Large Building Airtightness and Appropriate Air Barrier Strategies
An airtight building enclosure is an important part of a modern building. Proper airtightness can increase energy efficiency, improve durability, and allow greater control over occupant comfort and indoor air quality.
Airtightness is achieved with an effective air barrier system that is carefully designed and detailed, and then built and commissioned in the field. It consists of a continuous system of materials (building wraps, membranes, etc.), components (doors, windows, etc.) and accessories (tapes, sealants, gaskets, etc.). These air barrier elements must be individually airtight, and airtight when used together.
Increasing standards for airtightness
Airtightness requirements in building codes and energy performance standards are becoming increasingly stringent across North America.
Historically, many codes and standards have included requirements for just the air permeance of materials and components in building enclosure assemblies. The expectation was that if airtight materials were used to construct the building enclosure, then the end result would be airtight.
However, experience has shown that the majority of air leaks occur at joints and interfaces between these air barrier elements, and as a result, the requirements for how airtightness is specified and measured have shifted. More recent codes and standards have now begun to also include targets for whole-building airtightness as a way of consistently achieving higher levels of performance. To achieve these targets, airtightness must be considered through all phases of a building project, from design through construction to completion. To confirm these targets have been met, airtightness testing is done to measure the performance of the complete and installed air barrier system.
The increasingly stringent airtightness targets have led to broadened discussions surrounding air barrier elements that go beyond material airtightness. Materials and components should be considered in the context of the system they are part of and not in isolation. Considerations include the specific air barrier details, material compatibility, and long-term durability. Given the wide range of materials and details, there has also been an increasing awareness of the benefit of robust quality control and assurance protocols to verify the compatibility of materials and substrates, as well as the practical challenges faced during construction.
How do you test for large building airtightness?
Many people may be familiar with air tightness testing for small buildings like houses where a single “fan-door” or “blower-door” piece of equipment is used to pressurize and depressurize the space. This type of testing has become commonplace across Canada and the US for weatherization or energy retrofit work in existing homes and for commissioning of air barrier systems in new construction.
Large building testing follows the same concept but is performed at a larger scale in terms of the necessary teamwork involved, and often requires the use of more than one high-powered fan door to perform a whole-building test.
In jurisdictions where large building airtightness testing is mandated, ongoing test results are valuable insights into the design and construction of air barrier systems. There have been shifts in local construction markets with preferences to higher-performance air barrier systems, and increasing reliance on manufacturers to provide compatible air barrier material solutions from the roof to below grade.
There has also been a shift in the design and selection of appropriate air barrier strategies for taller buildings which are more robust and able to accommodate the higher air pressures caused by wind, stack effect, and mechanical systems in taller buildings.
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Where do today’s tall buildings currently stand? How are they holding up?
While current energy performance and airtightness standards set the requirements for overall building airtightness, it is useful to compare against the results of previous whole-building airtightness.
This data provides context for what airtightness levels are expected and achievable in larger buildings, as they have different air barrier approaches and considerations from houses, for example. Historical data can be easily compared and analyzed using the Normalized Air Leakage Rate metric. In a recent study of whole-building airtightness testing, the results of several hundred large building airtightness tests from Canada, the United States, and the United Kingdom were compiled and analyzed.
The chart below shows the distribution of the results of this study for buildings tested in Seattle, where air tightness testing is mandatory. For comparison, the chart shows a relative indicator of the levels of airtightness found in each test, rated as exceptional, good, moderate, and poor airtightness. Testing showed that most buildings met the minimum air tightness requirements, with many far exceeding them and achieving excellent results.
In the same study, newer large buildings in jurisdictions with mandatory air tightness requirements (United States Army Corps of Engineers (USACE) and Washington State) were compared with those with no air tightness testing requirements.
As shown in the chart above, the results indicate that buildings that are built with the intent of meeting a minimum air tightness requirements can consistently achieve and even exceed the minimum target, while new buildings built with no minimum air tightness requirements are generally less airtight. For buildings that must meet the USACE and the Washington state requirements, the majority are well below the minimum air tightness requirements, with only a few outliers.
The takeaway from this study is that high levels of airtightness can be achieved if the motivation is there – good strategies are readily available.
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Designing an effective air barrier system for exterior walls
The design of an effective air barrier system requires selecting materials, components, and accessories that can be combined to control air leakage. While relatively straightforward to achieve in the assembly, maintaining continuity of the air barrier at interfaces and penetrations of the building enclosure is critical to air barrier performance.
An effective air barrier should have the following attributes:
- Air impermeability
- Continuity
- Durability
- Strength
- Rigidity appropriate to the building type and expected service loads
Appropriate exterior wall air barrier systems and details are often the primary focus of design for architects and engineers, as they tend to be the most complex and detailed systems – though that doesn’t mean that the roof, below grade, and interfacing systems and details are any less leaky or unimportant. Roof air barrier systems for low-slope roofs consist of either the roof membrane or a supplemental air/vapor barrier membrane, and below grade waterproofing systems consist of applied liquid or sheet-applied products. Joints and interfaces are typically sealed with membranes and sealants. With exterior walls there are more choices, and different systems have different detailing and construction implications, so are worth further discussion.
Wall air barrier systems can generally be grouped into two conventional types: exterior air barrier systems, with the primary airtight elements placed at the exterior side of the enclosure, and interior air barrier systems, with the primary airtight elements installed at the interior side of the enclosure. And within these systems are various approaches and components used to achieve the air barrier.
