Feb 24

Rainscreen cladding and fire risk

Rainscreen cladding is designed to protect a building from the weather. However, the fundamental elements of the ventilated cavity that enables the rainscreen system, also make it a fire risk.

What makes it a fire risk?

The ventilation required to enable the system to dry out can act as a ‘chimney’ for the fire, causing it to spread. This causes a direct design conflict as the cavity needs to be closed off in the event of a fire, but left open to allow the system to ventilate in normal conditions. You need to design in a heat cavity barrier as part of the system that would expand to close the void when required.
Approved Document B of the Building Regulations (Part B) makes provision for this risk. But rainscreen cladding and fire is a minefield of regulation.
It’s a complicated area, so whilst we may cover some of the considerations and topics to think about, this is simplistic guidance. Each project needs to be reviewed in its own right and expert advice should be sought.
Part B has elements that contradict each other. The regulations are to ensure a cavity barrier around all penetrations: from ducting to door or window openings; at floor levels and at any compartment lines. For example, when designing flats, regulations require fire compartments are used internally and the protection must carry through to the cladding.
Due to the complexities in the rainscreen system there is a direct mismatch in regulations. In the vertical aspect, for the system to function correctly there has to be a cavity for ventilation to enable free airflow. However, in the horizontal aspect there has to be a complete block in the event of a fire. This, of course, causes a design conflict. To overcome this, a form of intumescent closure must be put in place that will allow air to pass through under normal conditions, but close up when in contact with heat from a fire.

Design considerations

It is important to note that a cavity barrier should not be confused with a ‘Fire Stop’. Whilst they are both parts of the fire strategy, they perform different functions and should be used in the appropriate way for each individual project.
According to Part B ‘the cavity barrier must provide 30 minutes integrity and 15 minutes insulation’. This means that the product chosen for the cavity barrier must remain intact and in place for 30 minutes, without allowing any heat to pass through the barrier. On the flip side, another section of the document states ‘1mm steel material to provide a cavity barrier’. This will give 30 minutes integrity but will not provide any insulating properties. Thus, it will only provide one of the 2 requirements.
Building Control enforcement on the interpretation of Part B varies between regions. Some Building Control Officers will accept a steel cavity barrier and others will only accept a barrier that provides both insulation and integrity.

Product choice for cavity barriers

There are two main materials for cavity barriers. The first is a dense ‘Rockwool’ solution with a heat-activated strip that allows expansion when heat is detected, giving both insulation properties and providing fire integrity.
The second option is an expanded mesh or perforated metal, coated with intumescent paint which, when exposed to heat, expands to fill the gaps in the perforations and closes the void. However this will not provide any insulating properties.

Installation considerations

The cavity barrier must be secure. A Rockwool insulating system could be installed into the cavity barrier, but it has to be fixed onto a robust backing. It is not uncommon to see a cavity barrier installed so tightly that the ventilated rainscreen system doesn’t have the ventilation required to dry out. This can leave water in the system that can cause the cladding to warp, and in extreme circumstances, even tear the fixings from the substrate.

In summary

All Building Control will do is ensure you conform to Building Regulations. Based on the conflict of ‘the cavity barrier must provide 30 minutes integrity and 15 minutes insulation’ but can be achieved by using a ‘1mm steel material to provide a cavity barrier’ interpretations can vary between building control areas.
The real issue is that these loop holes will enable a job to be priced cost effectively with steel. This could leave specialist subcontractors who have priced the job to achieve both integrity and insulation, appearing too expensive. The risk is that this will be picked up by Building Control and not passed, or the winning contractor will have to go back to the client and let them know the priced material may not pass inspection.
Approved Document B of the Building Regulations is in the process of being reviewed, but from Building Control to the specialist contractor, there is currently conflict.
You could design and install for integrity & insulation, at a cost, but a Building Control Officer could have signed off on integrity only with steel. Who’s to say what you should do for the best? Quality and performance, at a cost, or a cost effective solution that may tick the box…
Bailey would always recommend that the rainscreen is designed to provide the best possible performance and to meet both integrity and insulation.
Find out more about specifying rainscreen cladding systems.

 

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