The Benefits of Metal Building Systems From a Whole Building Perspective
Special Design Considerations
Frame shape and peak location: Both elements can be important components of an architectural design. The majority of buildings supplied today are a traditional rectangular shape, yet many other shapes are possible: L’s, T’s, U’s, and even octagons. The majority of metal buildings supplied are single slope or gable buildings with the ridge on center with the frame. The peak can be moved off center, however, to almost any location on the frame. A single slope building can be positioned with the high or low side facing the front, depending upon drainage or architectural requirements. When frames are specified with multiple ridges or with a valley instead of a ridge, an interior drainage system will be incorporated by the metal building manufacturer.
Bay sizes: Almost any bay size is possible today. The architect’s vision can define the solution, with mixed-bay patterns within a building also available. When the building has interior columns, these can be of different spacing patterns as well, ranging from 10 feet to 65 feet.
Column shapes: Tapered perimeter columns are often the most economical choice, but straight columns provide custom options to meet project specifications and architectural vision. Perimeter columns are normally fabricated as I-shapes and can be provided with parallel flanges or web tapered. Interior columns can be supplied in many different shapes, including hollow structural shapes, hot rolled I-shapes, and built-up fabricated I-shapes.
Column heights: Heights can vary to provide a step-in roof elevation or an in-floor elevation.
Accessories: Structural and nonstructural accessories include: insulation, gutters, downspouts, roof ventilators, roof openings, interior liner panels, wall vents, wall openings, windows, storefront glazing of all types, overhead vehicle doors, canopies, skylights, clerestories, fascia, and trim. These elements add aesthetic variety.
Expandability: Metal building systems are easily expanded. This usually involves the removal of an end or side wall, the erection of additional structural frames, and matching the existing wall and roof coverings to the addition. Manufacturers routinely perform assessments to add on to a metal building system, including structural and roof drainage analyses.
Energy Efficiency and Acoustical Considerations
Energy considerations and controlling condensation are important when providing an efficient building. Condensation can lead to long-term detrimental effects, including corrosion and degradation of insulation. Condensation occurs when warm, moist air comes in contact with cold surfaces, such as framing members, windows, and other colder regions within the building envelope. Warm air, having the ability to contain more moisture than cold, loses that ability when it comes in contact with cold surfaces or regions. When this happens, excessive moisture in the air is released in the form of condensation. If this moisture collects in the insulation, the insulating value is decreased since wet insulation has roughly the same heat conductance as water. Condensation problems are most likely to occur in climates where temperatures frequently drop below 35 degrees Fahrenheit over an extended period of time.
Here are four practical ways to assure energy efficiency and protect against condensation within a wall or roof assembly:
- Monitor the air temperature inside and outside of the building, as well as the relative humidity.
- Install an integral vapor retarder on the warm side of the assembly (under winter conditions). The main function of the vapor retarder is to slow down the flow of moisture through a roof or wall assembly. Permeability is measured in perms. The lower the perm rating, the more effective the vapor retarder is. A 0.10 perm rating (minimum) is recommended for normal building conditions.
- Install sufficient insulation between the outside skin and the vapor retarder to keep the temperature of the vapor retarder above the dew point temperature inside the building.
- Consult a mechanical engineer to properly size the HVAC system to assure adequate ventilation and humidity control while considering the end-use and insulation level of the building. Today’s buildings are much tighter in terms of air-leakage due to changes in energy codes and project specifications. HVAC equipment must be properly sized to account for moisture removal in addition to temperature control, taking into account the building’s air-tightness. Oversized HVAC equipment may handle temperature, but it won’t run often enough to control moisture accumulation.
Acoustics: Another important consideration in selecting insulation is acoustics. Both the interior and exterior noise levels can be greatly reduced by several types of thermal insulation. The insulation’s noise absorption properties help prevent the transmission of exterior sound, such as rain, hail, aircraft, and traffic. In addition, insulation helps to quiet noise within a building by absorbing reverberating sound. This is especially important in manufacturing facilities where there may be particularly high noise levels. The type and thickness of insulation both have a large influence on the thermal efficiency of a building because insulation traps still air and slows down conductive heat transfer. Two types of insulation are commonly used for metal building systems: fiberglass and rigid foam board.
