Part 1 - Introduction
By
Sean M. O’Brien, P.E., LEED AP
Michael B. Waite, LEED AP
1. Introduction
As energy resources are diminished, energy costs increase, and the detrimental environmental effects of combustion emissions become more pronounced and better understood, optimizing building energy performance becomes essential to building owners, the global community and the planet itself. The building design and construction industry is becoming more aware of the effect of the building enclosure on energy use, particularly with the increasing prominence of “green” building rating systems, such as LEED, higher stringency in energy efficiency standards, such as ASHRAE 90.1, and stricter enforcement of building energy codes. However, with only a few exceptions, rating systems, standards and codes in the United States all but ignore air barriers. And, despite numerous studies and extensive analysis showing the benefits of air barriers, and many architects and engineers espousing the advantages of reducing air leakage, the industry has been slow to adopt air leakage control as a priority, producing buildings with admirable intentions, but marginal performance.
2. Energy
Enclosure air leakage can increase heating and cooling energy use of buildings. Many buildings are designed to maintain a slight positive air pressure (relative to the exterior environment), so the greater the air leakage through the enclosure, the greater the volume of ventilation air necessary to maintain the required pressure differential. Typically, this air needs to be either heated or cooled to reach the system’s supply air temperatures. In some cases, the air may need to be dehumidified even when HVAC zones demand heating, which requires the ventilation air to be cooled (and dehumidified) before it is heated.
Air infiltration due to negative building air pressure in southern climates, or during the summer throughout most of the U.S., carries both heat and moisture into buildings. This can result in moisture-related problems, but also increases the burden on the building’s cooling systems. Air conditioning systems will be required to provide more sensible and latent heat removal than would otherwise be needed with lower levels of air leakage. Consequently, the balance between latent and sensible cooling may be different than that predicted for design conditions, especially since exterior air will often contain significantly more moisture than interior air. This may result in mechanical systems being unable to maintain the required interior temperature or relative humidity levels without operating more frequently.
The effects described above are significant, but the most substantial impact of air leakage on building energy use is generally an increase in building heating requirements in cold climates. Verifying this effect is simple: stand by a leaky window on a cold windy day or feel the air rush in behind you when opening the door of a tall building during the winter. The negative implications of air leakage are not limited solely to cold northern climates. A 1995 study performed by NIST (National Institute of Standards and Technology) found that 15% of the heating load in commercial buildings nationwide is caused by air leakage. The conclusions of this study are particularly noteworthy because the authors found that, perhaps counter intuitively, this percentage is higher for newer buildings than for older buildings. This does not necessarily mean that air leakage is higher in newer buildings, but it likely indicates that other enclosure characteristics (e.g. insulation levels and passive solar heating design concepts) are improving.
3. Moisture Problems
Moisture-related problems due to enclosure air leakage can be caused by humid interior air coming into contact with cold surfaces while exiting the building, or by humid exterior air finding a detrimental path into the building. As with many issues with air leakage, the problems typically occur at the details, such as transition areas between envelope components and assemblies, rather than in the field of the wall.
When buildings or portions of buildings are under negative pressure, the air flow is inward. As discussed above, exterior air can contain a significant amount of moisture. When this air is pulled into the building through wall cavities, ceiling plenums or similar spaces, moisture in the air can condense if it passes over cool surfaces, which are common in air-conditioned spaces. Water can accumulate on materials that are known to be conducive to mold growth, such as paper-faced gypsum wallboard.
When building or portions of buildings are at positive pressure relative to the exterior air, the air flow is outward. In most buildings, this may not be a problem as interior humidity levels are not high enough in the winter to cause significant problems. However, in high humidity buildings, such as museums and natatoriums (pool structures), the interior air contains ample moisture to cause condensation if it reaches cold surfaces. The most visible cases of condensation in these buildings are where thermal bridging – high conductivity materials bypassing low conductivity materials – occurs, such as at window frames and structural members. In positively pressurized buildings with air leakage paths to the exterior, humid air often “finds” hidden cold surfaces within the enclosure. The air flow paths are typically within concealed spaces, allowing moisture to accumulate over time undetected, and potentially causing catastrophic failures.
