The newly renovated façade of the tower is believed to have been built as follows:
exterior cladding: aluminium sandwich plates (3 mm each) with polyethylene core
a standard ventilation gap (50 mm) between the cladding and the insulation behind it
an insulation made of PIR (polyisocyanurate) foam plates (150 mm) mounted on the existing facade
the existing prefabricated reinforced-concrete facade
new double-glazed windows of unknown type and material, mounted in the same vertical plane as the PIR foam insulation plates
Both the aluminium-polyethylene cladding and the PIR insulation plates failed fire safety tests conducted after the fire, according to the police.

Earlier in 2014, safety experts had cautioned that the planned insulation was only suitable for use with non-combustible cladding.
The Guardian saw a certificate from the building inspectors' organisation, Local Authority Building Control (LABC), which stated that the chosen insulation for the refit should only be used on tall buildings with fibre cement panels, which do not burn.
Combustible panels with polyethylene were put up on top of insulation known as Celotex RS5000, made from polyisocyanurate, which burns when heated, giving off toxic cyanide fumes.

Despite the above, the Royal Borough of Kensington and Chelsea certified the Grenfell tower building work as allegedly conforming to "the relevant provisions".
Council building inspectors visited the site 16 times from August 2014 to July 2016.
Kooltherm, a phenolic insulation, was also used on Grenfell.
Kooltherm was never tested with polyethylene core aluminium panels according to the manufacturer.
The manufacturer, Kingspan, "would be very surprised if such a system [...] would ever pass the appropriate British Standard 8414 large-scale test".
Kooltherm's LABC certificate states phenolic products, "do not meet the limited combustibility requirements" of building regulations.

The combustible materials used on Grenfell Tower were considerably cheaper than non-combustible alternatives would have been.
There appear to have been intense cost pressures over the Grenfell refurbishment.
In June 2017, it was stated the project team chose cheaper cladding that saved £293,368, after the Kensington and Chelsea Tenant Management Organisation mentioned in an email the need for "good costs for Cllr Fielding Mellen [the council's former deputy leader]".

A building control officer from the Royal Borough of Kensington and Chelsea reportedly passed the cladding on Grenfell Tower on 15 May 2015, though there was a nationwide warning that the combustible insulation used should only be used with cladding that does not burn.
The newly renovated façade of the tower is believed to have been built as follows:
exterior cladding: aluminium sandwich plates (3 mm each) with polyethylene core
a standard ventilation gap (50 mm) between the cladding and the insulation behind it
an insulation made of PIR (polyisocyanurate) foam plates (150 mm) mounted on the existing facade
the existing prefabricated reinforced-concrete facade
new double-glazed windows of unknown type and material, mounted in the same vertical plane as the PIR foam insulation plates
Both the aluminium-polyethylene cladding and the PIR insulation plates failed fire safety tests conducted after the fire, according to the police.
Earlier in 2014, safety experts had cautioned that the planned insulation was only suitable for use with non-combustible cladding. The Guardian saw a certificate from the building inspectors' organisation, Local Authority Building Control (LABC), which stated that the chosen insulation for the refit should only be used on tall buildings with fibre cement panels, which do not burn. Combustible panels with polyethylene were put up on top of insulation known as Celotex RS5000, made from polyisocyanurate, which burns when heated, giving off toxic cyanide fumes.
Despite the above, the Royal Borough of Kensington and Chelsea certified the Grenfell tower building work as allegedly conforming to "the relevant provisions". Council building inspectors visited the site 16 times from August 2014 to July 2016. Kooltherm, a phenolic insulation, was also used on Grenfell. Kooltherm was never tested with polyethylene core aluminium panels according to the manufacturer. The manufacturer, Kingspan, "would be very surprised if such a system [...] would ever pass the appropriate British Standard 8414 large-scale test". Kooltherm's LABC certificate states phenolic products, "do not meet the limited combustibility requirements" of building regulations.
The combustible materials used on Grenfell Tower were considerably cheaper than non-combustible alternatives would have been. There appear to have been intense cost pressures over the Grenfell refurbishment. In June 2017, it was stated the project team chose cheaper cladding that saved £293,368, after the Kensington and Chelsea Tenant Management Organisation mentioned in an email the need for "good costs for Cllr Fielding Mellen [the council's former deputy leader]".
A building control officer from the Royal Borough of Kensington and Chelsea reportedly passed the cladding on Grenfell Tower on 15 May 2015, though there was a nationwide warning that the combustible insulation used should only be used with cladding that does not burn.
Grenfell Tower fire