Fiberglass blankets are the most common type of insulation used in the roofs and walls of metal buildings because of their low cost, fire and sound resistance, and ease of installation. The insulation is often placed on the outside of the purlins and girts and the panels are applied over it, with higher performance systems adding thick insulation in between purlins and girts. In the case of a standing seam roof, a thermal spacer block may be placed over the purlins to reduce heat loss at that location due to the compression of the fiberglass blanket insulation. The American Society of Heating, Air-conditioning and Refrigerating Engineers (ASHRAE) publishes ANSI/ASHRAE/IES Standard 90.1, “Energy Standard for Buildings Except Low-Rise Residential Buildings.” It contains performance requirements for all commercial buildings, including typical metal building assemblies. The performance impact caused by the compression of the insulation is accounted for in the ASHRAE standard, making finding and selecting code-compliant metal building assemblies for any end-use building relatively easy.
A second form of insulation is rigid foam board, typically made from polyisocyanurate (poly-iso) foam. Most often, a metal liner panel is installed over the purlins, and then the rigid foam board is laid in place with a vapor barrier. The standing seam roof is installed over it. The roof clips rest on a steel bearing plate over the foam insulation and a fastener is installed through the board insulation into the underlying structure. This is often referred to as a composite roof. Similar details for using rigid foam board insulation are available for metal building wall applications.
Life-Cycle Analysis
The Athena Sustainable Materials Institute’s Impact Estimator for Buildings software is used by design teams across North America. It allows them to explore the environmental footprint of different material choices and core-and-shell system options. The Impact Estimator provides a cradle-to-grave life-cycle inventory (LCI) profile for a whole building, including the projected flows from and to nature: energy and raw material flows plus emissions to air, water, and land. The software is available as a free download through the Athena Sustainable Materials Institute.
In 2015, MBMA released a study, conducted by Walter P. Moore and Associates Inc., which compared the environmental impacts of a metal building system against other forms of construction using the Athena Impact Estimator software. The study compared the environmental impacts for the structure and building envelope in 10 case study buildings. These included both metal building systems and other forms of construction in three separate climate zones. The purpose of the study was to determine how metal buildings compare to other construction types in a whole building life-cycle analysis.
The study showed that, for the types of project where metal buildings are most economical, they typically also perform better in life-cycle analysis and have the least material impact on the environment. Metal buildings performed better than concrete, masonry and traditional steel construction types, especially for long span buildings. Metal buildings also showed lower environmental impacts when comparing structural and envelope materials to load bearing masonry walls, concrete tilt-up, and traditional steel framed construction for structures with the same building footprint and functional equivalence.
Image courtesy of MBMA
The International Accreditation Service (IAS) is a wholly owned subsidiary of the International Code Council (ICC). IAS developed the Accreditation Criteria for Inspection Programs for Manufacturers of Metal Building Systems (AC472). This comprehensive, third-party accreditation program is based on the requirements of Chapter 17 of the International Building Code (IBC) and provides building owners, specifiers, and code officials with a means to approve the inspection programs of manufacturers that fabricate metal building systems.
Tom Gilligan, past chairman of the MBMA, offers his perspective on the AC472 program: “Imagine building manufacturer having their facilities, policies, and processes inspected twice a year by two independent parties—both the International Accreditation Service and an independent IAS Accredited Inspection Agency. Both apply strict criteria and run through a checklist of hundreds of items to confirm that everything within the manufacturer’s facilities are running according to absolute quality standards. Accreditation is not an easy process, but our industry has embraced it because quality assurance is of the utmost importance to the industry, the design public, and our end-customers. The benefits of the program are tremendous. AC472 should be a mandatory requirement in every architect’s specification.”
Some highlights of the program follow:
- Monitors manufacturing documents, design and detailing, and raw material usage and tracking.
- Monitors quality assurance and verifies the credentials of a manufacturer’s technical team, such as licensed engineers, certified welders and inspectors.
- Requires two on-site plant inspections per year.
- Provides compliance to International Building Code (IBC) Chapter 17 (Special Inspections) requirements. (AC472 accredited companies are considered approved fabricators.)
- Manufacturers provide letters of certification and erection drawings for each project.
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