Over the past few decades, the building industry has developed some understanding of the causes of moisture-related problems. However, much focus has been placed on vapor retarders without adequate attention to the effects of air leakage. Vapor retarders are intended to prevent the migration of moisture through building materials by diffusion. In many cases, drawings call for vapor retarders where a continuous air barrier system is necessary. Humid air can carry up to 100 times more moisture than can be transferred by diffusion through typical porous building materials. The discrepancy between the widespread application of vapor retarders and the generally poor design of air barriers is likely due to the prevalence of vapor retarder requirements and the near-complete lack of air barrier requirements in U.S. building codes.
4. Codes and Standards
State building codes typically reference or adopt ASHRAE Standard 90.1 – Energy Standard for Buildings Except Low-Rise Residential Buildings, or the International Energy Conservation Code (IECC), which references ASHRAE 90.1 itself. The most widely used and referenced energy codes and standards have well-developed requirements for wall and roof insulation, and for glazing assembly thermal transmittance and solar heat gain. They contain requirements for maximum allowable conductive thermal transmission through the envelope. Every few years these values are evaluated and, often, slightly increased. These requirements may be approaching – many would argue they have reached – a point of diminishing returns for which very little additional performance is gained by adding insulation or by reducing solar heat gain through glazing.
Neither the IECC nor ASHRAE 90.1 contains quantitative requirements for air barriers. While there are requirements that seams and transition areas be sealed, these provisions are difficult to enforce and contain no performance requirements for the sealing materials. The lack of air barrier requirements in energy standards is a significant oversight, as reducing air leakage can have a greater impact on building energy use than the incremental increases in insulation or reductions in glazing SHGC typically included in new editions of codes and standards.
Voluntary guidelines are also falling short of their stated goals. LEED, a widely accepted guideline for sustainable building design, references ASHRAE 90.1 for its energy efficiency prerequisite and credits, but contains no separate provisions specific to air barriers. While some benefit can be obtained through the LEED energy performance credit, the Performance Rating Method – outlined in ASHRAE 90.1 and used by LEED to compare a proposed building design to a “baseline” building – requires that a building not be given credit for reducing air leakage (i.e., designers can specify an air leakage rate, but that rate must be the same for both the baseline and the “improved” buildings). This process used to exhibit anticipated improved energy performance prescribes that the air infiltration input in the energy model be the same for the proposed design and the baseline building.
Some states have begun adopting air barrier requirements, yet even the most stringent among them do not meet the code provisions implemented in some other countries (e.g. Canada and the UK), or the recommendations of enclosure design professionals and energy efficiency experts. The U.S. Army Corps of Engineers recently began requiring air barriers (and quantifiable enclosure air leakage performance) for all new and renovated buildings, and federal building design guidelines now contain air barrier provisions. However, the lack of a national standard air barrier requirement prevents the concept from taking hold on the scale we have seen with insulation, thermally-broken windows and vapor retarders.
5. Summary
The importance of air barriers in building design is clear and well-documented. This is particularly true now that enough of the the “low-hanging fruit” in energy efficient enclosure design – insulation and improved glazing performance, for example – has been picked, bringing us near the point of diminishing returns on those improvements. Sustainable building design considerations – an essential aspect of all building design – can no longer be compartmentalized. As with other building features, air barriers need to be designed and constructed as a system, and integrated with all other building systems.
The next article in this series will discuss methods and materials that can be used to produce acceptable enclosure air leakage performance.
Part 2 - Design Guidelines for Air Barrier Systems
By
Sean M. O’Brien, P.E., LEED AP
Michael B. Waite, LEED AP
1. Air Barrier Basics
The basic function of an air barrier system is to prevent uncontrolled air leakage through the building enclosure. To this end, an air barrier must be a complete system of materials and components that work together to provide a continuous barrier to airflow. Even small discontinuities in an air barrier can significantly reduce its performance, since air will follow the path of least resistance regardless of its location. Air barriers must resist air pressure caused by wind, stack effect, or mechanical pressurization of a building, so they must also be relatively rigid or have solid backing capable of resisting moderate to high pressures.
