Open State Cavity Barrier

An open state cavity barrier for ventilated façade systems and timber frame construction.


AIM Open State Cavity Barriers (OSCB's) are manufactured from high density Rockwool stone wool and faced with an intumescent strip and colour coded for ease of identification.

Open state cavity barriers allow the ventilation necessary within the wall construction. In the event of a fire, heat activates the intumescent strip which expands quickly to fully close the cavity.

AIM OSCB’s have been designed and tested for use within ventilated rainscreen façades and timber frame construction, allowing free airflow and drainage.

Open State Cavity Barriers offer an effective fire barrier for ventilated voids up to 425mm in width. Having been tested to TGD 19 & the general principles of BS EN 1363-1, they offer a superior fire rating of up to 120 minutes insulation & integrity with ventilated air spaces within the cavity.

The AIM OSCB range is available to provide either a 25 or 44mm air gap. For simplicity the range also offers either a 60 or 120 minute rating for both Integrity and Insulation.


OSCB’s are a polythene sleeved ROCKWOOL® stone wool and intumescent material designed for use within open state cavities (e.g. Rainscreen systems), allowing ventilation and drainage of the cavity under normal conditions while providing up to 120 minutes insulation and integrity performance in the event of a fire.

The AIM OSCB range has been successfully tested to TGD 19 & the general principles of BS EN 1363-1 with a variety of substrates so that they can be used in a multitude of cavity wall constructions.

The OSCB 60/25 has also been tested for use vertically in a masonry cavity up to 425mm.

The AIM OSCB range has been tested to provide solutions that accommodate cladding rails interrupting the line of the barrier.


  • Provides a 25mm or 44mm airspace
  • Fire rated solutions for cavity voids up to 425*mm
  • Heat activates the intumescent strip which expands quickly to fully close the cavity
  • Tested in accordance with ASFP Technical Guidance Document 19 (TGD 19) & to the general principles of BS EN 1363-1
  • Provides up to 120 minutes integrity and insulation performance
  • Galvanised steel fixing bracket supplied as standard
  • Stainless steel metal brackets available as a tested option

* The OSCB25 range can be used in cavities up to 600mm where the barrier is supported by Rockwool Duoslab insulation or equivalent


  • In the event of a fire, provides an effective barrier to the passage of hot smoke and fire behind the cladding system.
  • Designed to enable a continuous airflow behind a rainscreen and timber frame cladding system thus helping to prevent problems of condensation.
  • Flexible specification: Choice of four OSCBs deliver insulation and integrity performance up to 120 minutes.
  • Quick and cost effective installation.
  • Easy to install: Simple fixing procedure; OSCB25 range incorporates spring steel screws and steel hanging brackets. No specialist tools required.
  • Colour coded for ease of identification


OSCB Rainscreen
OSCB Fixing Brackets
AIM Intumescent Mastic
Coarse wound (Pigtail) Screws

Galvanised steel fixing brackets are supplied at a rate of two per metre length. Brackets are packaged in a separate cardboard box located at the bottom of a pallet - the location will be marked with a label.

Fixing Brackets are designed to be easily re-profiled by hand on site, and should be cut as necessary to ensure they penetrate the barrier by at least 50% of its width.

Stainless steel brackets are available as an option.

Coarse wound (Pigtail) Screws are required for AIM OSCB 60/25 and OSCB 120/25, and are used to secure the front-facing intumescent strip. They are supplied at a rate of 3 per metre length and will be packaged with the fixing brackets.

Care should be taken to ensure that the Coarse Wound Screws protrude from the front face of the firestop by a maximum of 25mm.


• Thickness - 90mm
• Width - Total cavity size (up to 425*mm) less 25 or 44mm according to the barrier required.
• Length - 1000mm
• Max Air Gap – 25 or 44mm
* The OSCB 25 range can be used in cavities up to 600mm where the barrier is supported by Rockwool Duoslab insulation or equivalent.


AIM OSCB's are generally packed into cartons and stretch wrapped onto wooden pallets with a showerproof polythene pallet cover and high quality edge protectors.


AIM OSCB’s are supplied pre-cut in 1000mm lengths, 90mm thick with widths varying according to the void size.

The product is supplied pre-cut product complete with the required fixing clips.


