By James Hulbert: Product Manager – Rock Mineral Wool (Knauf Insulation Northern Europe)
‘Eighteen’, what’s our obsession with the number; years old perhaps? You can vote, buy alcohol and get a tattoo, though not necessarily in that order! Staying ‘current’ in X-factor parlance, ‘Eighteen’ is also a One Direction song penned by ginger haired pop mega minstrel Ed Sheeran… I could go on, but I’ve a word count to hit and a more interesting topic to talk about.
Eighteen is a height, in metres, that should (and does) resonate through the ears of many when mentioned in the same sentence as rainscreen cladding.
Why should one number be so embedded in people’s minds?
The Building Regulations is always a good place to start: ‘External Fire Spread B4.(1)’ states ‘The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height,’.
‘Having regard to the height’ is also referred to in Approved Document B v.2 (AD B2), source of the hallowed 18m figure, ‘12.7: In a building with a storey 18m or more above ground level any insulation product…used in external wall construction should be of limited combustibility…’
This option offers the simplest, risk free route to compliance, so what constitutes a material of limited combustibility?
Limited combustibility is a material with a Reaction to Fire of Euroclass A2 or better (that being A1). By installing a non-combustible (A1) insulation, for example rock mineral wool, compliance with AD B2 12.7 can be achieved. Simple….
What materials don’t fulfil this option? BCA Technical Guidance Note 18 (18 again) observes that, ‘Thermosetting insulants (rigid polyurethane [and phenolic foam boards]) do not meet the limited combustibility requirements’, ‘and so should not be accepted as meeting AD B2 paragraph 12.7’.
PIR and phenolic foams therefore need an alternate route to compliance and AD B offers this, stating external walls should, ‘meet the performance criteria given in the BRE Report Fire performance of external thermal insulation for walls of multi-storey buildings,’.
Upon selecting this route it’s critical that you are designing and constructing the exact build-up that has been tested, being mindful that variances in any element such as insulation thickness, sheathing board and façade will compromise system compliance with potentially significant ramifications in performance, time and cost.
Returning to the [extremely] tenuous One Direction link from earlier, there are four members of ‘1D’, and by sheer coincidence there are four options for compliance. With two down, the next two options ramp up the complexity.
BCA TGN 18 does offer a third option stating, ‘If no actual fire test data exists for a particular system, the client may instead submit a desktop study report from a suitable independent UKAS accredited testing body’, ‘stating whether, in their opinion, BR135 criteria would be met with the proposed system. The report should be supported by test data which the test-house already has in its possession. This option may not be of benefit if the products have not already been tested in multiple situations / arrangements. The report should also specifically reference the tests which they have carried out on the product.’
Of course with this and the previous method of compliance, any damage to the system in use or later modification of the building may have an impact on the relevance of the fire test results and the criteria that were considered when undertaking a desk study.
Three down, one to go, and one that involves the highest degree of complexity is also put forward by BCA Technical Guidance Note 18 and states that ‘if none of the above options are suitable, the builder may consider a holistic fire engineered approach for the entire building…’
The fire engineering approach may take into consideration factors beyond the fire safety of the rainscreen system, such as distance from boundaries, the use of building, the provision of active fire safety measures such as sprinklers and the specification of openings and other penetrations through the structure. All of the factors considered must be respected in the on-going use of the building.
Where does that leave the designer and contractor? Reassuringly there are multiple options available, but the easiest way of designing a rainscreen system compliant with fire safety requirements is to go down the route of non-combustibility i.e. mineral wool.
Supporting this principle, Zurich® Municipal’s School and Academy Design guidance states ‘as a guide, non-combustible insulation should be used…’ and ‘…rock mineral fibre and glass wool are inherently non-combustible. They do not contribute to fire growth and will be acceptable for many applications.’