In any building, drafts or air leaks can cause serious issues for efficiency, comfort, and safety. Achieving airtightness—resistance to unintentional airflows directed both in and out of a building—is essential for any architect or construction firm that wants to maximize building performance.
In practice, many different design mistakes and oversights can cause air leakage issues. Knowing how to avoid these pitfalls and measure your success will be essential in achieving an airtight building envelope.
The Risks of Drafts and Air Leaks
Air leaking out of the building envelope must be replaced by temperature-controlled air from the HVAC system, increasing the building’s energy consumption. Beyond reducing a building’s performance, air leaks can also put the health of building occupants at risk.
Drafts can bring condensation and moisture from outside the building in, potentially leading to corrosion of metal or helping to speed the growth of mold.
In polluted areas, outside air can also be harmful to occupants by default—airtightness helps ensure that occupants are only breathing ventilated and filtered air.
Ensuring building airtightness can improve building comfort, extend the lifespan of building components, and prevent health issues associated with drafts and air leaks.
Design Strategies for Airtightness in Buildings
Airtightness begins with planning. From the beginning of the building design process, designers should work towards a continuous airtightness barrier for the building. This barrier will be made up of various materials, flexible and sealed joints, as well as internal building components.
Typically, this process begins with defining an airtightness target and determining what level of airtightness is necessary for the building. This level may be determined by a combination of local regulations and client preference. Planners should also define the line of airtightness, which separates heated and unheated spaces in a building.
For example, a two-dimensional representation of a building may have a continuous red line that encloses the heated area of a house but excludes the unheated garage and attic spaces.
Planners will then need to identify junctions in the building where different components or materials meet. For example, a designer may make note of junctions between the walls and floors, between walls and roof, at window frames, and at access doors.
These junctions will need to be sealed during the construction process. Minimizing junctions in the design phase can help to make airtightness a more achievable goal.
Construction strategies that take advantage of digital design technology may be effective here, as digital airtightness plans can be more easily shared during the construction process.
Ideally, the airtightness plan will be fully developed before the planning stage is over and construction begins.
Managing Airtightness on the Construction Site
On-site, achieving airtightness requires close coordination between contractors and designers. Typically, designers will appoint an airtightness expert who will work on the construction site to guide operations and answer questions about the airtightness plan.
Choice of construction material can have a major impact on building airtightness. The designer will have to decide what material and design they will use for their air barrier system—potential choices include fluid-applied barriers, sheet-applied barriers, and flashing for junctions like windows and doors.
Even decisions like the choice of adhesive and sealant additives can have a major impact on a building envelope’s seal—kaolin and attapulgite, for example, can help to improve a sealant’s lubricity, suspension, and barrier properties. Using a sealant additive like these may be essential for some buildings where junction design requires sealants with improved filling properties.
Some designers will test the airtightness barrier once it is complete but before construction is done. This process can give contractors a chance to repair issues with the barrier before additional construction makes further changes difficult.
Guidelines and Regulations for Airtightness Testing
There are many standards that define how to perform airtightness testing, including several from the ASTM, an ISO standard, the Air Barrier Association of America (ABAA) standard, and the standards of the U.K.-based Air Tightness Testing & Measurement Association (ATTMA).
While these standards vary somewhat in approach, they all typically involve testing the airtightness of a building using controlled pressurization and depressurization of the building or part of the building. The designer measures building pressure as it changes to determine how well the building is sealed.
By measuring multiple parts of a building, designers can also check for varying levels of airtightness across a structure.
Airtightness Testing Methods
Most airtightness testing strategies will either use fan pressurization or tracer gas to test the building for leaks.
The fan pressurization test involves the temporary installation of a fan and blower door in a building entrance. Airflow through the fan creates a static and uniform pressure inside the building. Measuring the pressure differential throughout the envelope will give the tester a sense of how airtight the building is. Generally, the more airflow you need to increase building pressure, the less airtight the envelope is.
Tracer gas measurements are often used for building components, like air ducts. A known quantity of a non-toxic, odorless, and man-made chemical, like sulfur hexafluoride (SF6), is injected into the building. After a certain period of time has passed, a gas analyzer is used to measure the quantity of gas remaining in the building.
Typically, contractors will perform two tests: a pre-test, and a final test. It’s not unusual for buildings to fail the pre-test due to design oversights. Failing the pre-test can be an opportunity to uncover drafts and leaks that may be patched before the final test.
Airtightness building tests are not mandatory everywhere, though they are mandatory in much of the United Kingdom and a number of U.S. and Canadian states. In some areas, building designers must use the fan pressurization test to measure the airtightness of the building. In others, a building designer may have a great deal of control over how to test a structure for airtightness.
Essential Considerations for Airtightness in Buildings
Airtight buildings are more comfortable, more efficient, and less likely to cause health problems for occupants than drafty structures. Ensuring a new home or office complex is airtight can be a challenge, however, unless designers consider building airtightness from the very beginning of the building process.
Designers should pay close attention to the number and placement of building junctions during the design process. Once construction begins, the designer’s choice of air barrier and sealant type will be important. Testing multiple times will help contractors catch issues with the barrier before construction is complete.