Ready for retail

Industry has a way of setting trends, such as the design of a building or the ways in which that building will be used. Designing for retail structures is no different. Trends in today's marketplace have dictated the evolution of two primary forms of shopping mall designs. The first is the traditional, totally enclosed mall with common areas, atria, and anchor stores.

By Matthew E. Lininger, PE, LEED AP, Mechanical Engineer, Graef, Anhalt, Schloemer, & Assocs. Inc., Milwaukee June 1, 2008

Industry has a way of setting trends, such as the design of a building or the ways in which that building will be used. Designing for retail structures is no different.

Trends in today’s marketplace have dictated the evolution of two primary forms of shopping mall designs. The first is the traditional, totally enclosed mall with common areas, atria, and anchor stores. This design focuses on the interior environment of the mall to provide a safe and convenient shopping experience to patrons. Less attention is paid to the exterior environment, except to focus on public safety and efficient traffic movement.

The second trend of shopping mall design is the town center concept that is almost an opposite of the first concept. In this approach, the design focuses almost specifically and exclusively on the exterior environment created by the addition of small streets, avenues, and promenades.

There are advantages and disadvantages to both approaches. The totally enclosed structure offers the obvious benefit to the flexibility of the structure overall. Leased spaces can be remodeled, expanded, and divided with greater ease than in a town center approach. There also is the benefit of a totally enclosed and controlled building environment and patrons don’t have to leave the structure to move between retailers. However, benefits usually accompany some disadvantages or cost as a trade-off.

HVAC equipment selection

Typically, two predominant types of HVAC design are used in retail design.

Unitary rooftop equipment is the most common type of equipment and design strategy. There are several benefits to this type of system for a retail real estate developer. Packaged equipment offers a single solution to HVAC for a wide variety of applications with little adjustment required to accommodate different styles of retail outlets. This type of equipment also tends to be the most economical solution. These two factors often make package unitary rooftop equipment the design solution of choice for retail HVAC design.

Rooftop HVAC equipment offers simplified design approach when routing ductwork to and from conditioned spaces when roof and plenum space is not an issue. This approach incurs a lower initial cost to the developer and provides for quicker construction schedules. This type of equipment is less efficient, requires more maintenance, and typically has a shorter lifecycle. The cost associated with rooftop equipment often is passed to the leasing tenant as part of the lease, and this further reduces the initial cost of the project. Therefore, the decision to use rooftop equipment must be weighed against the priorities of the developer and the project as a whole.

The other dominant approach to a retail outlet’s HVAC design is the central plant concept. This system has hydronic components such as chilled water, hot water, and heat pump loops. Significant advantages for this type of design strategy exist when focusing on the efficiency, controllability, and flexibility of an HVAC system. Because the upfront capital investment costs are higher, the owner should focus on the extended energy efficiency and operational characteristics of the development in order to make that increased financial commitment.

A central energy plant can lower operational costs of large retail developments. The ability to maintain occupant comfort and improve the patron experience is enhanced through greater control of system components. The initial cost may be much higher, and most of this burden for infrastructure costs must be borne by the developer. This offset of operational costs and increased life expectancy of equipment can be balanced against developers’ higher initial costs to determine the right system for the facility. Other factors to consider are the flexibility of the system to accommodate remodeling and available space for back-of-house systems, such as economizer duct shafts and relief air systems.

Understanding what is important

Regardless of the approach taken during the design of large retail outlets, the hurdles involved with the design remain the same. HVAC designs for these structures often are faced with ambiguity.

Typical guidelines focus on limiting cooling loads, and defining the type of equipment in the space, exhaust provisions from the space, and the allotted electrical service size for the retail space. From these guidelines, it becomes the task of the engineer to plan for any type of retailer to occupy that space, from a jeweler with tremendous amounts of display lighting to a daycare facility with high amounts of sensible and latent loads from people in the space and outdoor air required.