The exterior air barrier: Approaches use an airtight layer, usually a dedicated membrane, installed over the exterior face of the building, and made continuous with tapes, membranes and sealants over joints, transitions and penetrations.
The interior air barrier: Approaches use an airtight layer applied from the interior of the enclosure, interfacing with the various interior elements, transitions, and penetrations.
In general, the exterior approach is a simpler and favored method for larger and taller buildings, since it does not interface with numerous interior elements like framing or service penetrations for electrical and plumbing. In addition, because the components of the exterior air barrier are often also used as the water-resistive barrier (for example a synthetic house wrap on walls), the effort and care required to achieve a continuous layer to resist moisture intrusion also contributes to the overall continuity of the air barrier.
For exterior air barrier systems, there are three primary generic approaches used on larger and taller buildings:
- The sheathing membrane approach (both mechanically attached and self-adhered)
- The sealed sheathing approach
- The liquid-applied membrane approach
Other proprietary materials and systems can also be grouped into these classifications in terms of how they are sealed. Whole building airtightness testing data of large buildings incorporating these three primary exterior air barrier approaches has shown that all three can meet current code requirements (often between 0.25 and 0.40 cfm of air leakage per square foot of enclosure area at a test pressure of 75 Pa). Testing has also shown that the more rigid and adhered sheet- or liquid-applied membrane approaches tend to be more reliably air tight and easier to maintain during construction, especially for taller and more exposed buildings. There are obvious material and labor cost differences between the various approaches which should be factored in as well.
The selection of materials and detailing of exterior air barrier systems must account for durability (both during construction and in service), the compatibility to adjoining materials, adhesion to substrates, adhesion of the product to itself, and constructability. It is also important to consider the expected weather, and the ability to adequately prepare the substrate to manufacturer requirements for each of the air barrier materials used.
The mechanically-attached sheathing membrane approach
Mechanically fastened systems use an airtight sheathing membrane, attached to the exterior sheathing with fasteners and washers. Joints, penetrations, and laps are made airtight using sealant, tape, and self-adhered sheathing membrane strips as needed.
This is a commonly used exterior air barrier system for low-rise wood frame construction. Care should be taken to ensure the sheathing membrane is adequately attached to the building during construction and it should be supported by strapping or cladding to avoid damage. This approach is not typically appropriate for taller buildings or those with higher design wind loads due to the pressures applied on the mechanical fasteners, tapes, and sheet interfaces.
The self-adhered sheathing membrane approach
Self-adhered sheathing membranes rely on adhesion to both the substrate and at membrane laps for airtightness. The membrane should also be installed onto a suitable substrate such as wood sheathing, gypsum sheathing, concrete, or concrete block that is dry and provides continuous backing.
Self-adhered membranes essentially make the membrane and backup material part of the air barrier system, from both a structural load and continuity standpoint. Compared to mechanically-attached sheets which are spot-fastened to the substrate, self-adhered membranes restrict lateral air movement behind the membrane, which helps with overall system airtightness, especially around details. Self-adhered membranes also easily span over joints and gaps within the substrate, unlike liquids which require special joint treatments and/or reinforcing.
The sealed sheathing approach
The exterior sheathing, when sealed at joints and interfaces, can also act as the primary air barrier element. This approach uses the exterior sheathing together with either sealant, a liquid-applied sheathing membrane, strips of membrane, or sheathing tape to create a continuous air barrier at the sheathing joints. A sheathing membrane is often required with this approach to provide the water-resistive barrier.
Within the past few years, a number of proprietary coated sheathing products that function as both air- and water-resistive barrier systems have been developed following this approach.
The liquid-applied membrane approach
Exterior liquid-applied membranes share many of the advantages of self-adhered membranes and are especially useful for complex detailing. Liquid-applied membranes rely on a supporting substrate to provide a continuous backing to achieve an airtight barrier. Joints typically require specific detailing considerations and often incorporate membrane reinforcement.
The substrate and weather conditions can affect curing time and adhesion, so manufacturer’s instructions should be strictly followed. Liquid-applied membranes are generally used as the water-resistive barrier, and must be installed and detailed as such.
Finding closure in your airtight building
Large building airtightness testing is quickly becoming commonplace in many jurisdictions across North America as a result of new building code requirements, or as part of enhanced commissioning programs.
Testing of large buildings thus far has found that it can be relatively straightforward to meet current airtightness targets – as long as you’re starting with good design and planning, appropriate materials and air barrier strategies, proper site implementation, and quality control – all of which is verified by commissioning testing.
There are hundreds of options and materials that can be used to form part of the air barrier system, and the appropriateness of different products and systems depends on the type and exposure of the building along with the expected loads during construction and in service.
Fortunately, with commissioning testing becoming more commonplace, people are quickly figuring out the better approaches towards airtightness, which is incredible for more efficient, durable, and comfortable buildings.
Learn more about airtight building enclosures at the DELTA® Academy:
About Graham Finch, Principal, Building Science Research Specialist:
Graham Finch, Principal and Senior Building Science Specialist, is a building science engineer who specializes in research and investigation work. His work experience includes a wide range of projects including building enclosure condition assessments, forensic investigations, research studies, energy assessments, building monitoring programs, field review, and testing services for new and existing buildings across North America. He has worked with numerous building product manufacturers on product research and development, performance monitoring, and field testing.
Graham has authored and contributed to many publications, including industry guideline documents related to durable and energy-efficient building enclosures. He is regularly invited by building industry organizations and clients to speak on practical and technical issues related to a broad range of building science topics, and actively presents technical papers and presentations at local and international conferences. Examples of his work can be found on the RDH Building Science Laboratories website.
