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HVAC and the Metal Envelope

Metal building envelopes are being studied more closely in terms of HVAC

Ma  Special  Feature  Aug17 2

A recent report says that government rules and tax incentive programs related to energy saving are supporting the growth of the global heating, ventilation and air conditioning (HVAC) market. The report, titled "HVAC Market-Global Industry Analysis, Size, Share, Trends, Analysis, Growth, and Forecast 2017-2025," was written by TMR Research, San Francisco, and analyzed the various factors supporting the HVAC market's growth. While energy conservation is driving HVAC growth, the report also says that HVAC demands are rising in response to burgeoning construction and infrastructure activities worldwide.

Because of this growth, the metal envelope is being studied more closely in terms of its relationship to HVAC. One of the metal envelope's roles is to separate the outside environment from the building interior via an efficient and tight boundary. An energy-efficient metal envelope produces a controlled environment enabling occupant comfort and energy cost reduction. HVAC relies on the metal building envelope to repel unwanted heat gain/loss, and contain the conditioned or heated air created by the building's mechanicals.

"This efficient boundary can minimize the fan and/or pump energy needed to bring heating or cooling from the interior of the building to the exterior zones to offset the thermal heat transfer to/ from the outdoor environment," says Don Posson, corporate director of engineering, SmithGroupJJR, Washington D.C. "There are many properties of a metal envelope that must be considered to understand its impact on a building's HVAC energy efficiency, including thermal properties, moisture migration, exterior color, mass, emissivity, etc."

Because selecting the right HVAC system for a building is very much impacted by the building's envelope, Posson always starts by studying the overall building massing and orientation to drive the overall HVAC loads down. This saves money on HVAC first costs, as well as yearly energy costs. He then moves to optimizing each envelope component (including metal walls and roofs) using parametric energy analysis to optimize the performance of the components, providing selections with the best overall life cycle costs. "The performance of each building envelope component also influences the correct selection of our HVAC systems, since we are always striving to obtain HVAC systems that can perform both efficiently from an energy standpoint and from a thermal standpoint, meeting all the comfort criteria for the spaces inside these building walls," he adds.

Jeff Hamstra, president, and Wade Hamstra, vice president of Hamstra Heating & Cooling Inc., Tucson, Ariz., approach metal buildings by performing full heat loss and heat gain calculations per Air Conditioning Contractors Association (ACCA) and/ or American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards using Wrightsoft design software from Wrightsoft Corp., Lexington, Mass. "Metal buildings tend to be more susceptible to air infiltration due to poor envelope air sealing, and if an HVAC contractor fails to take this into consideration they may undersize the system(s) or end up with a space that struggles with humidity control," they say. "We factor in the building construction type/materials into our engineering calculations. Naturally, a metal envelope provides less insulation value than stucco or block, so we must factor that in. We've been involved with projects that successfully built a building envelope with metal sheeting that were very well insulated and provided an opportunity for efficient HVAC system design."

Phillip Gentry, PE, partner/principal-in-charge, director of building systems at Valley Engineering, Mt. Crawford, Va., says when configuring HVAC for a metal building envelope, it all begins with designers using the latest version of ASHRAE 90.1, starting first with Table B-4 to determine the correct climate zone number and letter for the project site. Then, once the correct climate zone number and letter have been obtained, Table 5.5-1 Building Envelope Requirements for Climate Zones is used to determine the minimum building envelope requirements.

"There are specific minimum requirements for metal buildings in Table 5.5-1," he adds. "Once the building envelope requirements have been obtained, heating and cooling load calculations must be completed to determine appropriate temperature control zones and equipment sizes. With heating and cooling calculations complete, the engineer of record can select the right HVAC system based on budget, accuracy of temperature and humidity required, and capacities of HVAC equipment chosen."



Not only does the metal envelope play a pivotal role in building energy efficiency, it influences HVAC equipment sizing. "How the insulation is applied to the building is critically important-i.e., continuous insulation versus non-continuous," says Dan Gill, PE, studio principal, mechanical engineering at Little Diversified Architectural Consulting, Durham, N.C.

"In addition, the thermal breaks between the exterior and interior of the building will have a significant impact on the sizing of the HVAC equipment, and the energy performance of the building as a function of the run hours for the HVAC system. The energy conservation code in effect in the area of construction will make a determination of which type and how much insulation is needed. In my experience, metal buildings are meeting the energy code by a prescriptive approach and not a performance approach due to the size and typical simplicity of the HVAC systems."

