Air barriers versus moisture retarders

Better workmanship can improve the watertightness of mortar joints, but an air barrier and a moisture retarder are still needed to prevent moisture from entering the building.

Many designers recognize the need for a moisture retarder on the exterior face of the CMU, but they overlook the importance of an air barrier. According to a study by the National Institute of Science and Technology (NIST), the energy savings from a well-constructed air barrier can be as much as 33 per cent of the annual energy cost.

Insulation in cavity walls

Three types of insulation are used for masonry cavity walls:

• Extruded polystyrene (XPS) moisture-resistant rigid board.
• Foil-faced polyisocyanurate (polyiso) rigid foam.
• Closed cell sprayed polyurethane foam (SPF).

XPS insulation offers moisture resistance and long-term durability. It is lightweight, easy to handle, and can be cut into smaller pieces. XPS comes in 406 mm (16 in.) widths and
1.2 m (4 ft) lengths which can be pressure-fit between wall ties. A misgiving about XPS is its rigid configuration, making it difficult to enclose irregular shapes.

Dual foil-faced polyiso rigid foam insulation in 406 mm widths and up to 3 m (10 ft) lengths is recommended for cavity wall insulation. It has a higher R-factor than XPS at the standard test temperature of 24 C (75 F). However, studies have shown the R-value of polyiso steadily declines as the temperature goes below 15 C (59 F).

Closed-cell SPF insulation has sound/air barrier and moisture retarder properties. The adaptability of SPF to foam into irregular openings and cover irregular shapes makes it a potential complement to the rigid products in the development of air barriers. This adaptability helps simplify the design of cavity drainage walls, air-vented cavity walls, and especially pressure-equalized cavity walls. Cautions about SPF have been published in ASTM STP1549, Dimensional Stability Considerations in Spray Polyurethane Foam Air Barriers.

In a paper published in 2014, the ASTM made note of “the short- and long-term shrinkage variations in foam systems that cause problems when the SPF product is used as an air barrier particularly when applied to the exterior of the building.” Regardless of these concerns, SPF has an important place in the design of pressure-equalized rainscreen walls and air-vented cavity walls to make airtight building envelopes.

Understanding the extent of the bond between brick and mortar

There are two characteristics of the bond between brick and mortar: the extent of bond and the strength of bond. The more important of the two is the extent of bond, which refers to how completely the mortar is spread in intimate contact with the brick across the entire surface of the head joint. The extent of the bond between the brick and mortar is a direct result of the initial rate of absorption of the brick, the water retention of the mortar, the skill of the mason, and the workability of the mortar. Mortar bond can be a problem with flat stone or manufactured stone veneers due to lower stone porosity. An experienced mason can feel if the mortar is sticking to the brick or stone properly and adjust, if necessary, by rewetting the brick or retempering mortar which has gone dry on the board.