What is a Cavity Wall? | Components & Properties

A cavity wall is two wythes of masonry that is separated by a cavity of varying dimension. A wythe is a continuous vertical section of masonry one unit in thickness. The wythe units can be made of solid brick, structural clay tile, or concrete masonry units. Both the wythes are connected together as a single unit by using wall ties.

Cavity wall construction was first developed in the United States in the late 19th century. It gained property due to its superior property to resist rain penetration, high temperature, sound transmission, and fire. 

More research and development on cavity wall construction have made load-bearing walls not only in low-rise buildings but in high-rise building construction in the U.S.

Read On: Wall Ties in Masonry Construction - Features & Functions

Structure of a Cavity Wall

Figure. 2., shows a typical cross-section of a masonry cavity wall. The thickness of the cavity ranges from 2 to 4 1/2 inches in width. The cavity may or may not be provided with an insulation membrane. It is dependent on the requirement of the wall. 

Properties of Cavity Walls

The properties of the cavity wall are:

1. Resistance of moisture penetration
2. Thermal energy efficiency
3. Fire resistance
4. Structural properties

1. Moisture Penetration Resistance of Cavity Walls

Cavity walls are designed and built as a moisture-deterrent system. As mentioned in the structural elements of a cavity wall, there is exterior and interior wythe in a cavity wall unit. The moisture has the possibility to penetrate into the wall through the exterior wythe surface. The water that runs down the exterior wall surface is drawn into the inner cavity between the wythe, due to the wind pressure acting on the exterior of the wall as well as the negative pressure present within the cavity. 

Flashing & Weep Holes for Cavity Wall - Moisture Penetration Resistance

The presence of clean air space in the cavity permits the moisture to flow unobstructed down the cavity face. Installation of flashing lets this moisture be diverted back to the building's exterior through weep holes.

2. Thermal Energy Efficiency of Cavity Walls

Initially, cavity walls were built for structural and moisture qualities. But, the increasing life cycle cost of buildings, demanded the need for energy-efficient walls.

A cavity in walls is an excellent space to incorporate energy efficiency improving materials like insulations. Insulation in cavity walls helps to minimize heat loss and heat gain. Here, both the wythes act as heat reservoirs and positively affect the heating and cooling modes. 

The introduction of air space allows a large amount of heat to be absorbed and dissipated in the outer wythe and cavity before it enters the interior wythe or the building space. 

Cavity Wall- Thermal Insulation R-Value Calculation

The R-value of a typical cavity wall ranges from 14 to 26. The value varies based on the thickness of the insulation installed in the cavity.

The figure above shows the cross-section of a cavity wall that is provided with 2'' insulation and air-space. The total R-value of the whole unit is calculated as the sum of the R-values of each individual element composing the wall. 

Table-1

The table-1 shows the R-values of several materials constituting a cavity wall construction.

3. Fire Resistance of Cavity Walls

As per the test results of ASTM E-119, cavity walls possess very high fire resistance. The resistance is measured in terms of fire rating as per the Fire Protection Planning Report ( CMIFC). Cavity walls possess a fire rating of 4 or greater.

4. Structural Properties of Cavity Walls

A solid masonry itself is a good load-bearing unit. In cavity walls, each wythe helps to resist the wind loads, by acting as a separate wall. The horizontal joint reinforcement transfers direct tensile and compressive forces from one masonry wythe to the other. 

Joint reinforcement also helps in transferring shear around 20 to 30 percent across the wall cavity.

Insulation for Cavity Walls

Insulation is generally fixed to the external face of the internal skin of the cavity wall. This allows the remainder of the cavity to be ventilated without significant heat loss.

In certain situations, where insulation is retrofitted, the entire cavity is filled. Here, care must be taken to ensure that a path is not created for the penetration of moisture between the outside and inside.

A sound structural design, and employing quality materials and workmanship results in high-performance cavity walls. 

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