Metal Buildings and Energy Loss

by Marcy Marro | February 1, 2023 12:00 am

“One misconception people have is that metal building systems are subject to different energy conservation requirements than other forms of construction,” says Tony Bouquot, general manager of MBMA. “That’s simply not accurate. Metal buildings must meet the same stringent requirements of IECC and ASHRAE 90.1 that all low-rise, non-residential buildings are required to meet.”

Thermal Bridging

While metal building systems are subject to the same energy codes and requirements of wood-built buildings, there are certain things designers should be aware of when it comes to maximizing their energy efficiency. For instance, since metal transmits heat better than wood, thermal bridging mitigation is crucial, as it’s the most common source of thermal loss in metal buildings.

The American Institute of Steel Construction[1] defines thermal bridging as “the loss of building energy through thermal conductivity of elements that ‘bridge’ across the insulation of a wall or roof enclosure of a conditioned space when the outside temperature is warmer or colder than the interior space.”

In other words, Bill Beals, district manager of Therm-All Inc.[2], North Olmsted, Ohio, says, thermal bridging occurs when a more conductive (i.e., something with a poor insulating value) element creates an easy pathway for heat flow across a thermal barrier. The end result can lead to heat loss and the potential for condensation.

While thermal bridges tend to be more common in older metal buildings that are poorly insulated, they can also occur in new construction. “Technically thermal bridges can occur anywhere within a wall or roof assembly,” Beals says. “In metal building construction, the most common catalysts for thermal bridging are steel structural members such as rafters, purlins, girts and sheeting, as well as any discontinuities within the assembly (e.g., compressed rolls of fiberglass). Thermal bridging can happen in both the roof and wall assemblies, which means that it has a major impact on the overall energy efficiency of a metal building.”

More important than the material creating the bridge, is the number of bridges within a system. Stacey Fisher, senior product development technician and PM lead, energy lead at Nucor Buildings Group[3], Charlotte, N.C., says the spacing of studs in wood structures are often 16 inches apart, far more frequent than the spacing of metal building members, which is often 5 feet on the roof, and possibly even greater on walls. “This greatly reduces the number of thermal bridges, which in turn increases the energy efficiency of the system itself. Thermal bridging is something that is always going to be present in any building, the key is understanding ways to mitigate the number of bridges wherever possible.”

Reducing thermal bridging is a challenge in any type of construction, says Bob Zabcik, technical director of the Metal Construction Association[4] (MCA), and while energy codes are beginning to address this now, he recommends utilizing manufacturer-supplied suggested details as much as possible. While the possibility of bridges remain, Zabcik notes that most metal panel manufacturers develop products with this in mind, therefore mitigating potential issues. And, many manufacturers have included thermal transmission values (i.e., psi and chi factors), saving architects from having to recreate the wheel.

Nathan Fries, PE, senior sales engineer at Chief Buildings[5], Grand Island, Neb., adds that thermal bridging can be easily overcome in metal buildings by using thermal blocks and space for insulation to provide the barrier for thermal bridging. Gary Edgar, specification manager for Pittsburgh-based PPG Industries Inc.’s[6] Building Products, adds the use of thermal breaks in window and door systems will stop the transfer of the exterior temperature to the interior, while also preventing moisture from forming, especially in cooler temperatures.

“Thermal blocks and thermal tape are simple yet critical components to the thermal performance of a metal building envelope,” explains Beals. “Thermal blocks are placed within the roof assembly while thermal tape is utilized in metal building walls. Usually consisting of a polyisocyanurate or polyurethane foam core, thermal blocks come in a variety of widths and lengths and are most commonly used in standing seam roofs.”

Insulation

Metal buildings use the same insulating materials as other forms of construction. These include fiberglass, rigid foam boards, insulated metal panels and spray foam. “Like all forms of construction, the key is locating that insulation in the proper place relative to the building framing to maximize thermal performance,” Bouquot explains. “Insulation may be located between, below or in front of framing members. We encourage architects to work with their metal building supplier to ensure the insulation type and location meet the needs of the project.”