Unfortunately, the system concept is often ignored and designers “specify an air barrier” by including a specification section for self-adhered or spray-applied membranes or spray-applied foam insulation – sometimes as an addendum or afterthought once the rest of the building has been designed. This article discusses performance criteria for air barrier systems and highlights common problems found in air barrier specifications.
2. Air Barrier or Vapor Retarder?
A major stumbling block for many designers has been appreciating the difference between air barriers and vapor retarders. This is made more difficult by the fact that many air barriers are also vapor retarders, such as the ubiquitous “peel-and-stick” membranes that are used in some way, shape, or form on nearly all new construction projects. The danger in confusing these two systems is that the proper location for a vapor retarder is dependent on both the interior and exterior environments, while an air barrier can typically be located anywhere within the building enclosure as long as it is continuous. Table 1, below, summarizes the differences between air barriers and vapor retarders.
Table 1 – Summary of Differences Between Air Barriers and Vapor Retarders
| Vapor Retarder | Air Barrier | |
| Purpose | Control of water vapor flow via diffusion through building materials. | Control of water vapor flow via air movement, primarily through gaps or cracks in the building enclosure. |
| Requirements for Continuity | Does not need to be completely continuous; can contain small gaps, holes, or unsealed laps without significant loss of performance. | Must be continuous to be effective; even small discontinuities can significantly affect performance. |
| Location | Typically installed on "warm-in-winter" side of insulation (some exceptions apply depending on climate). Improper location can exacerbate condensation problems. | Can be installed anywhere in the building envelope if vapor permeable, otherwise follow guidelines for vapor retarder. |
| Structural Support | No structural support. | Must be continuously supported and be capable of resisting forces from wind, mechanical pressurization, and stack effect. |
| Detailing | Minimal detailing required to achieve design intent. | Careful detailing of transitions and changes in material are necessary to support proper system installation and meet the design intent of an air barrier system. |
Another area of confusion is the concept of air barrier materials vs. air barrier assemblies and systems. The following definitions are presented to establish the difference between materials, assemblies, and systems :
• An air barrier material is a primary element that provides a continuous barrier to the movement of air (e.g., self adhered membranes)
• An air barrier assembly consists of the air barrier materials and accessories that provide a continuous designated plane of resistance to the movement of air through portions of building enclosure assemblies. Air barrier assemblies typically consist of both air barrier materials and connections to adjacent materials, as well as penetrations, laps, seams, etc.
• An air barrier system is the combination of air barrier assemblies installed to provide a continuous barrier to the movement of air through building enclosures.
3. Performance Criteria
Specifying appropriate (and more importantly, achievable) performance criteria for air barrier assemblies and systems is a surprisingly challenging task. Established performance criteria exist for nearly all aspects of the building enclosure, such as windows and doors, curtain walls, and roof systems. This is the result of many years of work by designers, testing companies, and industry organizations. Over the course of several decades, unrealistic or unverifiable performance criteria such as “windows shall not leak under any conditions” have gradually been replaced by criteria such as “windows shall not experience water leakage at a test pressure of 5.5 pounds per square foot (psf) when tested according to ASTM Standard E1105”. Performance criteria for air barrier assemblies and systems are still developing; the third article in this series summarizes the current testing and performance standards for air barriers.
3.1 Materials
Although performance criteria for air barrier materials are relatively well established, problems with air barrier systems rarely develop as a result of air leakage through the field of an air barrier sheet or membrane. Further, air barrier products such as spray-applied or self-adhered membranes often have leakage rates that are orders of magnitude lower than the generally accepted criteria of 0.004 cfm/sf at 0.3 in. water for air barrier materials , making leakage through the field of the barrier unlikely to be a significant problem.
3.2 Assemblies
Since most air leakage occurs at details and transitions, the air permeance of the primary air barrier material(s) is often unrelated to the overall air leakage through a building. The Air Barrier Association of America (ABAA) recommends a maximum air leakage rate of 0.04 cfm/sf at 0.3 in. water for air barrier assemblies, which takes into account seams and penetrations and is more representative or real building conditions. The 2005 National Building Code of Canada recommends (but does not require) a slightly more conservative value of 0.02 cfm/sf at 0.3 in. water for buildings that maintain interior relative humidity levels between 27 and 55% - typical of most buildings with the exception of cold storage facilities and natatoriums. Since even “seamless” systems such as fluid-applied membranes will still have transitions and penetrations, such as brick ties or other cladding attachments, applying the more stringent air barrier material criterion to air barrier assemblies is unrealistic.
