LEEDS OFFICE: 0113 460 5906 LONDON OFFICE: 0203 773 8320
  • NEWS

Innovation Fire Engineering - Fire Safety Engineers

  • HOME
  • ABOUT US
  • SERVICES
  • PROJECTS
  • NEWS
  • CAREERS
  • CONTACT
0
Steph
Monday, 04 May 2020 / Published in Fire Engineering Articles, Innovation Fire Article, Innovation Fire News

Cavity walls and the risk of fire spread

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.

About Innovation Fire Engineering

Innovation Fire Engineering is an award-winning, independent company, specialising in the design of innovative fire engineering strategies for buildings.

 

If you would like to discuss a project or have any questions about fire engineering solutions, then our friendly consultants are happy to help. Call us on 0113 460 5906 to find out how we can support you.

 

 

 

What you can read next

Iceland
Guggenheim Museum
Towering Above Toronto

Recent Posts

  • Happy New Year!

    Wishing you a Merry Christmas and Best Wishes f...
  • The Good, The Clad and the Ugly – Fire Engineering and Fire Safety

    Innovation Fire Engineering will discuss design...
  • Care Home and Specialist Housing

    Research on fire safety in specialised housing and care homes

    Award winning fire engineers working on fire sa...
  • Shortlisted for the Midlands Residential Property Awards!

      “Shortlisted for Midlands Residential Pr...
  • 65,000 Visors for the NHS!

      We are very proud of our Associate, Hele...

Categories

  • Fire Engineering Articles
  • Innovation Fire Article
  • Innovation Fire News
  • Uncategorized

LEEDS OFFICE

North Lane House
North Lane
Leeds
LS6 3HG

Find us

General Enquiries

0113 460 5906

LONDON OFFICE

Floor 3
6-8 Bonhill Street
London
EC2A 4BX

Find us

General Enquiries

0203 773 8320

MENU

  • HOME
  • ABOUT US
  • SERVICES
  • PROJECTS
  • NEWS
  • Privacy and GDPR
  • CONTACT

FOLLOW US ON:

  • Facebook
  • Twitter
  • Google +
  • Linkedin

AWARDS

© 2017 Innovation Fire Engineering LTD | Registered in England: 6972239 Registered Office: North Lane House, North Lane, Leeds. LS6 3HG.
Website Designed by Bam Web Solutions LTD Web Design Leeds
TOP
Privacy and GDPR data policy: Find out more.