Air Infiltration

Many design and construction professionals, and prior versions of the energy codes, typically focused building envelope strategies toward heat transfer as the primary feature to be addressed. This includes insulation levels, amount and size of glazing, heat transfer through fenestration, and similar approaches. In more recent versions of the codes, there are also specific requirements to address the other major aspect of building envelope performance, namely control of air infiltration. This is based on the recognition that a well-insulated building can still perform poorly if unwanted air infiltration (i.e., drafts) is entering or exiting freely and counteracting the gains made in insulation. Since this aspect of energy performance is not easy to calculate in a building design without making some wholesale assumptions, the codes do not rely on computer programs to demonstrate compliance. Instead, there is prescriptive list of mandatory air-infiltration requirements that must be followed regardless of which of the three energy-code compliance paths are followed. Some of these requirements apply to aspects of all buildings (e.g., walls and fenestration), while others apply to specific conditions only when they exist in a building (e.g., loading docks).

The codes offer essentially two alternatives to show that a constructed building meets the air-infiltration requirements. For buildings within certain size ranges, an on-site air-infiltration test can be carried out. This involves following a standard protocol and a “blower door fan” that measures the pressure drop between the inside and the outside of the building when the fan is turned on. A computer processer then calculates the amount of sustained air infiltration at the specified pressure level. If the infiltration rate is too high, remediation measures (e.g., caulking, sealing, etc.) may be needed to bring it into compliance. Note that the codes allow for larger buildings to be segregated into smaller spaces to test and demonstrate compliance. In cases where the blower door test is not practical, the codes require that everything on the prescriptive list is in place and verified to the performance levels stated.

Daylighting

There is another aspect of the energy codes that is sometimes not clear to designers. There is no stand-alone energy-code requirement for daylighting to be used in commercial buildings. Rather, appropriate daylighting can be used to enhance building performance. In particular, the prescriptive requirements of the energy codes are based on a maximum glazed wall area (e.g., 40 percent in the IECC and ASHRAE 90.1). The percentage is for the whole building, not just for a single facade, but if the objective is to have daylight penetrate the interiors of a building floor plan, daylighting may help.

Under the codes, the glazed area can be increased up to an additional 10 percent of wall area, provided the design can demonstrate that clear “daylight zones” are created. These zones are defined for both wall and roof areas and carry some additional requirements. The first is that the glazing does not cause overheating of the building, so projections that shade the glazing and the use of glass that controls solar heat gain are described in the codes. The second is that the daylighting actually decreases energy usage. This means any electric lighting within a daylighting zone needs to have sensors and controls to dim or turn off the lights automatically when there is adequate natural light available.

Proper use of daylighting is a credit option under LEED and can help a building achieve certification. The principles for incorporating daylighting into buildings are essentially the same between the codes and LEED; however, the details are a bit different to show compliance. Therefore, a review of both the relevant energy code and LEED requirements and options is warranted to ensure the proper information and documentation is provided.

With an understanding of how metal buildings are designed, fabricated, and constructed as well as the relevant energy-code provisions, we turn our attention now to the areas of primary architectural significance: how metal building systems can provide high-performance wall and roof assemblies. In so doing, they can be used to comply with the minimum levels of energy performance or specified to achieve higher levels of performance for LEED and other criteria.

Daylighting can be incorporated into metal building systems in many different ways and coordinated with lighting controls to reduce electrical energy use in buildings.

High-Performance Walls in Metal Buildings

The exterior walls of metal buildings can be designed as part of a total metal building system package or a hybrid construction incorporating some traditional materials and assemblies. Either way, the level of energy performance is under the control of the designer in all of the following areas.

Insulated Opaque Wall Areas

Metal buildings can incorporate any of a multitude of high-performance insulation options, from fiberglass batts to rigid boards or even spray-on insulation. The only types commonly provided by the metal building system manufacturer are fiberglass insulation systems or insulated metal panels. The usual criteria of thickness, R-value per inch, and cost all come in to play when selecting the best insulation for a particular project. The nature of metal building wall systems, however, allows for more options in terms of accommodating the different choices and sometimes can more easily accommodate thicker insulation than standard stud construction for walls, all for a more economical installation.

Using insulated metal walls makes it easy to show code compliance. Since metal building systems are recognized as a typical method of construction, the prescriptive approach is very straightforward to use. This is true whether using the R-value approach of the insulation or the U-factor approach of the entire wall assembly. Since everything is based on known standards and principles, it is easy to show how a particular wall insulation design can exceed code requirements.

Fenestration

Several types and styles of windows and doors are available from metal building manufacturers, but the architect may also specify the fenestration based on commonly used products from other suppliers. These can include windows, storefront systems, or even curtain-wall systems where appropriate. This means that all of the standard and high-performance options for glass, glazing, and frames can be selected from and incorporated into wall facades. As such, the energy performance can be directly controlled to show code compliance or support higher levels of conservation. This wall fenestration can also be used for daylighting by spacing it appropriately along the walls to achieve the desired results. There are no inherent restrictions in fenestration placement by using metal buildings, and in fact there may be fewer compared to conventional construction since long-span structures may require fewer columns to work around in the walls.

Air Infiltration

Since the typical wall assemblies of metal building systems are fairly well known, it is reasonable to test these for air infiltration to determine what would be commonly found in the field. Accordingly, the MBMA conducted a series of tests at the National Association of Home Builders (NAHB) Research Center in 2011 to determine if typical insulated wall systems would meet or exceed the 0.04 cfm/ft² air-barrier limits for assemblies of materials provided in the IECC and ASHRAE 90.1. The results are summarized in the report titled “MBMA Air Infiltration Testing,” which utilized the ASTM E283 test method at various pressure differentials. The results that are summarized in Table 2 showed that typical metal building walls readily perform better than the energy-code-required air-leakage requirements. Part of this is achieved by the materials themselves (i.e., sheet steel, sheathing, insulation, etc.), and part is from sealing joints and connections using tape, sealant, gaskets, caulk, or other common construction measures.

An additional aspect related to air infiltration is loading docks on buildings. The IECC and ASHRAE 90.1 require the area around loading dock doors to be equipped with weather seals in some climate zones to restrict infiltration when vehicles are parked in the doorway. These seals are attached to the building and allow trucks to back up tightly against them so that air transfer between the enclosed loading dock area and the outside is cut off. This approach has become quite standard for loading docks constructed of all types of materials. Metal buildings are no exception to this and have been used very successfully in many applications.

Insulating a metal building walls can be done in a variety of ways both inside and outside of metal wall girts.

High-Performance Roof Systems in Metal Buildings

The basic principles of insulating and air sealing metal roof systems are the same as those for metal wall systems. The full variety of common roof insulation choices remain available, and air-infiltration performance is determined largely by attention to the materials used and sealing the joints between them. Nonetheless, there are some notable aspects of metal roof construction of which to be aware.

Different metal building wall assemblies were tested for air infiltration according to industry standards and found to be readily code compliant.