The effects of air leakage through windows, doors, and curtain walls (i.e., fenestration) are rarely considered when evaluating air barrier assemblies. This is a significant oversight, as most of the air leakage through a properly designed air barrier system will likely occur through these components. Established values for air leakage through fenestration range from 0.06 cfm/sf at 1.2 in. of water for glazed curtain walls to 0.4 cfm/sf at 1.2 in. of water for operable windows. Maximum air leakage rates are included in most building/energy codes as well as industry standards from organizations such as ASHRAE and AAMA. “Typical” values for air leakage through fenestration are somewhat difficult to determine as the various (local, state, and national) codes and standards attempt to reach a consensus.
Although the test procedures for air barrier assemblies (ASTM E2357) include the air barrier connections at windows, the window opening itself is “blanked off” during the test so that only the perimeter is evaluated. Consider the example of a 10 ft x 10 ft air barrier assembly containing a 4 ft x 4 ft double hung window. Specifying a maximum assembly leakage rate of 0.04 cfm/sf would result in an allowable airflow of 4 cfm through the assembly. For a typical code-compliant window meeting the performance criteria of 0.4 cfm/sf, the window leakage alone would be 6.4 cfm, exceeding the allowable value for the entire assembly without even considering leakage through other air barrier components. If a window is specified in assembly testing, a modified value for assembly leakage that considers the inherently “leakier” windows must be used.
3.3 Systems
To account for the wide range of materials, details, and transitions in the air barrier of any particular building, it is often more useful to speak in terms of system (i.e., whole-building) air leakage than material, assembly, or component leakage. This is especially true for purposes of energy simulation or HVAC load calculation, where the global quantity of air leakage is the primary concern. Unfortunately, there are very few established standards for whole-building air leakage that designers can reference. The 2009 ASHRAE Handbook of Fundamentals, Chapter 16 notes three “levels” of air leakage for typical buildings. These are 0.1 cfm/sf at 0.3 in. water for “tight” buildings, 0.3 cfm/sf for “average” buildings, and 0.6 cfm/sf for “leaky” buildings. These general classes of air leakage were first presented in the results of a study of 8 commercial buildings in Canada, ranging in height from 11 to 22 stories, and clad with glazed aluminum curtain walls. Despite being based on a small sample size and very specific building types, these “classes” are frequently cited in discussions of typical building airtightness or building performance criteria. A more recent study of approximately 200 low rise commercial and institutional buildings in the United States found an overall average leakage rate of 1.55 cfm/sf at 0.3 in. water – over 5 times greater than the “average” value of 0.3 cfm/sf noted above. Unfortunately, neither study clarifies if the buildings were designed with continuous air barriers. Considering this limitation, the average value of 1.55 cfm/sf from the 2005 study could be seen as a maximum value for building air leakage, as a new building with a dedicated, continuous air barrier is likely to provide greatly improved performance. For buildings designed and constructed with continuous air barriers, ABAA currently recommends an overall building air leakage rate of 0.4 cfm/sf at 0.3 in. of water. More stringently, the U.S. Army Corps of Engineers specifies a maximum leakage rate of 0.25 cfm/sf at 0.3 in. of water for some of their projects, and is considering the use of that criteria as a standard for all new buildings (although no formal design guide has been developed to include airtightness criteria at the time of this writing).
In 2002, the United Kingdom added a requirement for whole building/system air leakage to their “Building Regulations for England & Wales” for commercial buildings greater than 10,760 sf (currently 5,380 sf in the 2006 code). The established value, which is required to be verified through whole-building testing, is 0.547 cfm/sf at 0.2 in. of water. Preliminary findings have shown a marked improvement in airtightnes of more “standardized” building types such as warehouses and retail stores, with many buildings exceeding the code-required value. This is a significant improvement in airtightness, as typical values for air permeability of the same building types prior to the 2002 code change were on the order of two to three times higher than values achieved in recent years. However, less standardized building types, such as offices, schools, and hospitals, have exhibited a much lower “passing” rate. This is most likely attributed to the general lack of attention to air barrier detailing at conditions for which typical practices are not well established, in contrast to less unique building types for which a large body of detailing experience exists.
