Reducing uncontrolled air leakage through the building envelope is one of the most cost-effective energy efficiency strategies—for both new and existing buildings.

Why air leakage matters

Air leakage into and out of a building can contribute significantly to building energy consumption. Air leakage can also accelerate envelope degradation, increase occupant discomfort, and cause a host of other problems. Not surprisingly, recent building standards and codes have expanded requirements for air barriers and, in some cases, quantified maximum allowable levels of air leakage for materials, assemblies, and buildings.

How air leakage occurs

Unintentional holes or openings in a building envelope (walls, roof, windows, etc.), coupled with pressure differences between the inside and outside at that site, cause air movement from outside to inside (infiltration) or inside to outside (exfiltration). Pressure differences occur for a variety of reasons including temperature differences between inside and outside, stack effect, wind pressure on the building, mechanical system pressurization, and exhaust system depressurization.

Virtually all types of building envelope construction are vulnerable to air leakage if appropriate measures are not incorporated into their design and construction. Leakage points are commonly found at the  ntersection of different envelope assemblies (roof/wall, wall/foundation, window/wall, etc.), joints between differing construction materials, joints between panels, and through materials that are known to be air permeable (e.g. batt insulation).

A 2005 study funded by National Institute of Standards and Technology (NIST) simulated infiltration/exfiltration reductions in buildings and predicted that potential annual heating and energy cost savings ranged from 2% to 36%, with the largest savings occurring in heating-dominated climates and the smallest savings in cooling-dominated climates.

Why air leakage occurs

Some types of construction are more susceptible to leakage than others, but unfortunately, most existing buildings and many very new buildings have high levels of air leakage. This is true even for many buildings that originally targeted energy efficient design following green building codes, standards, and/or rating systems. Lagging industry standards and code enforcement for building air barrier design and construction contribute to the overall problem. The most commonly referenced building energy efficiency codes and standards have only required continuous air barrier systems for buildings since the 2010/2012 versions. Earlier versions required component air sealing to reduce air leakage, but lacked any requirement for a comprehensive air barrier.

Stack effect

As heated air rises it creates a pressure differential inside a building and increased pressure differences between the interior and exterior. This is referred to as the stack effect or chimney effect. The taller the building and the larger the temperature difference between the structure’s interior and exterior, the greater the stack effect. This image shows the effect of higher pressure at the top and lower pressure at the bottom of a building and the resulting air leakage from stack effect. In leaky buildings, stack effect can mean that large volumes of mechanically heated air leak out at the top of the of the building, with cold air leaking in at the bottom, thereby causing occupant discomfort, increased energy use, and possible damage to the envelope, and interior finishes.

Preventing air leakage

For new buildings, energy codes require a continuous air barrier. To control air leakage, a continuous air barrier composed of compliant air barrier materials and assemblies must be included as an integral component of the envelope assembly. The air barrier must be clearly defined in the construction drawings and installed per manufacturer’s specifications. To ensure compliance with building code air leakage requirements, a whole-building pressurization test is recommended as part of the required field testing. Compliant air barrier materials must be industry tested to meet maximum allowable air permeance ratings. Many common building materials meet the material requirements including various types of sheathing of minimum required thickness (e.g. plywood, extruded polystyrene insulation board, gypsum sheathing); various types of membranes (e.g. fully adhered single-ply roof membrane); spray foams with specified minimum density and thickness; solid enclosure materials (e.g. cast-in-place and precast concrete); etc. The air barrier materials of each assembly must be connected—in a flexible manner—to the air barrier material of adjacent assemblies, allowing for the relative movement of these assemblies and components. To seal seams between differing materials, seams between panels of like material, and any penetrations through the air barrier materials, a flexible and durable tape, sealant, or gasket must be used to form the continuous air barrier.

Creating a continuous air barrier in an existing building is more challenging than in new construction. Sources of air leakage are often hidden behind ceilings or walls, and not visible to the naked eye. The best method to detect hidden air leaks: depressurize the building using calibrated fan equipment (known as a blower door test) and perform a walkthrough with an infrared (IR) camera to locate air leakage points and seal them. Some materials are specifically designed as air barriers, while others may serve multiple purposes such as moisture barriers and/or insulation. If membranes such as polyethylene or building-wraps are used, they must withstand air pressure in both directions without displacement or damage. Fluid-applied elastomeric coatings, spray-on coatings, trowel-on materials, peel-and-stick membranes, and some spray-on foams can also function as air barriers. All must meet code-specified maximum allowable air permeance ratings.

Bower door tests are used to test air leakage through the building envelope

Placement of air barrier

The location of the air barrier in an envelope assembly will influence that air barrier’s properties. In heating dominated climates, like Illinois, if the air barrier is placed on the side of the insulation that will be warmer in winter (inside), specify a low water vapor permeability so it also serves as a vapor retarder. (7) However, if the air barrier is located on side of the assembly that will be colder in winter (outside), the material should be vapor permeable (5-10 perms or greater). (8)

New testing standards

The U.S. Army Corps of Engineers (USACE) has developed an air leakage standard and measurement protocol for testing a whole building. The USACE testing protocol includes infrared thermography as a part of the required air leakage testing. USACE Air Leakage Test Protocol for Building Envelopes provides guidance on air leakage specifications, how the test should be conducted, and standards applicable to the testing. This testing protocol is based on ASTM E-779-03.

Conclusion

Changes in codes and standards reflect the new focus in the design and construction industry towards tighter building envelopes. Properly-designed and constructed air barriers will create more durable, comfortable, and energy efficient buildings. Furthermore, HVAC systems can be downsized, indoor air quality will improve, and the impact of new buildings on the environment will be reduced. As the building envelope construction becomes more air tight, mechanical ventilation systems become increasingly important.

Further reading

The Center of Excellence for the Air Barrier Industry, Air Barrier Association of America (ABAA)

Impact of Air Leakage on the Building Envelope, The Construction Specifier, February 2011

Determining Air Barrier Performance, The Construction Specifier, December 2009

Air Barriers: Research Report - 0403, Building Science Corporation, April 2004

Air Barrier Systems Technology Brief, INTERNATIONAL MASONRY INSTITUTE, November 2009

How Do Buildings Stack Up?, Building Science Corporation, February 2014

END NOTES
  1.  2014 International Energy Efficiency Scorecard July, 2014, Executive Summary, American Council for an Energy Efficient Economy (ACEEE): http://www.aceee.org/files/pdf/summary/e1402-summary.pdf
  2. Oak Ridge National Laboratory has several studies on occupant discomfort that use hydrothermal envelope modeling. See: http://www.ornl.gov
  3. Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use, NISTIR 7238, June 2005; http://www.fire.nist.gov/bfrlpubs/build05/PDF/b05007.pdf
  4. Parapets: Where Roofs Meet Walls, Building Science Corporation, June 2011 (Rev. 04/2012), see http://www.buildingscience.com/documents/insights/bsi-050-parapetswhere-
    roofs-meet-walls/at_download/file
  5. ASHRAE 90.1 2010 § 5.4.3.1 Continuous Air Barrier, p. 24
  6. Technology Roadmap: Energy efficient building envelopes, International Energy Agency (IEA), 2013. https://webstore.iea.org/technology-roadmap-energy-efficient-building-envelopes
  7. Water vapor permeability is the property that allows fluids to diffuse through it. A perm rating of < 0.1 is considered an impermeable vapor retarder.
  8. Whole Building Design Guide Air Barrier Systems in Buildings http://www.wbdg.org/resources/airbarriers.php