With today’s high cost of energy and concerns about indoor environmental quality, air barriers are one of several building systems with a critical role to play. Recent studies, such as “Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy Use” by the National Institute of Standards and Technology, Gaithersburg, Md., show air-barrier systems in commercial and industrial buildings are estimated to reduce air leakage by up to 83 percent, save on gas bills by more than 40 percent and cut down on electrical consumption by as much as 25 percent. However, to achieve these sizeable energy savings, the air barrier must be designed, constructed and tested with diligence.
In preparation for an air-pressurization test, exhaust stacks, which are known to be an air pathway, are sealed.
What and When
The U.S. Department of Energy, Washington, D.C., estimates the operation of buildings, not including process loads within those buildings, accounts for roughly 40 percent of the nation’s energy expenditure. The majority of this energy is used to condition the indoor environment. A proper enclosure must be constructed to create the separation between the outdoors and conditioned environment. An air barrier is a system of building materials, such as membranes, coatings, foams, boards, etc., within the building enclosure. The air barrier is designed, installed and integrated to stop the uncontrolled flow of air into and out of the building enclosure. In addition, the air-barrier system reduces the following:
• Occupant discomfort because of drafts
• Degradation of building materials caused by interstitial condensation
• Poor IAQ caused by the ingress of fumes, dust, etc.
• Inability to achieve required pressure conditions in controlled environments
• Difficulties in balancing HVAC systems
• Noise transfer through leakage paths
• Microbial growth within building cavities
From a design perspective, the continuous air barrier must be included in the initial phases of the project to ensure the building is acceptable at delivery. In design-build cases, the continuous air barrier must be factored in during the original request for proposal response, designed into an acceptable wall/roof/slab system and accurately priced in the RFP submittal.
The design team must prepare plans and specifications that clearly translate the project’s needs and goals to the contractor performing the work. With the design documents in place, the next step toward creation of a successful air-barrier system is the construction phase. The standard of the industry does not currently construct to ensure air tightness, regardless of what the design documents say. This is not caused by unwillingness to perform work with integrity but rather a communication breakdown between the design team and contractor. It is essential to include a briefing of goals and expectations at the pre-construction meeting. This may include construction of a mock building section to familiarize the contractor with materials selected and ways they are incorporated into the building enclosure.
Throughout the construction phase, on-site observation by an air-barrier consultant is recommended. This allows work to be observed for defects or errors, opportunities for sample tests of portions of the building or mock ups, immediate answers to questions, and an on-site liaison between the contractor and materials manufacturer. The on-site observation is similar to that of a quality-assurance engineer but specific enough to require an individual or group specialized in air-barrier consulting.
Once construction of the building is complete, the performance of the air-barrier system must be verified. Unlike other programs in the building community that support certifications through checklists or theory, the whole-building pressurization test is a representation of actual measurable conditions of building performance. Much like water-penetration testing, pressurization testing gives a measurable/gradable result.
Air leakage is measured as the rate of leakage per square foot of external envelope per minute at an artificial pressure differential through the envelope (cubic feet of air flow per minute per square foot of envelope). The Washington-based U.S. Army Corps of Engineers, the first nationwide organization to require a continuous air-barrier system for its new and renovated facilities, has developed criteria to allow no more air leakage than the equivalent of 0.25 cubic feet per minute per square foot of envelope with a 75 pressure differential. This criterion is similar to other standards worldwide and will move the USACE building inventory toward greater energy savings. All wall, roof and floor areas that are exposed to the external environment are included within the calculated area, and floor slabs in direct contact with the ground are included because they are part of the enclosure. The building can be pressurized or depressurized to obtain results, but the most accurate result is produced by averaging the two.
Whole-building pressurization testing yields the overall leakage rate of the building and is accompanied by the use of infrared thermography to locate leakage points and paths of travel through the enclosure. As the pressure differential is increased, any air leaking through the enclosure will most likely undergo a temperature change. Prior to testing, heating or cooling of interior spaces can exaggerate this temperature gradient. Although infrared cannot see the leaking air, the surrounding building materials’ surface temperatures are affected by its flow, which easily can be detected by infrared scanning. Thermographic air-leakage patterns often are very subtle and diffuse; without proper equipment and expert personnel, air-leakage pathways easily are missed. In addition to locating air-leakage pathways with infrared thermography, smoke testing and invasive testing—physical dissection of the envelope to verify as-built conditions—are incorporated.
Leaks often are found at wall-to-roof or wall-to-slab connections. Penetrations in the envelope also are prone to air leakage. Proper design and specification can help alleviate these errors, but on-site observation during construction must be conducted to ensure proper implementation of the design and specifications. Air-sealing measures can be performed on a constructed facility by filling gaps, holes and cracks with materials, such as spray foam or sheet metal, but the most cost-effective solution is to construct it properly from the beginning.
Against the Wind
Today, a building team’s goal should be to create building systems that yield proper separation of interior and exterior spaces, as well as alleviate premature degradation of wall and roof systems. Considering that walls and roofs account for approximately 25 percent of total building construction costs, bolstering the building envelope makes a lot of sense from the standpoint of life-cycle cost and building health.
An inferred thermogram of this commercial facility showed air leakage at the inward corner while the enclosure was pressurized
As continuous air barriers are integrated into the building envelope, there will be a steady learning curve until designers and contractors become more experienced and comfortable with the concept. U.S. design and construction firms lag behind in knowledge and proper application of air-barrier systems compared to Canada and northern Europe where these systems have been required and used for many years. These countries are experiencing great energy savings. It’s only right that many of their standards and design knowledge now are being implemented into the U.S. building industry.
Lee Durston is a senior building-science consultant at BCRA, Tacoma, Wash. The firm’s Building Science Group provides air-barrier consulting in all phases of project delivery and serves as an expert instructor for the Washington, D.C.-based U.S. Army Corps of Engineers’ Mold and Energy Workshops around the world. Durston can be reached at firstname.lastname@example.org or (253) 627-4367.