Air Barriers and Vapor Retarders
Masonry walls require air barrier systems to control air leakage as discussed in the section Design/
Air Barriers. Similarly, the design and installation of vapor retarders for masonry walls needs to
follow the guidance given in the section Design/Vapor Retarders. To make a masonry wall airtight,
one must reduce the permeability of the masonry wall itself and address the intersections between
the masonry and other building elements. Mortar joints can not be made airtight because
differential movements caused by temperature, moisture, shrinkage of blockwork and movement of
other building elements inevitably lead to cracks in mortar joints. Since masonry itself is ultimately
permeable to airflow, an air barrier material must be employed to seal the small openings at the
unit/mortar joints. Air barrier materials used in masonry construction include layers of mortar,
plaster, heavily textured paint or mastic, sheet material, interior gypsum board and various sealants.
In order to achieve a continuous air barrier system, seams and joints must be meticulously sealed.
Air barrier elements are also required at the interfaces between the masonry construction and other
envelope components and must be able to accommodate the differential movement at these
The following figures show air leakage defects in masonry construction, pointing out some of the
key points in achieving an effective air barrier in masonry wall. Figure 4.2.9 shows a situation
where air leakage occurred because the air barrier was omitted behind the convector cabinets
(Quirouette 1989). Because the block behind the convector cabinets was left unfinished, interior air
flowed through the unfinished block into the cold space behind the precast concrete spandrel
panels and the column covers, resulting in severe condensation, freezing and melting problems.
This case shows the importance of applying the air barrier continuously over the entire wall.
Air leakage through
unfinished concrete block
Figure 4.2.9 Air Leakage Through
Unfinished Block (Quirouette 1989)