Advanced Sustainability Potential Using Metal Building Systems

Sustainability and affordability come together using available technology
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Sponsored by Metal Building Manufacturers Association (MBMA)
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Sustainable Sites

All sites are impacted by a building being added to it—the goal is to minimize the adverse effects and maximize the benefits. Metal buildings can help facilitate both of those aims in several ways.

Site Development

Up to 2 credits are available in LEED for addressing sustainability issues in the development of a building site. Toward this end, it is helpful that metal building system components are all shop-fabricated and then erected in a controlled manner on the designated building site. This provides the ability to minimize the need to impact any site areas that are beyond the building perimeter. Further, portions of the metal building package can be sequenced to arrive as needed so that the staging area can be minimized, with reduced site impacts.

Rainwater Management

Up to 3 credits are available in LEED for addressing rainwater in natural and sustainable ways. Metal roofs provide an excellent platform to collect quality rainwater for non-potable uses. Rainwater from metal roofs tends to be cleaner and safer than water collected from other roof types, particularly those that use petroleum products.

Heat Island Effect

LEED provides for up to 2 credits based on the recognition that sunlight striking dark-colored surfaces such as roofs can contribute to “heat islands.” Hence, the use of lighter-colored roofs, particularly metal roofs, has become a common sustainability strategy. Research from the U.S. Department of Energy shows that one additional percentage of reflectivity in a roof coating, on average, will reduce roof temperature by 1 degree. A common calculation used to define a cool roof is the solar reflectance index (SRI) as defined in ASTM E1980: Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces. SRI is a method to obtain an index for relative surface temperature with respect to a standard white (SRI = 100) and a standard black (SRI = 0) under standard solar ambient conditions and wind speed.

To determine the SRI of many materials, The Cool Roof Rating Council (CRRC) administers a Product Rating Program in which companies can label roof surface products with radiative property values once radiative roofing tests and aged field tests are conducted by CRCC approved facilities. Metal roofs (i.e., cool roofs) are ideal for reducing heat island effects with various paint coatings to achieve the intended Solar Reflectance Index (SRI) value. A lower surface temperature reduces the heat gain in the structure below. In the hottest months of the year, a standard roof surface can reach 150°F. A metal roof with a solar reflective coating can remain 50°F cooler under the same circumstances. This applies to building roof and parking lot canopies for short term and long-term SRI performance. Further, coatings that have high solar reflectance, used for both roof and wall panels, generate lower environmental temperatures which in turn lower building energy costs.

The coatings used on metal structures can also reduce smog. Catalyzers used in certain coatings use UV light from the sun and humidity from the air to break down harmful nitrogen oxides, known as NOx gases, into harmless salts.

Water Efficiency

Water use is needed in virtually all occupied buildings. Where that water comes from makes a difference. As already noted, capturing rainwater from a metal roof is a very achievable option for many buildings. That rainwater can then be stored and put to use both for outdoor and indoor non-potable water use.

Outdoor Water Use

Up to 2 credits are available in LEED to reduce potable water use on building exteriors. By developing a rainwater management program, quality, non-potable, captured rainwater may be implemented to reduce municipal water use for landscaping use.

Indoor Water Use

Since more water is often used indoors, LEED allows for up to 6 credits for demonstrating reductions in indoor water use. Here again, a rainwater management program can also be used to capture quality non-potable rainwater from a metal roof to reduce municipal water use for toilet and urinal flushing, along with some custodial uses.

Energy and Atmosphere

Energy use in buildings has been, and continues to be, one of the primary focus areas for increasing the sustainability of buildings. This includes conservation and efficiency strategies for new and existing buildings with metal buildings able to address all relevant aspects.

Optimized Energy Performance

Since a major component of sustainable green buildings is the responsible use of energy, LEED offers up to 18 credits in this category toward achieving some of those credits. Metal buildings also provide the ability to design the envelope for energy conservation and reduce energy demand. This is done through the proper levels of insulation, mitigation of thermal bridging, and controlling air leakage in opaque wall, floor, and roof areas of building enclosures. It also includes attention to details at fenestration, openings, and penetrations in these opaque areas to address the continuity of building enclosure barriers.

Energy codes provide minimum requirements for insulation in exterior walls and roofs and recognize metal building systems as a distinct construction type. However, manufacturers and architects often exceed those requirements through design decisions that promote higher levels of energy efficiency. Any of a multitude of high-performance insulation options are possible—from mineral fiber batts to rigid boards or even spray-on insulation—all of which can meet or exceed energy conservation requirements. The most commonly used insulation products for metal building systems help control condensation, thus reducing the effects of corrosion to the metal building or degradation of the insulation, all of which helps maintains overall energy performance.

Standard methods to achieve higher insulation levels have been developed for the metal building industry. Instead of using only a single layer of fiberglass insulation, it is now routine to use a double layer as recognized in ASHRAE 90.1 section A2.3.2.2: “The first rated R-value of insulation is for insulation installed perpendicular to and draped over purlins. The second rated R-value of insulation is for unfaced insulation installed above the first layer and parallel to the purlins and then compressed when the metal roof panels are attached. A minimum R-3 thermal spacer block between the purlins and the metal roof panels is required unless compliance is shown by the overall assembly U-factor.” Other options, such as foam plastic insulation may also be used whether in the form of rigid boards or spray foam in conventional or hybrid insulation manners. In some cases, insulated metal panels (IMPs) are also a very good option. This approach consists of a thermally broken panel with rigid insulation sandwiched between an inner and outer metal face that serve as interior and exterior surfaces.

In terms of coordinating energy criteria with other code requirements such as fire resistance, MBMA recently released six new UL fire-resistance-rated designs for wall-roof joints and intersections in metal buildings. These meet more stringent energy code requirements while maintaining fire safety. The new head-of-wall assemblies still call for a layer of insulation between the metal roof panels and the top of the wall, but allow for additional insulation, glass fiber, or mineral wool, as well as a vapor barrier, as found in filled-cavity insulation systems and liner insulation systems. Overall, more than 20 fire-resistance-rated assemblies are contained in the UL fire resistance directory for metal buildings. They are also available on the Fire Protection page of the MBMA website.

Using the strategies described above, metal building envelopes can be insulated at (or well above) the base-energy codes. Combined with cool roof coatings, metal buildings can fully optimize energy performance as validated through energy models. Further, HVAC systems may be downsized within a thermally efficient building to help reduce purchased energy loads.

 

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Originally published in April 2022

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