What Is a Cavity Wall?

From the outside, most brick, block, or stone structure walls look like one solid piece of construction. However, hiding inside those walls is often a gap, stretching the entire length and height of the wall. This gap isn’t an error in the wall’s construction or an oversight. In actuality, these cavity walls (the common term for this type of wall with a gap) serve some very important purposes. 

What is a cavity wall?

Plainly put, a cavity wall is a masonry structure made up of two separate walls (called leaves) running parallel to each other. These walls are typically known as leaves, and they’re tied together with blocks or metal ties. Between those two walls is a gap (usually under 10cm or just under 4 inches). 

Cavity walls can be load-bearing or not, allowing for use anywhere in a structure. For situations where a cavity wall might carry a load, the inner wall is typically thicker than the other wall. This allows the inner wall to do most of the work while the outer wall serves more as a facade. 

Many older masonry structures, whether they’re commercial or residential, utilize cavity wall construction. For modern buildings using this method, the gap is typically filled with cavity wall insulation made from foam or mineral wool. This insulation allows the home or building to retain heating or cooling better using modern insulation materials.  

Uses for cavity walls

Cavity walls are most common on external walls, and the gaps within these walls create a bit of a moisture barrier. Since the two walls don’t touch, moisture is unable to bridge the gap from the exterior of the building to the interior. This makes cavity walls a viable solution for damp areas where solid walls would wick up moisture and transfer it inside the space.

Air is actually quite good at insulating, as space filled with air prevents thermal bridging. In cavity walls, the only thermal bridges available are the metal ties or brick ties that connect the two walls. If the cavity is sealed tightly, the air serves as thermal insulation that prevents heat loss or gain. In other cases, designers might call for a cavity wall specifically for the ability to add insulation between the two leaves. By itself, a cavity wall provides half the heat flow rate of a solid wall. Adding modern insulating materials between those leaves can only add to the comfort, making this type of wall better suited for colder climates than solid masonry walls.

It’s also possible to find cavity walls on the interior of the building, as well. Even in wood-framed structures where wood floors and walls exist, a secondary interior leaf can provide additional structure, support, and insulation. 

Advantages and disadvantages of cavity walls

When it comes to stone and brick structures, cavity walls are still a relatively popular building method. With this building method comes several advantages and a few disadvantages.

Advantages of cavity walls

  • Moisture prevention. The main advantage of a cavity wall is moisture prevention. Without a solid wall to wick up all the moisture from the external walls and bring it in, the interior leaf stays dry. And, with a damp-proof course (a row of bricks protected by a water-resistant membrane), cavity walls can offer all the moisture protection necessary. 
  • Heat insulation. The ability of a cavity wall to insulate better than a solid wall of the same overall thickness is a big advantage. The trapped air and lack of thermal bridging reduce heat loss in the winter and help prevent conditioned air from leaving the space in the hotter months. 
  • Sound insulation. The lack of interior structure between the leaves also prevents the transfer of sound. This is helpful for creating soundproof rooms or preventing excess street or neighbor-related noise from making its way into a home or office.
  • Material savings. Leaving a gap between the two leaves requires fewer materials. Rather than building a wall with a third course of bricks or using wider blocks, the gap (coupled with the metal ties) provides a strong yet economical method for building. 

Disadvantages of Cavity Walls

  • Expertise required to build. Most framing crews can build a straight wall with ease, but it takes an experienced (and more expensive) mason to build a straight, evenly spaced cavity wall. This means more money spent on the project and possibly longer lead times waiting for a craftsman that can do the job.
  • Sediment and dirt accumulation. In older buildings, sediment such as dirt, debris, flaking mortar, and other materials can accumulate in the cavity. This sediment can allow for thermal bridging, removing one of the biggest benefits of cavity wall construction. 
  • Expensive repairs and modifications. In buildings where some added insulation is desirable, property owners must call experts to drill holes in the cavity walls and fill them with insulation. Going this route for insulation is expensive compared to pulling down drywall for a DIY insulation project. One other point to consider is that the metal ties holding the two leaves together can rust and rot over time. This is especially true when the exterior wall takes on water or experiences condensation, rusting the brick ties apart. This can lead to a safety hazard as the leaves can separate and weaken the structure.

MT Copeland offers video-based online classes that give you a foundation in construction fundamentals with real-world applications, like how house framing works. Classes include professionally produced videos taught by practicing craftspeople, and supplementary downloads like quizzes, blueprints, and other materials to help you master the skills.

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