Brent Schipper, AIA, LEED AP, principal of ASK Studio, Des Moines, Iowa, acknowledges that contemporary metal buildings now have a superior envelope and thermal breaking, which he feels makes HVAC system design "a little generic." He views this as being a good thing. "Metal buildings are now seen as superior envelopes for fit, finish and water resistance," he adds. Gary Davis, director of marketing at Zahner, Kansas City, Mo., contends that as the metal envelope has become better designed and installed, the need for large HVAC systems has decreased.

HVAC system size is affected by the metal envelope because a building with poor thermal performance will require increased HVAC capacity to maintain the indoor air within a tolerable comfort range. Similarly, "Excessive moisture migration through the exterior walls either through diffusion or uncontrolled air leakage beyond the capacity of the mechanical equipment to overcome it will result in an excessively damp or dry interior. Which, again, may vary outside the typical comfort zone boundaries," says David Cook, principal architect, CTLGroup, Skokie, Ill. "This will result in uncomfortable building occupants. Excessive air leakage has been shown to be a major transporter of thermal energy as well as moisture vapor both in and out of a building. Therefore, air leakage must be controlled to help maintain indoor air comfort levels without increasing the size and capacity of the building's mechanical equipment."



Air infiltration and metal envelope leakage can easily become one of the biggest factors in a commercial building's energy usage. Tighter metal building envelopes help reduce air leakage which helps to reduce HVAC system size. One of the more difficult challenges a designer deals with is infiltration of outdoor air via cracks around doors, windows, garage doors and main entry areas. Reducing uncontrolled infiltration provides more stable temperature and humidity control. Tighter building envelopes have become one of the biggest energy conservation measures that are studied early in a project's design.

Davis agrees that overall building assemblies have become tighter because of metal envelopes. "Well-designed metal rainscreen envelopes have a vented column of air behind the metal surface," he says. "Face-sealed metals systems are not as prevalent any longer and are not germane to the 21st century, in my opinion."

Ventilation air is required in all types of buildings. This amount of air is governed by the local mechanical code and will depend on the use of the space. Gill says by using packaged direct expansion (DX) units, outside air can be directly introduced into the unit and not require additional penetrations of the space that would be necessary if split-system units are used. "In the event of evaporative cooling, in which the space is cooled via 100 percent ventilation air, a relief air path will be necessary to maintain a reasonable pressure within the building," he says.

The Hamstras see the tightening of metal building envelopes to be "a great trend" for HVAC. So much so, at their own metal building that they work in, they installed a damper system on their fresh air intakes with wall-mounted carbon dioxide sensors to best control fresh air intake and control indoor humidity during the summer monsoon season.

"Prior to implementing this damper system, running with intakes wide open, we struggled with indoor humidity control in July/August," they say. "We sized our fresh air vents for a very tight envelope, which enlarged them compared to an average envelope, and led to intake of too much humid outdoor air. Now, our CO2 sensors only open the dampers when required and we avoid over-intake." And for all of the buildings it works on, Hamstra Heating & Cooling modifies it air charge rate based on the tightness of the building envelope. "For very tight structures, we use 0.11 air change rate, for semi-tight, we use 0.22 and for average tightness, we use 0.32," they say.



The HVAC contractor is integral to the building design, responsible for the methods involved in creating a quality construction and for getting the HVAC systems operational, including coordinating the final testing and balancing, and commissioning of the systems. "The earlier in the design process the HVAC contractor can be brought into the design, the better, as it allows them to better understand the fundamental design intent of the systems," Posson says. "At a minimum, we like to have a constructability review by an HVAC contractor at the design development phase of a project."

As an HVAC contractor, Hamstra Heating & Cooling wants to be involved in the design-build process as early on as possible. "A struggle of ours is in gaining access to the building owner/occupant early on in the planning stages," the Hamstras say. "Without this access, it is challenging for us to accurately identify the customer's needs and details regarding their intended use of the space. The HVAC design is compromised if we start from any other point besides identifying the end customer's desires."

One variable affecting the HVAC contractor's involvement is whether the building project is design-bid-build or design-build. "If the project is design-bid-build, the HVAC contractor has very little role in actual HVAC systems design because the engineer-of-record is generally hired by an architect or owner," Gentry says. "If the project is designbuild, then the HVAC contractor is directly involved in designing the HVAC system for a metal building because the contractor is the engineer of record." Contractors, in addition to owners, designers and the industry as a whole, are all paying more attention to energy efficiency, understanding the metal building envelope must be effective to reduce HVAC costs, meet industry standards and attain happier occupants.