Horizontal open state mineral fiber fireblocks incorporating intumescent strips are a relatively new passive fire protection solution for rainscreen facades in the USA.
Sometimes referred to as open state cavity barriers (OSCBs), they permit ventilation and drainage under normal conditions and inhibit fire spread once the intumescent material expands to fully seal the concealed cavity after heat activation at approximately 266°F/130°C.
However, it is vital that they are securely fixed to the internal substrate to prevent them from becoming displaced and subsequently compromising the passive fire safety of the facade.
Buildings are subjected to forces such as wind loads, settlement and temperature changes that can cause them to move.
In a fire, heat-induced air pressure and thermal expansion can cause further stress on the structure.
If the fixings used to attach fireblocks to the internal substrate lack sufficient tensile strength or retention capability, these factors can cause complete or partial detachment.
In addition to thermal and moisture management issues from fallen fireblocks obstructing ventilation and drainage flow in the cavity, any gaps that form between the substrate and the fireblock will allow fire to spread vertically through the unseen cavity.
Misalignment of the fireblock can also lead to its intumescent layer not expanding within the limits of its stated air gap range, thus failing to form a complete seal against the opposing surface.
To mitigate these risks, open state fireblocks must be selected with fixings that are designed to keep them securely in position even when subjected to movement or fire-induced stresses and that allow for the intumescent to fully expand when activated.
Fixing approaches for open state mineral fiber fireblocks with intumescent are as follows:
Direct through-fix to substrate using non-combustible fixings (small voids): Usually stainless-steel countersunk head screws fixed at a sufficient depth into substrate with a washer which sits flush with the intumescent face
Mechanically fixed partial penetration brackets (large voids): Usually stainless-steel spikes that penetrate the width of the fireblock by typically 50-75% and fixed to the backing wall using non-combustible stainless-steel fixings, sometimes with pigtail screws piercing through the intumescent into the fireblock
Mechanically fixed full penetration brackets with bent tails (large voids): As above without pigtail screws but with ‘tails’ piercing through the intumescent on the leading edge of the fireblock. These are bent flat, or ‘counter folded’, to the outer face to positively retain the fireblock and the intumescent
Section 718.2.1 of the 2024 International Building Code (IBC) requires “batts or blankets of mineral wool, mineral fiber, or other approved materials” to be “installed in such a manner as to be securely retained in place”, whilst Section 718.2.1.4 says the “integrity of fireblocks shall be maintained.”
While it doesn’t provide a specific fixing methodology, guidance from industry bodies advocates for positive mechanical retention.
For example, the Centre for Window and Cladding Technology (CWCT)’s Guidance on Built-up Walls advocates for OSCBs used within the concealed space of ventilated exterior wall assemblies to be fixed with brackets that penetrate the full thickness of the fireblock with tails bent over to provide positive restraint:
“Steel spikes are used to fix the barriers, and these should penetrate the full thickness of the barrier with the tails turned over to positively hold the barrier in position.”
It says failure to do so may allow the barrier to fall down the cavity, making it ineffective as a fire barrier.
Section 58 of its Technical Note 116 Fire performance of façades – Application to built-up walls, further reinforces the difference in effectiveness between full penetration and partial penetration brackets: “Fixings that provide positive restraint to open state cavity barriers such as spikes that penetrate the full thickness of the barrier and are then bent over to restrain the cavity barrier are preferable to spikes that merely penetrate part way through the barrier.”
It also highlights the practical benefits of these products, as “in addition to providing a more secure fixing, the return remains visible making it easier to check that the fixings have been installed at the required frequency.”
Fireblocks whose retention is reliant on the exterior wall covering being installed can require a section of the covering to be deconstructed to check how they have been fitted.
Meanwhile, the visible tails eliminate the need for destructive inspections.
Full penetration brackets that have retaining bent tails enable the fireblock to be firmly secured to the substrate, preventing it from moving laterally during its service life.
They can also reinforce its structural strength by allowing its weight to be supported by the inner leaf, instead of relying on the tensile strength of the bracket alone.
Additionally, an equilibrium is created between the fireblock’s outer face and the bent tails when the bracket is fixed to the substrate above the fireblock, which can increase structural strength and prevent sagging.
As they do not rely on the external wall covering for retention, they can also be fitted at an earlier stage of construction.
They are much less likely to be dislodged or disrupted during the installation of the external envelope.
It is much easier for installers and inspectors to confirm if the fireblocks have been installed correctly.
Meanwhile, some types of open state fireblocks require hand-screwed pigtail screws to be wound out until they touch the inside face of the cladding, after it has been installed.
The fixing method is key to the performance of intumescent open state fireblocks, so it is important to look for products that have been tested with their fixing methods.
This includes identifying products with a defined design life, and that have been put through tests that replicate their intended application.
The Association for Specialist Fire Protection (ASFP) Technical Guidance Document TGD 191 and the European standard EN 1364-6: 2025 both outline the test configurations and failure criteria for open state cavity barrier products.
Whilst these tests can determine standalone product performance, it is only through large-scale testing that we can fully understand how complete assemblies might perform, along with the impact of details such as fixings, joints, gaps and penetrations.
Therefore, data from large scale systems tests such as NFPA 285 can be invaluable.
Even with positively retained fireblocks, systems can fail if the exterior wall covering allows the fire to spread up the outside of the building causing the panels to move, break down or detach and fall.
Therefore, it is vital to always refer to the latest specialist advice for facade design and specification, and to ensure high quality workmanship at every stage.
For open state mineral fiber fireblocks to perform effectively under both fire and non-fire conditions, they should be mechanically fixed using full-penetration brackets with bent tails that have been engineered to withstand serviceability movements and thermal expansion.
For further information contact Siderise’s Fire Safety Technical Support team via natech@siderise.com or visit www.siderise-us.com.