Given the results of recent studies in the United States, the average commercial building significantly exceeds this target. Until additional studies of more recently constructed buildings (designed with continuous air barriers) are available, system air leakage criteria may be difficult to enforce due the lack of knowledge about what level of air leakage is typical and achievable for new buildings. In addition, the acceptability of leakage criteria is likely to fluctuate as new data becomes available and more testing is performed.
4. Specifications of Air Barrier Systems
At present, there is a significant disconnect between the criteria contained in most specifications for air barrier system and the actual performance achieved in the field. This is due to a combination of factors, including designers’ unfamiliarity with air barrier systems, poor understanding of how air barriers function, and misunderstanding regarding test procedures and limitations.
The Air Barrier Association of America has proposed a new specification section to establish the administrative and procedural requirements necessary for the construction of a complete air barrier system in a new building. Since the air barrier system consists of multiple materials covered under several specification sections (including windows, doors, curtain walls, roofing systems, and exterior wall air barriers), this specification seeks to establish some kind of connection between the various sections and provide a means of coordinating the different trades involved in the construction of the air barrier system. As the popularity of air barriers has grown in the past few years, so too have the number of projects where air barriers were added to the scope during design (or even early construction) by the inclusion of a single specification section for a sheet or spray-applied membrane. This approach creates an air barrier in name only, and does not address the numerous connections of that material/assembly to other components in the building. Details such as window perimeters and roof-to-wall joints are critical to the performance of the overall air barrier system, and require much more coordination and planning than is likely to happen during the construction process, when trades may be running behind schedule and design/consulting budgets may have been exhausted.
The following common mistakes should be avoided when specifying air barrier systems:
• Failure to coordinate air barrier components, such as specifying windows that are difficult to integrate successfully with the air barrier, or specifying air barrier materials or assemlblies with conflicting performance. Since the air barrier is only as strong as the weakest component, specifying high performance windows in a building with a poor (or no) air barrier will do little for overall airtightness. The same is true of specifying an air barrier in a wall but not in the adjacent roof, while requiring the entire building to pass an airtightness test. Quantitative testing of air barrier systems installed as part of a building addition where the “base” building has no such systems is also generally of little value, unless the addition is separated by airtight interior partitions to make it a truly separate volume.
• Failure to provide sufficient details for the air barrier system, especially at critical locations such as window perimeters and roof-to-wall interfaces. Many specifications provide only general information or do not show sufficient detail on the drawings, but may include language intended to place the detailing design burden on the contractor. Air barrier systems are complex and require careful design to be effective. Just as we would not allow the contractor to design the structural system for the building “on the fly”, it is unreasonable to expect contractors to assume the role of primary designer of the air barrier details.
• Specification of impossible or unrealistic test criteria. Some specifications require that air barrier assemblies be tested in the field to verify performance, but do not take into account the numerous issues associated with qualitative testing that may make testing impractical or unlikely to yield useful results. Some specifications contain incompatible test criteria, such as including a window in the air barrier assembly that is tested but not adjusting the assembly criteria to account for the inclusion of that window. Given the differences between criteria for windows and criteria for air barrier assemblies, it may be impossible to meet the “typical” assembly leakage of 0.04 cfm/sf due to leakage at the window.
• Specification of system performance criteria that are not backed up by research or practical experience. Given the lack of whole-building airtightness data on relatively recent buildings that include air barrier systems, specifying a system leakage rate can lead to confusion or disagreement if the building fails to achieve the test criteria. Without realistic established values for system leakage (with the exception of the 2006 United Kingdom Building Regulations, which is still in its infancy), it may be difficult to enforce compliance with a seemingly arbitrary requirement.
