CPD: Open state cavity barriers for rainscreen
Above: Positioning of horizontal and vertical cavity fire barriers for rainscreen cladding
Open state cavity barriers are vital to the prevention of the spread of fire in high-rise buildings with ventilated rainscreen cladding, says Chris Hall.
Modern ventilated rainscreen cladding systems have become the preferred choice for architects across the world for high-rise structures, providing design flexibility as well as weather protection. However, what lies beneath the skin is equally important in producing high-rises that are as safe as traditional low-rise buildings.
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One of the principal challenges is how to use a ventilated rainscreen system that allows a facade to function by the continuous movement of air, while ensuring the cavities formed can be closed in the event of a fire, stopping flames and smoke travelling from one part to another.
This CPD offers an overview of the legislative and performance requirements for the provision of cavity barriers in ventilated rainscreen, with guidance on system selection.
Ventilated facades comprise a wide range of panel materials, complex designs and interfaces.
In very simple terms, a ventilated system consists of an outer panel, a ventilated cavity, insulation and an inner leaf. This allows ingress of air at base and egress of air at the top, circulates air to expel moisture, can be drained and ventilated, and sometimes pressure equalised.
There is what might be considered to be a chimney effect within the cavity. This needs to be effectively compartmented in order to prevent unchecked fire spread.
In England & Wales we are familiar with Approved Document B (ADB), which gives us guidance on ways to satisfy the Building Regulations and relevant performance requirements. In Scotland we refer to the Technical Handbook, Section 2, and in Northern Ireland, the Technical Booklet E.
For the purposes of this CPD, with respect to ADB Volume 2, the provision for cavity barriers is illustrated in Diagram 33 (see box below). Contrast here the internal compartmentation, where we require a firestop to be installed to maintain continuity of fire resistance of the compartment element, which is the case when we install a perimeter barrier firestop between a slab edge and the interface with curtain walling.
As the temperature rises, the intumescent material activates, creating a seal to prevent further spread
Where rainscreen is concerned, we are more concerned with the subdivision of the external cavity. Any building with a compartment floor above ground floor needs cavity barriers. Their use is independent of building height and/or use category. From ADB, the simplest definition of a cavity barrier would be a construction to close a concealed space against penetration or spread of smoke or flame.
The Building Regulations state specific functional performance requirements in terms of integrity and insulation criteria. In England, Wales, and Northern Ireland a cavity barrier is required to provide 30 minutes integrity (E) and 15 minutes insulation (I). In Scotland there is no requirement for insulation.
Fire engineered and insurance-driven specifications increasingly require an EI 30 performance requirement: 30:30 and higher has become market standard. The regulations also require that cavity barriers are tested to meet that performance for integrity and insulation.
In parallel with regulations, Centre for Window and Cladding Technology (CWCT) performance standards provide a framework for specifying building envelopes. The National Building Specification and National House Building Council Guides also refer to the CWCT standard for systemised building envelopes.
220.127.116.11 Cavities in Rainscreen Walls states that cavity barriers shall be provided:
- On the line of any compartment wall or floor.
- To close the cavity around penetrations through the rainscreen for windows and doors.
- To subdivide the cavity with horizontal and vertical barriers.
Position of cavity barriers
For rainscreen walls, ADB states that cavity barriers are to be provided:
- At the junction of the wall with a compartment wall or floor;
- To close the edges of cavities including around window openings; and
- To limit the maximum dimension of the cavity to 20m where the surface/product exposed in the cavity need to meet National Class (BS) of Class 0 or Class 1, and should a Harmonised Product Standard exist then EN Classification of A1 or A2-s3-d2 or B-s3-d2 or c-s3-d2 and 10m in other cases. (This includes shear walls).
Design of cavity barriers
How do we bridge the basic needs of rainscreen – ventilation and drainage – with the need to prevent the spread of smoke and fire? Conventional “full width” seals will prevent ventilation and drainage, leading to moisture ingress and damage. The accepted market solution,acknowledged by the CWCT standard, is the use of intumescent materials which allow the cavity to be maintained under normal circumstances but seal it in the event of a fire.
The barriers are sized and installed to maintain the desired ventilation air gap. In the event of a fire, the intumescent will activate at a critical temperature, typically around 130°C, and will begin to expand. For an initial period, smoke will bypass the cavity barrier until the intumescent seal is fully closed. The “integrity” criterion is re-established after the initial closure. The expansion continues to fully close the void.
Provisions for cavity barriers
1. Close top of cavity 2. Compartment floor 3. Wall forming bedroom or protected escape routes 4. Close around openings 5. Sub-divide extensive cavities 6. Compartment wall 7. Sub-divide extensive cavities 8. Close around edges
For guidance in terms of required air gap size, we refer to the CWCT standard. The standard states that these minumum dimensions ensure that no blocking of the cavity occurs. Additionally it states that the “area of the path shall not be reduced by more than 50% at fire barriers or support rails”.
