With cavity barriers being a vital component of passive fire protection, FPA Technical Director, George Edwardes looks at an alternative test method
A cavity barrier is a non-combustible barrier placed inside a cavity to prevent the spread of fire through a building. Cavities are found within concealed spaces, such as wall cavities, floor voids, and roof spaces.
There are two main types of cavity barrier, referred to as open-state and closed-state. A closed state cavity barrier is a non-combustible barrier which always blocks off the cavity. An open state cavity barrier will leave the cavity open during normal operation, to allow ventilation, but will expand and close off the cavity during a fire.
Cavity barriers are typically made from mineral wool to meet the insulation and integrity requirements from fire resistance tests. They can also be made from fire resistant boards, solid metal plates, or metal gauze. Most open state cavity barriers use an intumescent material to expand and fill the air gap when exposed to heat and some cavity barriers rely on the thermal expansion of metal components.
What is the current test method?
The requirements for testing a cavity barrier depend on its use. Most cavity barriers are used in line with the edges of fire compartments, to continue that compartmentation through the cavity and are therefore subjected to a similar fire resistance test as the internal fire compartment. Fire resistance tests involve using a furnace that is capable of following a temperature curve, reaching temperatures in excess of 1,000°C. The test component is placed in the wall or ceiling of the furnace and subjected to the furnace temperature on one side and ambient temperature on the other.
The British Standards Institution (BSI) publishes furnace test methods for fire doors, plasterboard, joint seals, and other products, but there is currently no specific fire resistance test for cavity barriers. Most tests are done by adapting the furnace, following the guidance in ASFP TGD 19 - Fire resistance test for open-state cavity barriers used in the external envelope or fabric of buildings. This guidance is in the process of being converted into a European Standard (EN 1364-6) and a draft is currently out for public comment.
This test subjects the cavity barrier to a slowly ramping temperature and measures the point at which the barrier fails to maintain insulation or integrity. To maintain integrity, the cavity barrier must remain intact and prevent the passage of flames or hot gas. To maintain insulation, the outside temperature must remain within 180°C of ambient. The test allows the cavity barrier five minutes to close off the cavity and as such any failures recorded before the five-minute mark are disregarded.
What are the limitations of ASFP TGD 19?
ASFP TGD 19 is effective at testing the insulation and integrity of a cavity barrier once it has closed and sealed off the cavity and this is what fire resistance tests in a furnace are intended to assess. There are limitations, however, due to the use of a fire resistance furnace. A furnace slowly ramps up the temperature, creating the ideal conditions for an intumescent barrier to expand and seal, but a rapid or instant exposure to flames can cause an intumescent to become brittle and break apart easily.
One of the scenarios cavity barriers need to be prepared for is a room in flashover breaking through a window and instantly exposing the cavity barrier to high temperatures. ASFP TGD 19 does not assess the ability for the cavity barrier to close and seal before the cladding material ignites as the initial heat source is not large enough. Even if a system does fail before the intumescent has sealed, TGD 19 disregards any failures before the five-minute mark.
Being a furnace test, the heat is applied uniformly along the full length of the cavity barrier and the intumescent seals across the full length. In reality, cavities can be up to 20m wide and subjected to a point heat source, which will reduce in temperature along the cavity barrier, and this may never fully seal. ASFP TGD 19 does not assess the activation temperature of the intumescent or help predict how far along the cavity the intumescent will expand and seal it.
FPA proposal for open-state cavity barriers
When assessing an open-state cavity barrier, it needs to be prepared for flames suddenly breaking into the cavity and the extent to which it prevents the spread of flame. The key questions to assess are:
- How long do flames pass the cavity barrier before it seats in the location of the flame?
- Is that duration sufficient to propagate fire on the other side of the cavity barrier for the given material collection?
- What is the horizontal seating rate of the cavity barrier?
- What is the critical temperature at which the cavity barrier seats?
