basic shapes, with round being the most compact shape. The compactness of a building is calculated as the ratio between the building envelope surface area (A) and the conditioned volume (V). The compactness (C), or A/V ratio, is expressed by a simple formula: C = A/V. Other expressions of building compactness also exist. The design goal of a compact building isn’t to squeeze everything together at the occupant’s expense, or to make buildings that look like grain silos. The idea is to shrink and simplify the building envelope, thereby minimizing the ratio of the surface area to the volume of the building, to enjoy the benefits discussed above. Consider a building with an atrium. The atrium is always unoccupied but adds to the conditioned volume and causes the building envelope to have more surface area. Eliminate the atrium, and a building with similar shape can have equal occupiable space with a much smaller conditioned volume, and smaller envelope surface area. With a smaller conditioned volume, supply air fans don’t need to move as much ventilation air-the more compact building uses less energy to cool, heat, and ventilate.

Compact building components

When building compactness is a goal, selecting the building systems and components gains an extra layer of value. For example, many HVAC system types exist, each using different methods to deliver ventilation and comfort to occupants, each with different footprints, heights and clearance requirements.

Architects usually allow a couple of feet of interstitial space to make room for trades-including mechanical trades, to properly install their components. Opportunities exist to reduce the interstitial space, while still allowing installation and service clearance for HVAC, electrical, lighting, communication, fire-safety and other systems. Duct systems usually need the most space, so they deserve extra attention.

Duct systems

Many of the various HVAC systems deliver conditioned air to occupied zones through ducted diffusers. To reach diffusers, ducts usually traverse the ceiling horizontally, but still need to connect to a diffuser vertically. To achieve the rated performance for comfort, pressure drop, and noise, a length of straight ductwork is needed on the final approach to the diffuser. But that isn’t what usually happens. To save space, or to use less ductwork, or because of poor planning, the ductwork approach to the diffuser is often abrupt, with sharp angles immediately upstream, sometimes resulting in pinched ducts that constrict airflow, which increases the duct pressure and fan energy use, and decreases occupant thermal and acoustical comfort. The diffuser shown below is ducted from above, and even though the diffuser profile is less than 3", the installation requires clearance for the duct, approximately 24".

Round ducts are the most compact, but the shape doesn’t always contribute to building compactness. Flexible round ducts are popular, but their flexibility allows them to be easily misapplied. A well-planned interstitial space benefits from rectangular ducts. European style commissioning boxes eliminate any possibility of a sharp duct bend impacting diffuser performance by facilitating horizontal connections while adding other benefits like duct access, ease of commissioning, sound attenuating and balanced air discharge profiles.

Chilled beam systems

Chilled beam systems offer another opportunity to intentionally shrink the interstitial space. Chilled beams provide most of the space cooling and heating by recirculating air from the occupied zone, so less supply air from the air handler is needed for occupant comfort, resulting in smaller ducts. A room that would need a 12” duct to ventilate, cool and heat in an all-air system may only need a 5” duct to provide the same comfort using a chilled beam. Supply air is delivered to chilled beams through a side inlet, so no extra vertical duct clearance is required. Many examples exist of buildings being designed to reduce the floor-to-floor height of a building because of the reduced interstitial space requirements of chilled beams. Specifying chilled beam systems for buildings can make the buildings more compact.

A compact chilled beam is shown with the supply air duct connection on the side, so no top installation clearance is required. The duct diameter is only 5", less than the 8" height of the chilled beam, requiring up to 16" less vertical space than the ducted diffuser discussed above.

Chilled beam systems are one example of a building system that can be chosen to reduce vertical space requirement and contribute to a more compact building.

Compact systems reduce lifecycle costs

Compactness of a different sort can also appeal to a building design team. Maximizing occupiable space is valuable. Commercial building space value is expressed in dollars per square foot, so any space freed up by more compact mechanical systems yields increased revenue for the owner. Consider air handling units, required in most buildings for ventilation. Manufacturers offer many models that meet the mechanical project requirements. Some have a footprint that is intentionally small without sacrificing performance, and options to run the ductwork vertically. If air handlers are installed indoors, and if an air handler with a smaller footprint is chosen, architects may reduce the mechanical room area to offer the owner more floor space to lease. For example, if a project requires 10 air handlers, and the team's mechanical engineer specifies air handlers that are just 1 foot shorter than another model, it is reasonable that at least 5-10 square feet are freed up per air handler, 50-100 square feet in total, enough for an extra rentable office. Selecting vertical, instead of horizontal air handler duct runs easily doubles the amount floor space made leasable. Over the life of a building, this extra floor space can substantially contribute to the owner’s income.

By striving for the most compact design, and with intentionality, building specifiers can lower the material and environmental costs while potentially enhancing the buildings financial performance. The extent to which aesthetics is balanced with compactness is of course up to the owner and the design team.