Properties and Advantages of Brick Cavity Walls

Words: Steven Judd, Ian Moffatt, Interstate Brick
Photos: Interstate Brick

Many compound words are self-explanatory "descriptive names." Sunglasses are glasses that block the sun. A fireplace is a place to build a fire. A cavity wall is exactly that: a wall assembly that contains a cavity.

Previously, this concept was used primarily for multi-wythe brick walls, with a cavity between two wythes of brick that could be left as air, filled with grout and reinforcing, or filled with insulation. While multi-wythe cavity walls are still an acceptable practice, they are less common today compared to the more popular option of brick veneer cavity walls with masonry ties. When one refers to a brick masonry cavity wall in today’s world, they are typically referring to a wall where the outermost layer of full-bed-depth brick is separated from (and tied back to) the rest of the wall assembly by an air gap.

The introduction of an air cavity to a brick wall has many advantages. Take a look at just a few of the reasons why this wall assembly adds value and flexibility to any project.

Assembly
Anchored veneer cavity walls can be used with any standard construction backing with a large range of prescriptive freedom, and a seemingly endless range of veneer applications when using engineered methods. Wood and steel studs allow up to 30 feet (38 feet at a gable) of brick veneer construction without additional supports, per the 2021 and earlier building codes, with unlimited height for brick veneer over concrete or masonry with due consideration for differential displacements. More recently, the unlimited height of brick veneer is allowed over light wood or steel stud-framed walls and other backing walls in the 2024 building codes, as long as differential movement is properly addressed and accommodated, which can take some special detailing.

For each of the wall types, typical masonry tie spacing (for common wind and/or seismic loads) requires a tie every 2.67 square feet, which can be accommodated using the 16” x 24” rule: 1.33’ (16”) x 2.0’ (24”) = 2.67 sq. ft. This easy rule of thumb is perfect for typical stud spacings by placing an anchor every 24” along a stud when using framing at 16” on center, or an anchor every 16” when the stud framing is 24” on center. High wind pressures or high seismic forces require a closer (more dense) tie spacing as determined by code or the design professional.

Different veneer tie types are used for different backing systems, such as sheet metal and corrugated sheet metal ties being used for light wood framing only, whereas adjustable ties are an acceptable choice for any structure. Corrugated sheet metal ties can only be used with a 1” air gap from exterior sheathing to the back of the brick veneer. The Masonry Society code (TMS) outlines all of these specifics as well as the maximum cavity widths per tie type. Adjustable ties, for example, through prescriptive methods allow a 6” maximum cavity width (from the face of exterior sheathing to the back of brick). The minimum air gap to be maintained is 1”, which allows a designer up to 5” of room for continuous insulation within a wall assembly. The maximum prescriptive air gap for standard ties is 2”.

All brick ties, regardless of the type, must be embedded into the mortar bed joint a minimum of 1-1/2”, but still leave 5/8” of mortar cover from the outside surface. To ensure that these minimums are actually met, the mason should, as part of his quality assurance program, assess or survey the backing wall construction to make sure it complies with the project requirements, specifically location and associated tolerances. If the backing wall is out of place, tie embedment may be compromised, or the ties acquired for the project may not be installed in compliance with the code. Any backing wall that is found to be out of place should be brought to the attention of the general contractor (or owner) so that corrective action can be taken BEFORE the backing wall is covered by the brick veneer. A formal RFI (Request For Information) may need to be initiated by the mason to resolve the issue.

Air Gap As A Water Barrier
Water penetration into a building is a major design consideration, no matter what material is chosen for a project. A brick cavity wall comes with the added benefit of a water drainage layer (air gap) as part of the wall assembly. When flashing is properly detailed with weep holes properly located, such as above openings or at the base of the wall, any water that somehow finds its way through the veneer is passively removed with little worry of causing any harm to the building. It is also recommended to use a mortar capture device to work with the weeps to collect all mortar droppings that may occur during installation.

These concepts are not only beneficial throughout the life of the building but also during initial construction. While installation methods do include procedures to prevent damage, such as tarping work at the end of each day, cavity wall construction provides additional peace of mind that no additional labor should be necessary to remedy the site after rainfall. In fact, rainfall and moisture considerations in wetter environments can sometimes be the driving factor in an architect's design.

