FLAMMABLE insulation panels which also give off toxic gases when set alight were used in the fateful Mackintosh building refurbishment rather than more expensive mineral ones which do not burn.

Fire inspectors are investigating whether these exacerbated the spread of the fire, which broke out in the Glasgow School of Art building on June 16, and which firefighters described as a "fireball" and of unprecedented severity. These panels were similar to those used in Grenfell Tower.

The Sunday Herald has asked several times from all those involved in the project for full details of the insulation materials installed in the "Mack", and also submitted questions under freedom of information regulations. However, the art school, Historic and Environment Scotland, which was responsible for managing the project, and the architects Page/Park have refused to divulge them.

Sunday Herald View: The wall of silence over the art gallery school fire must be destroyed

Instead, they have repeatedly referred our queries to Glasgow City Council planning and building control departments which had no involvement. As the Mack was Category A listed, only improvements and additions – and not like-for-like replacements – were required to be vetted by the council. A council spokesman confirmed that none of the bodies involved in the refurbishment had consulted the council about the choice of insulation.

However, we can reveal that 100mm polyisocyanurate (PIR) was used, which is rigid plastic foam between two sheets of aluminium foil. The PIR is flammable with the foil designed to stop it catching fire. The underlay on the roof was an air and vapour permeable product called Roofshield which is also flammable. There is no suggestion that these materials breached any building regulations or British Standards.

PIR is up to 50 per cent cheaper than non-flammable products like Glasswool or Rockwool, which are made from recycled glass and strands of rock. This newspaper asked all of the parties involved whether this was specified because it is cheaper than the mineral equivalents, without response.

This type of insulation also gives off toxic gases, including hydrogen cyanide and carbon monoxide, when ignited. Professor Richard Hull, head of Chemistry and Fire Science at the University of Central Lancashire, has pointed out that some of the Grenfell fire victims were treated with antidotes to the toxic fumes, while others who died might have been incapacitated by them. He also pointed out that the UK has no regulations on the toxicity of fire smoke from construction projects.

Sunday Herald View: The wall of silence over the art gallery school fire must be destroyed

Despite the refusal to confirm or deny the type insulation used it was confirmed that PIR was installed by Page/Park depute of Conservation Iain King in the architects' publication RIBA Journal earlier this year. It was also revealed that in some of the studios pale green paint pigment and linseed oil washes were used, replicating what Charles Rennie Mackintosh specified when the original building was built 109 years ago. These are also flammable and may have also helped spread the conflagration.

On Friday, we asked King and Page/Park whether cost was the reason for the choice of panels. They have not responded.

Since the fire all of those involved in this refurbishment have refused to answer questions about the materials used. This is the second fire, and much more serious one, to rage through the building. In 2014, a fire started in the Mack when flammable gases from a foam canister used in a student project ignited. An overheated projector ignited gases from the expanding foam (which was banned from the building) used to fill gaps between three foam panels fastened to walls.

At the time the GSA director Tom Inns said: “The fire was an accident and, like any accident, it’s caused by many different factors coming together and conspiring against us on the day. There are a huge number of lessons that can be learned and we’ve been working very hard over the last six months on our health and safety procedures, training and so on."

No-one was held responsible for that fire and so far neither Inns nor the GSA trustees have established responsibility for this one. The day after the fire Inns emailed staff that no one should speak to the media and all questions should be referred to the public relations department.

In a highly critical article in the Scottish Review, Eileen Reid, a former senior member of staff, pointed out that this was not the first time the school had tried to silence its own employees and that in the wake of the earlier fire staff and former students who had raised safety concerns were silenced.

Sunday Herald View: The wall of silence over the art gallery school fire must be destroyed

The refurbishment of the Mack was costed at around £50 million, the majority of that from public money. After the fire in June the remaining walls of the gutted building were deemed to be unsafe and they were pulled down. The debate continues about whether a replica of the Mack should be built on the Renfrew Street site, at an estimated cost of more than £100m, or whether a new one should replace it.

A spokesman for the Scottish Fire and Rescue Service said last night that the investigation into the causes of the fire was "still ongoing".