Next article in the Air Barrier Systems series: Field Testing of Air Barrier Systems
Part 3 - Field Testing of Air Barrier Systems
By
Sean M. O’Brien, P.E., LEED AP
Michael B. Waite, LEED AP
1. Why Test?
For some enclosure systems such as thermal insulation, in-place performance is generally consistent with calculated performance. However, physical testing is often the only way to accurately assess the installed performance of air barrier systems. The sensitivity of air barriers to workmanship (e.g., sealing laps, making transitions, etc.) and their potentially large impact on building energy use, make in-place performance testing an important quality control measure for air leakage assessment of specific details, and verification on a whole building (or “system”) level.
2. Test Methods
2.1 Quantitative Testing
In this arrangement, the measured airflow is a combination of flow through the specimen and the test chamber. To separate these flows, the test (using an interior chamber) is initially performed with the exterior of the specimen sealed off, typically with an impermeable sheet material. After this initial test, a second set of measurements is taken with the specimen unsealed. The difference in measurement between these two tests is the leakage through the specimen. Chambers are typically constructed on the interior of the specimen for practical reasons (e.g., access), although they can technically be located on either side. For typical punched windows, air leakage, on the order of several cubic feet per minute CFM), can be measured reliably with equipment designed for use in the field.
Air barrier assemblies, consisting of several components, can be tested in a similar manner, but the testing is generally more difficult for several reasons. First, air barrier assemblies are typically much larger than discrete components such as windows and doors. Second, air barrier assemblies may contain unique geometries that make construction of an air-tight test chamber difficult, such as parapets, structural members or slab edges that interrupt the test chamber (or in the case of steel studs, create so many penetrations in the chamber that it must be constructed from the exterior). If performed during construction, scaffolding or other temporary constructions may interfere with access to the assembly. Third, testing of complete assemblies may not be practical due to the installation of different materials at different times, as in the case of a wall air barrier being installed long before the roof air barrier.
2.2 Qualitative Testing
Since a primary goal of air barriers is the reduction in air infiltration and corresponding reduction in heating cooling loads, a useful value to designers is typically the system air leakage rate. Knowing the leakage rates through individual components can be useful for verifying component performance (typically windows and curtain walls) or comparing the relative performance of existing vs. replacement components in retrofit applications, but for new construction these rates are less critical to determining overall building performance than the whole-building value. At the component or assembly level, knowing where air leakage occurs is often far more useful than knowing how much leakage is actually occurring, especially in the case of high humidity buildings, such as museums and swimming pools, where even small air leaks can cause significant condensation. Qualitative testing has the advantage of providing installers with the locations of defects in the air barrier that require repairs. Providing an air barrier installer with an air leakage rate through the overall system provides little practical information (e.g., locations of major air leaks) on how the system can be improved.
The test chambers and setups used for quantitative testing can be used for qualitative testing as well. However, chambers for qualitative testing are often easier to construct since then need only provide a level of airtightness sufficient to achieve the desired test pressure; this is in contrast to chambers for quantitative testing, which must be relatively airtight to allow for accurate measurement of the airflows in question. A basic test chamber can be constructed on the interior of the component (Photo 4), connected to a fan and differential pressure gage only. The chamber is typically necessary in new construction projects, where the building enclosure is still relatively open to the elements. For enclosed, or partially enclosed, buildings, a blower door or similar device can be used to place whole rooms or whole buildings under positive or negative pressure, eliminating the need for a specially-constructed chamber. Rather than measure the air leakage directly, visualization aids such as tracer smoke or infrared thermography (if temperature conditions allow) are used to locate leaks in the air barrier system
Tracer smoke is a relatively simple method of locating air leaks while a specimen or area has a pressure differential applied. The smoke will quickly reveal air leakage paths as it is drawn into gaps or blown away from them (Photo 5). An alternate method is to use a large smoke generator within the test chamber so that smoke is “blown out” through any breaches in the system.
3. Summary
A variety of test methods are available for both quantitative and qualitative evaluation of air barrier systems. A summary of the most common methods is provided in Table 1. Understanding the special requirements for these procedures, as well as the difficulties and limitations associated with the tests, are critical for performing successful tests that produce accurate and meaningful results.
