Coatings
Liquid air barriers are often favoured by contractors because they are easy to apply and can be either sprayed, rolled, or, in the case of a mastic, trowelled onto the surface. Usually not exceeding about 1 mm (40 mils) in thickness, fluid-applied air barriers form a continuous seamless covering around the wall and up to the roof even in more complex architectural structures containing protrusions or tight spaces as well as beneath ground level.
Liquid coatings are usually formulated with resilient water-based polymers that may or may not contain an emulsified asphalt to impart additional moisture resistance. Rheology modifiers are added to impart sag resistance to the coating.
There are products compliant with volatile organic compound (VOC) restrictions that can coalesce in cold environments at temperatures as low as 0.6 C (33 F), and can adhere to damp substrates.
Wraps and sheets
Air barrier membranes can be made by casting a butyl polymer (which is very sticky) on a high-density polypropylene, then rolled into a wide sheet that is nominally 1 mm (40 mils) thick. The butyl side is adhered to the wall panel. These can be cut to size and adhered to a wall surface with relative ease and provide suitable air infiltration resistance. The butyl has fantastic flexibility, even at low temperatures.
Sheet air barriers bridge gaps better than air barrier coatings, and no special equipment is needed to install one. Unlike liquid-applied barriers, applying sheets creates many seams that need to be covered with additional butyl tape or sealant to prevent air infiltration at those sites. Sheets applied around windows or protruding pipes must also be addressed in order to ensure a tight seal is achieved at the cuts in the sheet.
Air barrier systems
Air barrier coatings and sheets can be used in conjunction with other materials to create air barrier assemblies or air barrier systems. (To read Joseph Lstiburek’s 2006 article on the topic, “Understanding Air Barriers,” visit buildingscience.com/documents/digests/bsd-104-understanding-air-barriers.) For example, an exterior wall may consist of a painted gypsum board wall placed over a polyethylene air barrier and vapour barrier. This sheet is placed over the stud and cavity insulation that may comprise a polyurethane foam or fibreglass insulation. OSB covers the other side of the stud on the exterior side of the wall, followed by vapour-permeable building paper or house wrap and, finally, the siding. The latex paint, gypsum, and air barrier sheet is an air barrier system.
Air barrier manufacturer websites are useful starting points for determining the right product or system for the building owner. (For additional information, training, and technical literature on air barrier types, access NABA’s website at www.naba.ca.)
Conclusion
With the emergence of more stringent building codes requiring airflow be controlled in new construction, the use of air barriers is an effective means of curtailing the effects of air transmission through the building envelope. With the airflow in check, moisture, pollutants, or allergens that would otherwise be carried into the building through unprotected surfaces stay outside where they belong. This way, the air can be used for more benign purposes, like, say, filling balloons.
Jason Smith is the senior research and development chemist for The Garland Company in Cleveland, Ohio. He has been awarded numerous U.S. patents for roofing and has regularly published articles related to roofing science, coatings applications, and solvent regulations. Smith is a member of the Roof Coatings Manufacturers Association (RCMA) where he serves on the board of directors; he also co-chairs its Technical Committee. He received his undergraduate degree in chemistry from the University of Pittsburgh and his master’s degree in polymer chemistry and coatings from DePaul University in Chicago. Smith can be reached via e-mail at jasonsmith@garlandind.com.