Without a full understanding of the use of the space, the process becomes a futile effort of speculation and guessing. This is not the only consideration that must be made by the designer. The typical retail store might occupy a storefront space for 10 years. After that, a completely different tenant might choose to lease this space. Most large retail outlet designs need to account for this type of flexibility within the design and it becomes the main focus of decision-making conversions throughout the project life.

The leased spaces are not the only spaces that must be taken into consideration during the design. Back-of-house spaces are critical to the success of such properties. If the transport of goods or access to stock locations becomes too difficult, the property will struggle to find tenants. However, store frontage for retailers is valuable space and cannot be earmarked for building systems that require access through exterior walls.

The back-of-house areas should have well-organized utilities and coordinated services for a successful retail property. Space for ductwork, piping, conduit, and other services must be planned in the design. Access is equally important. Plenum and chase management plans are chiefly the responsibility of the design engineer.

Design for the unknown

HVAC design starts by understanding what building services the developer is willing to provide to each tenant to attract potential leasers to the property. Items such as centralized exhaust systems for toilet rooms and janitor closets, or outside air ducts for serving split system units can be part of the utilities run by a developer to improve the appeal of leased spaces. Other utilities that must be part of the collection are water, power, and natural gas.

All of these utilities have a common design hurdle to overcome: There is no clear direction as to what the end-use of the space will be. It is useful to determine the utility service strategy at the onset of the project in order to minimize the amount of potential rework that will be done to accommodate different tenant types.

The developer’s leasing plan will outline which retailer will occupy each leased space. This plan will provide invaluable guidance as to the requirements of the space.

Food service and clothiers with in-house capabilities for cleaning and pressing operations are the two tenant types that tend to be the most challenging. If not planned in advance, finding space in tight quarters for grease exhaust and steam-laden exhaust ductwork can be a challenge. A simple solution to the issue is to require that these types of tenants be identified prior to starting the design of the building systems. This is unlikely, because the signing of tenant lease agreements for most restaurants is dependent upon the other types of retailers in the same development.

Another compromise that might be initiated by the engineer would be another proposed restriction to the leasing plan document. Restricting where problematic tenant types might lease space can help avoid situations where code variances need to be obtained as a result of lack of planning for specific tenants requirements. This practice will help avoid significant costs incurred as a result of forcing a building system into a space not originally conceptualized for that use.

In town center-type developments, it is common for a developer or architect to provide a single specified storefront system for all of the retail spaces. This places more emphasis on the internal loads and their interaction with that glazing system for different occupancy types. When accounting for this type of envelope system, the center of glass values must be used to determine the thermal performance properties for estimating solar loads in the space.

ASHRAE Standard 90.1 Chapter 5 discusses the proper way to handle building envelope features like vertical fenestration and acceptable alternative methods for calculating thermal performance values. These methods should be used to provide the most accurate picture of solar loads to leased spaces. Air distribution in spaces also must account for poor thermal performance of storefront systems to ensure that condensation does not occur on the glazing in spaces with high latent loads such as spas, hair salons, and daycares. In all respects, special attention should be paid to these exterior components as energy code requirements for HVAC systems are met.

Fire and life safety

The rating of walls and the adverse effect this has on building utilities and services often is an overlooked requirement of retail design. To remedy this, small retail malls may require fire separation around storage rooms that are 100 sq. ft or larger. Fire-rated assemblies also are encountered around atrium areas and require special attention from HVAC design.

Both of these special types of occupancies are defined within the 2006 International Building Code (2006 IBC). It is important to verify that local codes comply with these requirements because fire and smoke protection requirements differ greatly at the municipal level and with the authority having jurisdiction. These assemblies can become a major cost issue if missed during design. Sprinkler systems or other approved fire-suppression devices would be needed on utilities that penetrate these fire-rated assemblies.