Table 1 – Common Tests for Air Barriers
| Standard | Type | Tested System |
|
ASTM E2178 - Standard Test Method for Air Permeance of Building Materials |
Quantitative – Laboratory Test | Air Barrier Materials (membranes, etc.) |
| ASTM E2357 - Standard Test Method for Determining Air Leakage of Air Barrier Assemblies. | Quantitative – Laboratory Test | Air Barrier Assemblies (membrane, including laps and penetrations but not including fenestration components) |
| ASTM E283 - Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen | Quantitative – Laboratory Test | Fenestration components |
| ASTM E783 - Standard Test Method for Field Measurement of Air Leakage Through Installed Exterior Windows and Doors |
Quantitative - Field Test |
Fenestration components; can be modified to test air barrier assemblies |
| ASTM E1186 - Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems | Qualitative - Field Test | All air barrier materials, assemblies, and components |
|
ASTM E779 - Standard Test Method for |
Quantitative - Field Test | Air barrier systems (i.e., whole buildings) |
Part 4 - Energy Analysis
By
Sean M. O’Brien, P.E., LEED AP
Michael B. Waite, LEED AP
The first article of this series discussed the problems with excessive air leakage, detrimental air leakage paths through the enclosure, and pressurization for certain building types and environments. In subsequent articles we presented materials and design strategies to minimize air leakage, and discussed how to quantify enclosure air leakage in existing buildings, and the challenges this poses. This article addresses the difficult question of how to predict the amount of air leakage in a new design, and the implications of inaccurate quantification of air leakage in new and existing buildings.
Underestimating air leakage could result in undersized mechanical equipment, but this is not typically the case since mechanical engineers employ safety factors in the design of cooling and, especially, heating systems. With the evolving implementation of sustainable design practices and the understanding of the effects of peak building energy on the required capacity of on-site equipment and the nation's energy infrastructure (i.e. the number of power plants), more sophisticated tools have gained prominence in sizing mechanical equipment and predicting building energy use.
By employing more advanced tools, there is a potential for smaller safety factors and, thus, smaller equipment. However, smaller safety factors coupled with inaccurate assumptions about building performance may cause problems. Complicating matters, building owners and designers often expect energy analyses developed during design to be absolute predictors of building energy use. This expectation is generally unrealistic due to the wide variation in actual versus assumed building operation. In addition, mischaracterizing air leakage rates will reduce the accuracy of energy models and may adversely affect design decisions that depend on whole building energy analysis results.
A theme throughout this series has been the importance of designing and understanding an air barrier as a system. In much the same way, the building enclosure itself is a system made up of many component assemblies, such as fenestration, insulation, waterproofing and air barriers. And, as discussed previously, the overall building is a system with far too many interdependent performance characteristics to allow for compartmentalization of our design evaluations.
Whole Building Energy Analysis
The use of whole building energy analysis (often referred to as "energy modeling") to evaluate design options is becoming increasingly widespread throughout the building design community, and particularly in high performance building design. Energy modeling allows users to include many more aspects of building design and operation than traditional HVAC sizing approaches. Naturally, it is very dependent on the accuracy of those inputs. No matter how simple or complex the tool, its performance relies on the user's understanding of the capabilities and limitations of that tool. The complexity and interdependence of building systems require that engineers constructing energy models understand how the systems interact and how the inputs and assumptions for individual systems relate to other systems and affect the predicted building performance as a whole.
Some practitioners in the building industry may expect energy models to be absolute predictors of building energy use. However, software developers (and most users) make no such claims. Still, the presence of engineers (or "modelers" who are not engineers or architects) who claim the ability to predict actual energy use has provided fodder for energy modeling's detractors. Though this article does not aim to examine - and certainly not adjudicate - the current deliberations in the industry over the proper role of whole building energy analysis, we think it is important to understand these tools have their shortcomings, but also their advantages. To evaluate design options, system control schemes and building operation considerations, whole building energy analysis is beneficial in its primary role as a comparative tool.
The assumed level of air leakage affects both the predicted energy performance of a building and what systems may appear attractive to a designer. That is, the relative performance of some energy efficiency measures is affected by the assumed air infiltration. The first item here is understood, and perhaps even intuitive, to the majority of designers. Mischaracterizing the air leakage performance of the enclosure in an energy model will likely affect the building energy use predicted by that model. The effect will be particularly pronounced in heating-dominated climates, where the effect of air leakage is more significant.
