Using Metal Building Systems to Meet and Exceed the Energy Code

High-performance results include insulation options and improved air-infiltration sealing
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Sponsored by Metal Building Manufacturers Association (MBMA)
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Climate Zones

The IECC and ASHRAE 90.1 recognize that different locations have different climate conditions that directly impact a building’s energy use. Therefore, they both identify eight different numbered climate zones: zone 1 in southern Florida progressing up to zone 8 in Alaska. It is important to look up which of these eight climate zones a building is being constructed in because certain energy-code requirements are different based on climate zones. Not understanding this point leads to confusion when the wrong criteria is being followed because the incorrect climate zone is referenced.

Metal building systems are a recognized type of construction in the energy codes with specific prescriptive requirements indicated.

Compliance Path Options

Both the IECC and ASHRAE 90.1 allow the design and construction team to choose one of three methods to demonstrate that the building meets the provisions of the relevant energy code.

  • Prescriptive requirement path: This is the most straightforward method but also the least flexible. Here, the codes identify specific construction components, elements, or systems all based on typical construction and state the minimum performance levels of each. In this method, the background calculations have already been done based on standard assumptions about the rest of the construction to achieve a baseline or minimum overall performance level for buildings.

    It is important to be aware that there are two forms of prescriptive requirements in the codes, mostly contained in tables that summarize them. The first set is based on R-values that are specifically for the insulation only and not the rest of the assembly. Alternatively, there are U-factor tables that apply to the entire assemblies. These are provided in the codes to allow the designers to use either one. The R-value tables already take into account the assumption of typical construction and set the insulation and other levels accordingly, albeit rather conservatively. If the designer prefers to calculate the U-factors of the total assemblies (i.e., insulation, sheathing, framing, siding, interior finish, etc.) are acceptable and, in some cases, may be more appropriate, particularly if any non-standard construction or unique materials are used. In using the U-factor approach, there are specific criteria that need to be followed and calculations submitted so that the claimed values can be verified by the code-enforcement officer if required. It should be noted that the traditional R-value to U-factor conversion equation, R = 1/U, is not valid regarding the insulation alone. In most instances, the assemblies identified in the U-factor calculations capture the effects of air films, insulation joints and compression, and unavoidable thermal bridges, while the R-value of the insulation alone does not directly include these influences.

  • Building envelope trade-off path: The codes recognize that sometimes a particular design or construction assembly may restrict the ability to meet particular prescriptive requirements in the building envelope. Hence, an alternate path allows for a deficiency in one part of the building envelope (i.e., too little wall insulation) to be made up somewhere else in the envelope (i.e., increased roof insulation) to achieve the targeted overall performance. Note that these trade-offs are limited to building envelope provisions only—mechanical and electrical system performance cannot be part of any trade-offs. The most common way to determine if the envelope trade-offs meet the minimum performance level compared to the prescriptive path is to use a free software program called COMcheck. This program has been developed by the U.S. Department of Energy (DOE) and allows designers to pick their location (climate zone) and the particular energy code they are showing compliance with (IECC, ASHRAE 90.1, year, etc.). It then asks for some very straightforward information to be input related to wall assemblies, fenestration, roof assemblies, and foundations. The program executes the calculations and indicates if the design as input passes or fails to meet the selected energy code, and by what percentage of difference. The COMcheck option has been embraced by the design community and code officials, so much that the DOE reports more than 90 percent of the states permit the use of COMcheck for purposes of demonstrating compliance with the adopted code.

  • Performance path: There will always be building designs that do not fit neatly into the mold of one of the paths above because of the building owner’s requirements for a more efficient building design, the use of non-standard construction methods, the use of unusual or complex forms, or of restrictions in terms of either the envelope or mechanical and electrical systems being used. The energy codes allow for all of these conditions by offering the alternative of having a complete computer simulation done to calculate the total annual energy performance of the building. In this case, the building design must first be modeled using all of the prescriptive criteria in the code to establish a baseline, comparative performance. Then, a second model is done using all of the proposed differences in the design. Comparing the two needs to show that the proposed design produces the same or better annual energy performance as the prescriptive baseline. The details of what computer programs are acceptable, how to establish the baseline, which items must remain the same, etc. are all clearly spelled out in both the IECC and ASHRAE 90.1. These details are different in each, however, so the correct process needs to be followed, and should be discussed with the code-enforcement officer ahead of time to ensure that everyone is on the same page. Note too that energy models used for LEED submission requirements are defined somewhat differently and will not normally be acceptable for code compliance without some modification to the baseline scenario.

Thermal Bridging

While the IECC and ASHRAE 90.1 have addressed some forms of thermal bridging, not all types of thermal bridges are identified in the codes. Thermal bridges occur when building insulation is interrupted by another building component or material that has a higher rate of conductivity, thus reducing the overall effectiveness of the insulation. A common example is the use of metal stud assemblies that only contain insulation in the cavity space. Heat is transferred directly through the studs, creating a thermal bridge between the inside conditioned space and the outside of the building. As a means to overcome this heat transfer, the prescriptive code requirements call for the use of continuous insulation based on the concept that the continuous insulation will break this transmission path through the assembly. It should be noted that ASHRAE 90.1 is currently developing specific thermal bridging requirements for its future edition of the standard, which will begin to address many other types of thermal bridges.

The energy codes recognize metal buildings as one of several common construction methods. Therefore, in the prescriptive insulation requirements, they acknowledge the different insulation options that are possible (see Table 1). In all of these cases, the intent is to provide insulation that mitigates the thermal bridging effect. For metal buildings, this typically includes holding a layer of insulation outside of the primary and secondary structural members but inside of the metal roof or wall covering. The code does not dictate any particular insulation material, just the performance that needs to be demonstrated. However, to achieve this performance, most manufacturers use a thermal spacer block to isolate the outer siding from the interior framing. These blocks are field installed but function the same way that thermal breaks do in aluminum glazing systems. The size of the thermal spacer block is directly related to the depth of the insulation being used, so it is very important that the architect and designers coordinate these sizes to achieve the intended outcomes. It should be noted that thermal spacer blocks serve a dual function in that they also prevent the compressed insulation beneath from distorting the pan of the standing-seam roof panel. Without these rigid blocks, poor aesthetics and bad modularity of the panels would result.

The principles behind insulating a metal building and overcoming thermal bridging are the same as for any other building: isolate the conductive materials (i.e., steel) from the exterior using a layer of insulation or through different construction techniques. Architects or other design professionals who need to understand the common ways this is done in the metal building sector are advised to access publications available from the Metal Building Manufacturers Association (MBMA) at www.mbma.com. In particular, the MBMA publications titled “Energy Design Guide for Metal Building Systems” and “Energy Code Compliance: A Guide for Metal Building Contractors” provide more detailed information on these topics.

Metal building systems can address thermal bridging and air-infiltration requirements of the energy codes.

 

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Originally published in Architectural Record
Originally published in December 2020

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