Smoke control systems are required in atrium areas serving more than two floors in a mall building. The interlock of supply air devices and exhaust air devices with the lockout of heating and cooling functions often is used to accomplish the smoke control requirements of the pressurization method outlined in the 2006 IBC. This requirement can be a difficult challenge to meet air change requirements, air velocity requirements, and space considerations of equipment. In most cases, the air change volume requirement will be the driving force of supply and exhaust fan sizing to meet smoke control air volume requirements even if outdoor air economizers are provided with HVAC systems. These requirements for higher smoke control airflows result in larger ductwork sizes and can make plenum management practices more important when routing these distribution systems.

Vestibules in retail design are non-existent except for large anchor stores. Individual lease spaces with access to the building’s exterior value the storefront space too much to incorporate vestibules or revolving doors to prevent infiltration to the space. This, again, becomes of particular concern when dealing with the town center type of mall development.

There are several exceptions to this requirement in the 2006 International Energy Conservation Code where the requirement for a vestibule is required at all publicly accessible entrances to spaces that are larger than 3,000 sq. ft of sales floor area. In some municipalities, another exception is permitted for spaces that employ the use of an entryway similar to a lobby to protect the main interior environment from direct contact with the exterior environment. To incorporate this exemption into a smaller retail space would require acceptance from the lease holder to modify the typical sales floor layouts used as part of the imaging policy for that company. The opinion of a local code authority also is required to determine if this type of strategy is acceptable on a case-by-case basis.

Structural elements

A final consideration for building system designers is structural elements within the building. Structural systems, such as post-tensioned slabs and shear walls, can prohibit the passage of distribution systems, such as ductwork and electrical conduits. Identifying these elements early can eliminate expensive rerouting of systems to reach tenant spaces.

It is best practice to establish routing areas for mechanical, electrical, plumbing, and fire protection systems, and to make structural designers aware that these areas will need access through the building structure. In extreme cases, an entire wall of a mid-rise building could be used as a shear wall system, eliminating the potential of using the building face for HVAC penetrations. If no space is available up through the building or on the roof of the building for HVAC systems, expensive penetrations through these structural systems will certainly result in budget overages.

Designing building systems for retail settings is a balancing act between prudent judgments and compensating for changes. Some important pieces of information such as up-to-date leasing plans, well-written lease agreement requirements, and clear understanding between real estate professionals and design professionals can help eliminate the ambiguity in large mall developments.

Top priorities for designers should be to maintain the flexibility of the HVAC system to compensate for inevitable changes in the project and strive to collaborate with architectural and structural components of the design as early as possible. Having an organized plan for building services will allow for tenant build-out of leased spaces easily and quickly, which translates into higher occupancy rates for the developer and quicker opening dates for retailers.

Author Information

Lininger is a project engineer and primary LEED consultant at GASAI, concentrating on improving the energy efficiency of retail and commercial building designs.

Sub-metering for success

Incorporating sub-metering into retail outlet design for utilities, such as natural gas, electricity, water, and sewer, is a decision that cannot be made by the engineer or architect alone. This decision needs input from the developer, real estate personnel, engineers, architects, and utility contacts.

To understand what is required for utility metering, the requirements of the local utility company must be investigated with respect to the anticipated distribution scheme of the engineering design. The additional cost of construction must be balanced against the potential benefit to the developer for easier billing tabulation of leased space utilities.

In some cases, local utilities might have requirements for restrictions against sub-metering: Sub-metering strategies for water might include meters located in each tenant space, and the developer is responsible for charging tenants for the utility service. In the event that a dollar/square foot cost is written into the lease of a space for water service, this could be interpreted as water utility service and therefore would require regulation by the appropriate governing body. This reinforces the fact that investigation of local requirements of code and utility requirements are incredibly important.

Individual metering of electrical and natural gas utilities offer more incentive to developers in the form of greatly simplified billing for these services. In some cases, these utilities might even be billed directly to the lessee of the space by the utility so the developing company can be removed from this paper trail. In this case, the benefit to eliminate this administrative cost can be easily weighed against the additional installation cost and educated judgments can be made. The decision should be made as early in the design process as possible to be incorporated into the “Article C” documents used by the real estate personnel leasing spaces and provide clear direction for designers of energy distribution systems.