- What is the system’s ability to prevent the passage of (a) flame, (b) heat, (c) smoke, (d) toxic products?
- How has the presence of the cavity barrier reduced the potential fire size?
To address these questions, the FPA has developed a test method which simulates a realistic fire scenario for a rainscreen cladding system. This test uses ducting to direct flames from a gas burner straight into the cavity, simulating a fire scenario that provides flames with instant access to the cavity barrier. It simulates a cavity which is 4.5m wide, and the depth of the cavity can be modified depending on the size of the cavity barrier being tested.
In this test, the cavity barrier is installed in accordance with the manufacturer’s requirements. A non-combustible cladding material is positioned to form the cavity at a distance to give the required test parameters of cavity depth, and closure depth. Fire is introduced through a simulated vent hole at a location below the cavity barrier, and thermocouples are placed along the length of the cavity barrier, both above and below, to monitor temperatures.
Through visual observation and temperature measurement, the response time of the cavity barrier to restrict the passage of flames and hot gasses is measured. Assessment is also made of the ability of the cavity barrier to remain in place once activated. The test rig has been designed to generate the following data:
- Initial activation time of Open-State Cavity Barrier (s)
- Max sealing distance (m)
- Average rate of cavity sealing (m/min)
- Material falling from the cavity barrier (visual observation)
Table showing example of test data
What is the aim of the test?
The initial aims for this test method were to produce a simple way of testing open-state cavity barriers with the intention of:
- providing an insight into the process of how open state cavity barriers work in practice, inside the confines of a representative external cladding cavity
- providing data on the extent (and rate of spread) of lateral activation of the intumescent materials (where used) within a cavity
- obtaining quantitative test data that could characterise the performance of open state cavity barrier products (‘fingerprinting’)
- obtaining test data that might be used to compare the performance of different open state cavity barrier products
- being able to assess the suitability of using a specific open state cavity barrier product with different types of insulation materials and void membranes that they might be used to protect.
Test evolution
The design of the test rig has been modified since its inception. Changes have been made to the following design aspects:
- Flame inlet position: This was moved from the centre of the rig to one end. This effectively doubled the width cavity barriers could be assessed over.
- Air supply: A forced air supply, with a disbursement method was introduced into the cavity to maintain flaming even when the cavity is fully sealed.
- Smoke collection hood: This was originally included to enable measurements for gas flow rate and smoke toxicity. It was removed to simplify the design and aid visibility, specifically the progression of flames past the cavity barrier. This was found to be of greater benefit than the smoke toxicity measurements.
- Height of the walls: The height was increased to conform with a multiple of the standard size of plasterboard sheets, making the test rig easier to construct and produce less waste.
- Ventilation: Multiple holes were included at either end of the cavity to improve ventilation.
- Thickness: The thickness of the cavity was modified to improve flame stability.
- Thermocouple location: The location relative to the cavity barrier and insertion depth were both optimised to maximise the temperature difference above and below the cavity barrier, to enable the most effective measurements for determining cavity barrier closure rates.
- Assessment metrics: The original set of 10 performance metrics was narrowed down to four simpler performance metrics, mentioned above, with the intention of still being able to achieve the main aims of the test.
Cavity barrier test rig diagram
Conclusion
The information collected from these tests provided a useful insight into the response of open state cavity barriers when subjected to a sudden increase in temperature as might be encountered if fire from within a compartment were to break into a cladding cavity.
The test rig and method was found to be easy to operate and produced potentially valuable data that cannot be collected through current test standards for open-state cavity barrier products. It also enabled a number of metrics to be derived that could be used to characterise the performance of open-state cavity barrier products, producing a fingerprint of their performance.
The test method and results have been presented to the European Commission for consideration as they develop EN 1364-6, a fire resistance test for open-state cavity barriers, and the test is also available for anyone who would like to learn more about the performance of their cavity barriers.
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George Edwardes is Technical Director at the Fire Protection Association