The key to increasing energy efficiency in a metal building, Fisher says, is understanding the need of the building and the options available and finding the right system to satisfy those needs. “These range from the most basic single and double layer of blanket insulation over the structure to varying methods of cavity filled and liner systems which utilize the cavity of the secondary members. There is also insulated metal panels (IMP) and specialty systems available.”

The deeper structural members of metal buildings range from 6 inches to 12 inches deep, creating more space for insulation, Fisher adds. “In addition to filling this space insulation is stretched over these members both on the roof and wall. This flexibility and increased capacity for insulation allows for more energy conservation.”

Since metal building systems are designed to incorporate all types of insulation in varying thicknesses, Fries says the ability to house additional insulation creates an energy-tight environment. “For existing metal building systems, insulation can be easily added and increased with the roof cavity without removing the roof,” he explains. Additionally, Edgar notes that using the proper R-value of insulation for both the roofs and side walls is important to preventing heat transfer inside and out.

Air Leakage

Once insulation needs are met, reduced air leakage is another way to help with building energy efficiency. And since air leakage is the next most significant contributor to energy loss, Bouquot shares that MBMA has done extensive whole-building air leakage testing to identify where leakage occurs and how best to eliminate it. “We’ve taken the findings from that testing and developed a best practices guide for minimizing air leakage from metal building systems. The guide is due to be released early this year.”

“With the current energy codes requiring filled cavity systems for the roof and walls,” Beals says, “additional thermal performance is limited by cavity depth. By adding air barriers and sealing all penetrations to reduce any air leakage, the system installed will perform at maximum levels of published thermal performance.”

To minimize conditioned air loss, Fries recommends air barrier testing using infrared cameras during and after erection, which allows for corrective action to be taken. “The best advice I could give anyone is to use a contractor experienced with buildings that have been whole building tested,” Zabcik says. “The process of testing and remediating a building is educational for them and they’ve learned what is important and what is not.”

Energy Codes

Energy codes continue to evolve with new iteration. Over the past several years, Fisher says the evolution has primarily revolved around increasing the thermal performance of a building. “Great strides have been made in this area and the focus has begun to shift more toward other ways to conserve energy,” he says. “One of the biggest topics right now is air infiltration and leakage. The industry is literally pressing the envelope to ensure that the amount of air leakage is kept to a minimum. This is being tested through different methods of pressurizing a building and measuring the amount of air leakage. This has pushed manufacturers and installers to take a hard look at sealing the envelope of the building. The critical areas are at intersections of planes, such as a wall to roof intersection and any fenestrations through a wall or roof. These areas present the greatest opportunity for air leakage. Creating systems to prevent gapping and ensuring the installation of the system is done properly are critical to this new movement.”

Beals adds that architects can expect to see the majority of 2021 IECC/ASHRAE 90.1-2019 updates in air sealing and blower door testing. The 2021 IECC also adopted a new requirement for whole-building air leakage testing for certain conditioned non-residential buildings. While it will likely be several more years before this code starts getting adopted by local jurisdictions, Bouquot shares the good news is that manufacturers already know how to make metal buildings comply with this requirement.

Preventing Energy Loss

Metal buildings offer architects a lot of flexibility in creating project solutions. And while energy loss is likely to happen in any building type, Fisher says the biggest key to preventing it is understanding how the different systems work, and how heat tries to move through those systems. “Once the system is understood, proper installation becomes vital to the actual performance of the system. Ensuring that that the envelope of the building is sealed, avoiding any gaps that could lead to heat loss and drafts, is one of the most critical aspects to prevent energy loss in any structure.”

Zabcik recommends finding a good test firm to administer the whole building air barrier testing and ensure they are involved in the preconstruction process, which will allow them to share knowledge with the contractor. “It’s harder to fix something than it is to do it right in the first place,” he says. “For thermal bridging mitigation, use thermally broken products like insulated metal panels and thermal windows. But when you do, make sure the product interface together properly. Most air leakage and thermal bridging both happen when one assembly or product touches another.”

Source URL: https://www.metalarchitecture.com/articles/metal-buildings-and-energy-loss/

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