In some instances it may be necessary to install a cassette insert to provide a flat service for the OSCB to close against, eg where cassette panels are being used. Please contact AIM our experienced sales team on 01293 582 400 or [email protected]

Stainless steel fixing clips are available on request.


Vertical useYesNoNo No
Horizontal useYesYesYesYes
Cladding railsProvide 60+
minutes integrity
Provide 60+
minutes integrity
Provide 60+
minutes integrity
Provide 60+
minutes integrity
Cavity sizes50mm to 425mm* 50mm to 425mm50mm to 425mm*50mm to 425mm
Polythene sleevedWhite colour codedBlue colour codedRed colour codedBlack colour coded
Fire rating (integrity / insulation)
Masonry to masonry60/6060/60120/120120/120
Sheeting board to masonry60/6060/60120/120120/120
Test evidence includes Euroclass EN 13501-1 A1 Sheathing Board, SFS Framework, Intersecting Vertical Rails.
Test run with combustible PIR, Rockpanel Façade, Extended Airspaces (30mm & 50mm - one hour only).
* The OSCB 25 range can be used in cavities up to 600mm where the barrier is supported by Rockwool Duoslab insulation or equivalent.
NOTE: Increased airspace reduces performance to 60 minutes.
Option for the OSCB 25 range without Coarse Wound Screws is available.
Option for the OSCB 25 range with a 'Through Fixing Clip' is avallable.
OSCB 60/25 has been tested vertically in a 425mm wide cavity, Stainless Steel Screws / Coarse Wound Screws as standard. Stainless Steel Clips available.
We hold R&D test data for Cassette Inserts & for FF102/50 to be screwed to vertical rails.
We hold test evidence showing the performance of AIM OSCB’s where the exterior cladding is a Rockpanel construction. Please contact our technical department for further details.
Test standards employed: BS EN 1363-1 & TGD19.
The OSCB range has been exposed to BS 8414 Fire performance of external cladding systems fire tests and assessed to BR135 to achieve pass results with a variety of third party cladding systems.
The AIM OSCB Range has been tested with masonry façades to establish the performance of the product itself without being influenced by the supporting structure. It may be used with a variety of façade types however this should involve the consideration and approval of a competent person.


Assessment ReportBasic Details of Test
A variety of test reports are available but the suitability of the OSCB
range is encapsulated within these assessments


More drawings available in the Technical Guide which can be downloaded above.


AIM Open State Cavity Barriers should be installed in accordance with the Fixing Instructions supplied with every order.

The installation sequence is outlined within our Technical Guide available to download here.


  • PPE abrasion resistant gloves
  • PPE impact resistant goggles
  • RPE dust mask - FFP3
  • Sharp knife
  • Tape measure
  • Insulation saw
  • Acrylic fire rated intumescent mastic (optional extra)


Products are typically supplied in cartons on wooden pallets with edge protection and a shower proof hood. Products should be stored away from the elements until ready for installation


This product does not contain moving parts and, if undisturbed in the cavity, requires no routine inspections or maintenance.

It is recommended that the integrity of the barrier is rechecked if further works are carried out, which may involve disturbing the product.


AIM fire barriers are chemically inert, will not sustain vermin and do not encourage the growth of rot, fungi, moulds or bacteria. They are compatible with all normal building materials. They do not degrade under the usual conditions found in buildings and will perform effectively for the life of the building.


Insulation products supplied by AIM are considered to be inert articles and as such are exempt from requirements to provide a Safety Data Sheet.

A Product Safety and Handling Information Sheet is available upon request.


Global warming potential = zero

For product recycling please contact: Rockwool T: 01656 868400 E: [email protected]


To order this product the following information will be required:

• Cavity depth in mm
• Fire Performance required
• Approximate quantity
• Delivery location

All AIM fire barriers are made to order. Products are typically supplied in seven to ten working days but lead times may vary depending on existing factory commitments.

There is no minimum order quantity or value although small orders may attract transport surcharges.


Technical Support is available from our experienced sales team on 01293 582 400 or [email protected]

Our Partners & Affiliates

AIM are partners with NBS. Our products can be found on NBS Source and have been authored to NBS specification standards and have both CAWS and Uniclass 2015 classifications.

AIM are members of Centre for Window and Cladding Technology & ASFP.

AIM working in partnership with Rockwool.