Rain Screens
Rainscreens are increasingly popular in buildings designed for moist environments and are even mandated by code in parts of the Pacific Northwest. To qualify as a rainscreen, a wall must include a cladding outer layer, an air space/cavity, a water-resistant barrier, and a back-up wall structure. A brick veneer cavity wall naturally qualifies as a rainscreen, making it an excellent choice for designers seeking a simple, long-lasting solution. Rainscreen detailing requires drainage and venting of the cavity behind the cladding.

A “vented” cavity wall only requires weep/vents at the bottom of each wall panel area. A “ventilated” cavity requires weep/vents at the bottom and top of each veneer panel area. Both vented and ventilated cavity walls are considered rainscreens. Installing weep-vents at the top and bottom of the cavity wall panel areas allows for the removal of both liquid water and water vapor. Rainscreen options include a variety of other materials, so proper design and installation of flashing should be taken seriously to continue the reliable reputation that brick has created for itself.

Design Flexibility
One of the many benefits of designing with brick is the freedom to experiment with bond patterns, color blends, and corbelling. Let’s explore how a cavity wall can support a distinctive design approach.

Using prescriptive methods, a designer may alter the finished face of a single-wythe brick veneer by up to half of the brick height or one-third of the brick depth per course, with a limit of one-half the brick depth for the maximum offset. For a modular brick, that is just over 1” of offset allowed in either direction, per course. Let’s say the designer chooses to recess every other course into the wall by 3/4”. Per TMS 402, the designer can establish the air gap to be 2”, recess every other course by 3/4”, and still maintain an air gap of 1-1/4”, satisfying the minimum requirement of 1”. Without this flexibility, the mason or manufacturer would have to cut 3/4” from the back face of half of the building’s brick, adding high cost and time to the project.

Thermal Properties
The minimum air cavity per TMS 402 is 1” for anchored veneer, and ASHRAE 90.1 states that any airspace of 1/2” or greater has an R-value to consider. Vented brick veneer rainscreens and ventilated brick veneer rainscreens have great thermal properties, contrary to popular folklore. The brick and air cavity do play a major role in the thermal performance of the wall. When navigating the tables of ASHRAE 90.1, one can find an R-value around 0.9 (ft²·°F·hr/Btu) for a vented ideal airspace with building materials on either side. This gives the layer of air a bigger impact on the overall R-values (or U-factors) than the gypsum board, plywood sheathing, and even the brick itself.

Further, relatively recent hot box testing by the National Brick Research Center in Anderson, South Carolina, has shown that the air cavity is relatively stagnant, even in a ventilated cavity wall with winds applied to the wall, allowing the mass of the brick to moderate and enhance the thermal properties of the wall. Technically, brick veneer used in a vented or ventilated cavity wall construction meets energy code requirements for a “mass wall” based on wall density and wall material heat capacity, which often reduces the required prescriptive insulation by as much as one-half or more.

As an example: for Climate Zone 5, a light wood framed wall or a steel stud framed wall have a prescriptive requirement to provide a total insulation value (combined in the stud cavity plus continuous insulation) of R-20.5 to -24, depending on framing type (wood versus steel), whereas a mass wall assembly (brick veneer cavity wall) only requires a prescriptive insulation value of R-11.4.

Cavity wall construction also allows the use of continuous insulation outside of the water-resistant barrier, rather than using insulation between the stud framing. Used in conjunction with thermal break ties and taking advantage of brick qualifying as a mass wall, a designer is able to maximize the efficiency of continuous rigid insulation to design a thinner, more efficient wall profile. An added benefit to this approach is being able to control where the dew point occurs in the assembly. Placing most, or all, of the insulation outside of the water resistive barrier drives the dew point outboard of the water resistive barrier and will keep more moisture out of the interior of the building. This can increase the overall lifespan of the building by preventing mold, mildew, and other moisture-caused defects in a wall.

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Properties and Advantages of Brick Cavity Walls

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