Cavity walls are widely used in buildings to control moisture and provide insulation to buildings. While they do offer obvious benefits, they also pose a fire risk if proper consideration isn’t given to cavity barriers or insulation.
In this article, we look at the risk of fire spread in cavity walls and question whether current recommendations are sufficient.
The evolution of cavity walls
Before the 1970s, the two skins of a cavity wall were typically constructed by masonry materials and no insulation materials were provided between the two skins. However, the skins of modern cavity walls now vary in design.
Weep holes are often provided at the bottom of a cavity wall or above windows. These allow wind to generate airflow through the cavity which drives moisture out and prevents it from penetrating the wall into the building.
Some innovative designs also use double-skin walls to assist air conditioning in buildings or to improve the thermal insulation performance of external walls.
Cavity walls – the risks
Introduction of a cavity to an external wall can help to solve the moisture penetration problem, but it also introduces a route for fire spread.
The following image shows how fire spreads through the cavity from a window opening.
Figure 1 Principle of fire spread into a cavity wall
Flame and hot air flowing out of a window will also flow into the cavity. If the cavity opens to other floors, the flame, hot air, and unburnt combustible gases generated by the fire can flow to other rooms of the building.
Recommendations
To stop fire spread, design guidance typically requires a cavity to be sealed around openings and for cavity barriers be provided. Cavity barriers block the airflow in the event of a fire spreading into the cavity.
A recent revision of the Approved Document B 2019 (ADB 2019) outlines recommendations for residential buildings. Where the two skins are made of brick or concrete of at least 75mm in thickness, the design will be acceptable if a cavity within the wall is closed at the top and around any openings.
A 75mm thick brick wall can typically achieve one hour of fire resistance, so on the surface, the code-compliant design appears robust.
The shortfalls
Unfortunately, the recommendation does not include guidelines for the materials used to close the cavity, meaning they don’t need to achieve a specific performance concerning fire resistance. In fact, any kind of combustible materials can be placed within the cavity.
This causes a problem.
A fire that breaks a window could be a well-developed fire that can melt materials such as aluminium. With the non-fire-rated cavity closer, non-provision of cavity barriers, and no limit on the use of combustible materials, a flame passing a cavity at a lower floor could quickly burn out the cavity closer and spread flame into the cavity. With the provision of weep holes, the supply of oxygen will be abundant, and a flame can quickly build up.
Numerical simulation of a Celotex Board fire in a cavity showed a flame grows from 0m to 9m within 30 seconds. With such a flame in the cavity, the cavity closers at upper floor windows could be quickly burnt out and a cavity fire could spread to multiple floors.
Furthermore, building walls are rarely airtight. Without cavity barriers, hot air, gases generated from compartment fires and burning combustibles within a cavity wall can find a lot of leak paths. This could result in a fire spreading to multiple compartments simultaneously.
Compared with recommendations outlined in the previous version of ADB, it looks like ADB 2019 has lessened the fire protection to external walls in certain building types.
Taking precautions
We suggest that for high-rise residential buildings, it’s necessary to enhance the ADB 2019 recommendation by including the provision of fire rated cavity closers and cavity barriers along compartment lines. This would reduce the potential of fire spread through cavities.
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