Insulation manufacturer Celotex – which itself has been accused of producing misleading market information about product performance – revealed a crucial email exchange regarding the use of fire stopping.

In emails between staff at the firms, a technical manager at Harley said: “there is no point in ‘fire stopping’, as we all know; the ACM will be gone rather quickly in a fire!”

In another email fire consultant Exova, accepted the zinc cladding being considered at the time would fail if there were external flames.

In response, an architect at Studio E said that “metal cladding always burns and falls off”.

In its evidence Celotex said: “None of the individuals involved in the March 2015 email exchange have sought to explain how their knowledge that the cladding would fail in the event of a fire with external flaming can be reconciled with their responsibility as designers, contractors or consultants to ensure, or exercise reasonable care to ensure, that the rainscreen cladding system on Grenfell Tower complied” with building regulations relating to the spread of fire.

The inquiry heard that there was no indication that any of those involved contemplated requesting a full-scale test of the proposed system.

Evidence from fire consultant Exova revealed that the Grenfell Tower was originally expected to be clad in zinc.

But value engineering led to the use of cheaper ACM cladding saving £454,000, according to a Harley estimate.

It said that when Rydon was appointed, it was on the basis of employer’s requirements which specified zinc cladding with an aluminium honeycomb core; and contemplated that alternative materials might be used but must comply with prevailing fire regulations.

“It was only later that – evidently guided by the pressure to cut costs – Rydon (together with Studio E and Artelia, and with support from Harley) pursued, and obtained client KCTMO’s approval for the change to ACM,” said Exova.

The fire consultant added that it was only then that work on designing the cladding system commenced, under a team that comprised Rydon, Studio E, and Harley.

The inquiry continues today with cladding manufacturer Arconic providing its opening statement.