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Yes Class O in accordance with BS 476 Parts 6 and 7 testing and criteria. And I understand how an aluminium enclosed combustible core can achieve this.

However, for buildings greater that 18m tall (and as referred to previously) the cladding/insulation materials should be of limited combustibility – Table A7 defines this as:

a. Any non-combustible material listed in Table A6 (which doesn’t include polyethylene);
b. Any material of density 300/kg/m’ or more, which when tested to BS 476-11:1982, does not flame and the rise in temperature on the furnace thermocouple is not more than 20ºC;
c. Any material with a non-combustible core at least 8mm thick having combustible facings (on one or both sides) not more than 0.5mm thick. (Where a flame spread rating is specified, these materials must also meet the appropriate test requirements);
d. Any material of density less than 300kg/m3, which when tested to BS 476-11:1982, does not flame for more than 10 seconds and the rise in temperature on the centre (specimen) thermocouple is not more than 35°C and on the furnace thermocouple is not more than 25ºC.

Or, meet the requirements/criteria of thermal insulation for walls of multi storey buildings (BR 135) for cladding systems using full scale test data from BS 8414-1:2002 or BS 8414-2:2005. The testing criteria includes:

a) extent of flame spread over the surface of the cladding system (both vertically and horizontally);
b) extent of flame spread and damage within any intermediate layers (both vertically and horizontally);
c) estimate of any flame spread and damage within the cavity, if present, (both vertically and horizontally);
d) extent to which the external face of the cladding system has burnt away or become detached;
e) details of any collapse or partial collapse of the cladding system.

So for the use of this particular cladding to not contravene of Building Regulations requirements, I understand that the above standards would also need to have been met for this particular use (building more than 18m tall).

I don’t see how a polyethylene core panel could have met these requirements. Also, does it not beg the question why the manufacturer/supplier would offer/produce a “less flammable” version if the standard Polyethylene product could suffice.

The use of this particular cladding on this particular building is surely a question for whomever (multiple) specified, justified and approved the use of the product assuming it was considered as part of the Building Regulations application.

Finally, although produced by manufacturers’ the following guidance is also useful to understand the requirements of guidance to the Building Regulations:
http://www.kingspaninsulation.co.uk/Knowledge-Base/Facades---Fire-Safety.aspx
http://www.kingspaninsulation.co.uk/getattachment/dc8cf2c7-5e23-4d9a-9a1f-96bdf571ecdd/Techncial-Bulletin--Routes-to-Compliance--Fire-Saf.aspx
https://www.inca-ltd.org.uk/wp-content/uploads/2016/09/INCA-Technical-Guide-01-Fire-PR-for-EWI-Systems.pdf
https://www.celotex.co.uk/assets/rainscreen-compliance_specificationguide_mar15.pdf

Also a presentation:
http://www.c-a-b.org.uk/wp-content/uploads/Facade-fire-testing-routes-to-compliance.pdf