- Sealed joints: 25mm air gap (drained and ventilated); 12.5mm reduced gap at fire barriers
- Closed joints: 25mm air gap (drained and ventilated); 12.5mm reduced gap at fire barriers
- Labyrinth joints: 38mm air gap (drained and ventilated or pressure equalised); 19mm reduced gap at fire barriers
- Baffled joints: 38mm air gap (drained and ventilated or pressure equalised); 19mm reduced gap at fire barriers
- Open joints: 50mm air gap (pressure equalised only); 25mm reduced gap at fire barriers.
Noting the open joints figure, we can see how the 25mm gap size became our design requirement when developing cavity barriers. All Siderise rainscreen cavity barriers are developed to function with this 25mm minimum gap.
In practice, due to the depth of cladding panels, location of supporting rails and practical site tolerances, we also have developed a range of barriers to seal 50mm air gaps.
Testing of rainscreen cavity barriers
We first began to test cavity barriers for rainscreen in the late 1990s; prior to that, only assessments were available. The ad-hoc BS 476-20 tests were conducted with the barrier fixed in a cavity between walls of fire-resisting construction.
The regime involves the standard time/temperature curve at standard pressure – but the barrier is installed to maintain a 25mm or 50mm gap. Activation time and expansion properties are critical to the performance of the seal. It is the “time-to-close” property that is considered paramount.
Intumescent materials react in different ways at different temperatures and pressures. For standard proprietary intumescent, there may be a significant time delay before they adequately form a seal.
With a growing market come new products, unfortunately, not all are ultimately fit for purpose. In 2014, the Association for Specialist Fire Protection (ASFP) responded to members ’requirements for a potential route to 3rd Party Certification to remove the uncertainty of other not so well tested products in the market. with the publication of a Technical Guidance Document (TGD) specifically for the testing of open state cavity barriers (see box below).
This gave a new formal definition of open state cavity barriers as cavity barriers that allow ventilation and drainage in the cold state, but which either close in a fire or are inherently fire resisting, providing fire separation in the cavity.
From the ASFP TGD we now have a standard performance for effective closure: Cavity barriers must close within five minutes from the start of the test as determined above or they will have been deemed to have failed the test.
Our current products activate and achieve closure quicker than the allowed five minutes.
Our standard position is that the cavity barrier is to be installed with interrupted thermal insulation above and below. Insulations may be “fire safe”, but not tested to specific requirements of a cavity barrier. To do otherwise introduces questions of integrity performance – either due to the material, gaps at joints or uneven surfaces.
By convention, the vertical barriers need to be installed first and take precedent. Suitable non-combustible fittings must be used.
- Cut material to allow compression to back of panel;
- Impale brackets into barrier;
- Fix brackets to wall;
- Foil tape to joints.
For detailing at an internal corner, horizontal barriers are carefully cut to form a simple mitre, tightly abutted, ensuring continuity of intumescent coverage. Seal joints with foil tape. At an external corner, vertical barriers are installed first at full cavity width for sub-division. Horizontal barriers are then installed with a tight abutted intersection. Seal joints with foil tape.
At an intersection, vertical barriers are installed first at full cavity width for compartmentation. Horizontal barriers installed with tight abutted intersection. Seal joints with foil tape. At windows, extend vertical barriers above and below window apertures. Horizontal barriers are tightly abutted between vertical barriers. Seal joints with foil tape.
An installation video is available here.
Testing open state cavity barriers
Guidance from the ASFP sets out the test configurations and failure criteria.
The Association for Specialist Fire Protection (ASFP) Technical Guidance Document 2014 outlines test configurations and failure criteria. It is based on the linear joint seal test, but modified with upstands to better replicate cavity construction.
In the section above, the cavity barrier is installed with thermal insulation interrupted above and below. A suspended thermocouple above the air gap record temperatures as the seal closes.
It’s worth looking at the failure criteria:
- Closure of seal by five minutes;
- Insulation: <180ºC rise above initial ambient after five minutes;
- Suspended thermocouple can exceed 180ºC rise before five minutes provided that it goes below 180ºC rise afterwards (good indication of seal closure);
- Integrity: cotton pad, and sustained flaming only (no gap gauge) after five minutes;
- Integrity failure relaxation before five minutes does not include sustained flaming of insulation on upstands.
Siderise continues to test to the ASFP TGD 19 standard, as well as collaborating with partners in the large-scale BS8414 tests, which test how all components in a cladding system perform.
Chris Hall is commercial development officer with Siderise.
This CPD is sponsored by Siderise. Visit www.siderise.com