Probably less intuitive is the fact that incorrect air leakage assumptions, even if they are consistent across all analyses, can affect the predicted improvement or increase in energy use for the design option (or combination of criteria) being evaluated. At higher leakage rates, the heating and cooling requirements are higher. Since the absolute increase or reduction in energy use is similar under most conditions, the relative effect of air leakage will be less at higher leakage rates (the denominator in the equation increases while the numerator remains the same). Generally, however, the opposite problem seems to present itself in many energy models we have seen: The predicted air leakage rate is much lower than that of the actual building. This can result in overstated energy savings, which can affect the economic analyses for a project.
In most climates, the effect of inaccurate air leakage modeling on the evaluation of design options will not be significant enough to significantly influence the decision-making process. Existing buildings, as is often the case in energy modeling, present unique challenges. On existing building projects, we are often trying to evaluate and predict the reduction in energy use associated with "tightening" the building enclosure. In these cases, improvements in thermal (or solar heat gain) performance are often coupled to reductions in air leakage in the same energy efficiency strategy (e.g. replacing windows). The best approach is to evaluate the envelope modifications with a range of air leakage rates to gauge the sensitivity of the building's predicted performance to the assumed air leakage. We have found that utilizing the testing techniques discussed in the previous article to be effective in evaluating an existing building and envelope improvements. Whole building tests establish a baseline and component tests can be used to quantify the contribution of specific envelope areas to the overall leakage rate. By assuming a leakage rate for these components after upgrading the envelope, the whole building rate can be adjusted in the energy model.
Air Barriers in High Performance Building Design Standards
The first article in this series touched upon the rise in popularity of "green building" rating systems, such as LEED, and the increasing stringency of energy efficiency codes and standards, such as ASHRAE 90.1. We discussed the lack of air barrier requirements in most codes and standards and the absence of any credit in LEED rating systems for reducing air leakage. These systems - and the codes and standards themselves - rely heavily on whole building energy analysis to show compliance or to exhibit improved energy performance. Due to the issues discussed above, the energy savings associated with various energy efficiency measures may be misrepresented if air leakage is modeled inaccurately. That said, the effect this has on certification under LEED, while worthy of discussion, is a secondary concern. A good, continuous air barrier is essential to acceptable enclosure performance in high performance buildings. However, the supporting infrastructure for projects attempting to achieve a sustainable design has not been formalized. The benefits of air leakage must be properly included in these standards and programs, and the high performance building design process must account for air leakage in whole building energy analyses.
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We have now developed an understanding of the importance of good air barrier design; outlined important design considerations, materials and methods; discussed approaches to quantifying and tracking enclosure air leakage in existing buildings; and presented the implications of inaccurate quantification or expectation of air leakage rates. We have touched upon the interaction between the enclosure and other building systems. We now must understand how a good air barrier changes overall building performance and how other systems need to be designed, constructed and controlled. The next article in this series will discuss the requirements for mechanical systems in tight buildings. We will focus primarily on how "business-as-usual" is often not an option and that code requirements may not be sufficient to provide an acceptable level of performance.
About the Authors
Sean O’Brien is a Senior Project Manager in the New York City office of Simpson Gumpertz & Heger Inc. Mr. O’Brien specializes in building science and building envelope performance, including computer simulation of heat, air, and moisture migration issues. He has investigated and designed repairs for a variety of buildings, from condominiums to natatoriums and art museums, and has published extensively on building science-related matters including moisture migration in masonry wall systems and condensation resistance of windows and curtain walls. He can be reached at [email protected]
Michael Waite is an engineer in the New York City office of Simpson Gumpertz & Heger Inc. He specializes in the interaction between mechanical systems and the building enclosure. He has designed and investigated a wide range of building types and has focused primarily on building energy performance, building enclosure design, and thermal and hygrothermal performance building enclosures. He is a member of ASHRAE SSPC 90.1 and its Envelope Subcommittee, as well as several other industry organizations. He can be reached at [email protected].