When moisture enters into an exterior wall, it can potentially cause problems with the various materials contained inside the wall cavity. As such, you may be wondering if ventilation is required for cavity wall construction.
Traditional cavity walls do not require ventilation. They are designed to be weathertight (airtight and watertight) in order to be thermally efficient and keep moisture out of the wall cavity. Instead, weep holes are provided along the exterior masonry to drain out moisture. Weep holes also allow wind to create an air stream through the cavity. This eliminates the need for additional venting.
Any water that does manage to enter into the wall is diverted away from the internal cavity through a combination of metal flashing at specific locations and weep holes spaced at a maximum distance of 2 feet (610 mm) on center (o.c.).
This system of construction is distinctly different from a rain screen wall system where air and moisture are intentionally allowed to enter through the exterior wall surface, and a secondary weathertight system is installed behind it.
What are the Advantages of a Cavity Wall?
The main reason for using a cavity wall system over other available systems is that it provides good insulation values. This is due to the fact that the wall system is broken up into two separate parts with an airspace in between.
The airspace is important because it provides a thermal break between the two separate wall layers. This results in less heat loss from inside the building to outside. Likewise, it reduces the bridging of cold air through the wall from the exterior.
Cavity walls are used extensively in residential and commercial masonry construction, in particular in coastal or colder climates. Often times, rigid insulation is placed within the wall cavity to provide additional thermal benefits. This helps the interior remain warmer during the cold season and cooler during the warm season.
How do Cavity Walls Work?
Cavity walls are a drainage-type wall that provides resistance to water penetration and air infiltration. They consist of two wythes of masonry separated by airspace in between.
Either or both wythes of masonry can be load bearing. However, it is more common for the interior wythe to be load bearing. This places less stress on the exterior wythe so that it doesn’t compromise the weathertight seal it is meant to provide.
The mass of the wall and discontinuity of construction, due to the airspace in between, make the cavity wall a good thermal barrier for most types of structures. This method of construction also provides optimal sound penetration resistance.
Do Cavity Walls Need Air Bricks?
Air bricks are masonry units that have a similar appearance and are made from similar materials as a standard brick but have openings to allow for ventilation between the interior and exterior of a wall.
Because cavity walls are meant to be weathertight, air bricks should not be used in a cavity wall system. They allow for both air and moisture to enter into the wall cavity, something which is undesirable for this method of construction.
Air bricks can be, and often are, used in rain screen systems in order to provide ventilation. However, as mentioned earlier, this type of construction is different in concept than that of a traditional cavity wall system.
The rain screen system is designed such that the second layer or interior wythe of masonry is watertight. This is why both moisture and air can penetrate the outer wythe without causing damage to the interior. Cavity walls, in contrast, are meant to have a watertight seal on the exterior wythe.
This is particularly important in systems that are designed with insulation inside the cavity. If the insulation comes in contact with moisture it can become damaged. This compromises the thermal and, potentially, the structural integrity of the wall.
What is the Minimum Width of a Cavity Wall?
Cavity wall widths vary depending on a number of factors. The actual airspace in between a cavity wall system should be a minimum of 2 inches (50 mm). However, it is common to see airspaces of up to 4 1/2 inches (110 mm) to accommodate for the insulation thickness when it is installed within the cavity.
If two wythes of standard brick masonry are used, 3 5/8 inches (92 mm) in width with the minimum airspace requirement, the total width of a cavity wall can be as little as 9 1/4 inches (235 mm). However, this minimum width may not be suitable for load bearing walls and may not provide enough thermal insulation for a building envelope.
More commonly, an exterior wythe of standard masonry brick is used in conjunction with a concrete masonry unit (concrete block) weight-bearing system. These units range from 5 5/8 inches (143 mm) in width to 7 5/8 inches (194 mm) in width.
In such a system, the total wall thickness would range from 11 1/4 inches (286 mm) to 13 1/4 inches (337 mm). This assumes a 2 inch (50 mm) air space. If the space is increased to 4 1/2 inches (114 mm) to allow for 3 1/2 inches of insulation, that total cavity wall thickness increases to 15 3/4 inches (400 mm).
Weep Holes in Brick
Weep holes are positioned towards the bottom of cavity walls, just above the metal flashing. Typically spaced at a maximum of 2 feet (610 mm) on center, they provide a mechanism for water to escape the inside of a cavity wall.
Other common locations for weep holes are at window or door openings and at slabs. Essentially, weep holes can be installed at most locations where metal flashing is used.
Usually constructed of plastic or metal tubing, weep holes are slightly angled to allow for water seepage. Weep holes need to be installed at an appropriate angle. If the angle is too steep, moisture will not be able to enter into the tubing. If too flat, moisture will not make its way out of the weep hole.
Cavity Wall Insulation
Insulation can be placed either within the airspace of a cavity wall, attached to the second (internal) wythe. Usually rigid foam is used, however other types of insulation such as granular fill are sometimes used.
If rigid insulation is used, a gap of 1 inch (25 mm) should be allowed between the insulation and the exterior masonry wythe. This creates a thermal break between the exterior wythe and the insulation and better protects it from moisture.
Insulation can be infilled within the internal concrete masonry units. It can also be placed on an interior furred stud wall as batt insulation. In cases where insulation is placed on an interior wall, the minimum 2 inch (50 mm) airspace should still be maintained in the cavity.
Masonry Ties and Anchors
Because cavity walls are comprised of two separate wythes of masonry, they need to be bridged structurally as a unit. Masonry ties are used to anchor the two separate wythes of masonry at the wall cavity.
Used in conjunction with other reinforcing systems, masonry ties provide lateral support for the cavity wall. They also allow for the separation of two wythe masonry units with minimal thermal bridging.
Masonry ties are usually made of non-corrosive metals. They are adjustable to compensate for the width of the cavity. Maximum spacing is 24 inches (60 mm) vertically and 36 inches (90 mm) horizontally.
Cavity walls do not require additional venting beyond what weep holes provide. Unlike rain screen systems which are intentionally designed to allow air and water penetration, cavity wall systems are weathertight.
In order to maintain the integrity of the cavity wall system, it’s important for the exterior wall surface to be as watertight as possible. This can be achieved with the use of mortar and silicone sealants at joints.
Since movement, deterioration, and potential moisture infiltration are inevitable, cavity walls make use of metal flashing and weep holes to drain out water. Weep holes have the added benefit of allowing wind to provide some degree of airflow in the cavity. This eliminates the need for additional ventilation.
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