“We have to scrape the mold off the walls.”

Every two weeks, Zoe Godrich scrubs black mold from the walls of her children's bedrooms.

She said her house now resembles a squatter home and it “is really painful not being able to provide for my children's basic needs – a safe and warm home.”

The mould started growing after she installed hollow-wall insulation in her Swansea home three years ago: Within a few weeks, she says, water was constantly running down the walls “like a little waterfall”.

The BBC has revealed that cavity wall insulation installed under a government-backed green energy scheme may have failed in hundreds of thousands of homes because it was not installed properly.

Zoe, a single mother, and her three children are currently sleeping on mattresses on the floor in one room, and Zoe says the other two bedrooms are uninhabitable.

She said she threw out her entire bed and wardrobe, which had rotted from damp and mould. She showed me a photo of her 6-year-old daughter's baby doll, which was covered in black mould and had to be thrown in the bin.

One of her daughters takes several types of asthma medication, and her son has severe eczema that causes cracked, bleeding skin. She says they weren't taking the medication before the insulation was installed. The NHS says damp and mould can cause respiratory and skin problems.

David Walter, a building inspector who has been inspecting homes across the UK for more than 25 years, visited Zoe's home in April and found the insulation was “poorly installed” and not properly ventilated.

He says there is nothing unusual about what happened at Zoe's house. “The industry is motivated by money. They get subsidies from the government to do this work, so there's an incentive for contractors to just hire people, get the job done and get the subsidy.” He blames a lack of oversight and regulation for the problem.

Since 2008, more than three million homes have had cavity wall insulation installed under the government's green energy scheme. Energy suppliers are responsible for inspecting 5% of these homes to ensure the quality of the installation.

Figures compiled by energy watchdog Obgem, seen by the BBC, suggest that hundreds of thousands of homes may have had their insulation fail because it was not properly installed.

A total of 15 million households had cavities filled.

A year after the problems began, Zoe went through a number of different companies and organisations looking for help. In early 2022, she spoke to Installers UK, the company that had carried out the installation, after they claimed the insulation wasn't to blame and then went bankrupt.

The company told the BBC that a hole in the roof was the cause of the problem, but could not provide a copy of the pre-construction report to back this up – standards body HMRC said that if there were problems with the property they would have been fixed before the work was carried out.

She then contacted City Energy Networks, the company that secured the financing for Zoe's house. The company's name appears on the warranty for the works, but City Energy says this is a mistake and that it has nothing to do with the warranty, which should have been issued by the company that did the work.

Zoe says they recommended she remove the insulation – and she took out a £7,000 loan to pay for it. She didn't realise that removing the insulation would void the work's warranty. Without insulation the house would be very cold – there would be no room to turn the temperature up, and damp and mould would increase even more.

City Energy told the BBC its only involvement was to provide a grant through a government scheme, and that it has no connection to Installers UK or involvement in the installation or removal process.

The government recognised the need to improve standards for several years and in 2019 commissioned independent organisation Trustmark to set up a quality assurance scheme.

But the problems Zoe faced raise questions about how well the scheme works. Installers UK, which carried out the works in 2021, and City Energy, which secured the grant, were both Trustmark registered.

Zoe says there is no resolution in sight: “It has sucked the life out of me. The last few years have been all I've done, fighting company after company.”

City Energy said it was under no obligation to provide any assistance but that it “recognizes and sympathizes with the situation Ms. Godrich is in” and has attempted to resolve the issue directly with her to assist.

In a statement, Trustmark told the BBC: “The issues at Ms Goderich's property are complex. Works were carried out to previous standards and did not take a 'whole house' approach as per current standards, which would not have prevented some of the issues seen here.”

“However, the conditions Ms Godrich is in are completely unacceptable and must be improved. We will continue to work with HMRC and City Energy to find the right solution to ensure Ms Godrich's home remains a warm, comfortable and healthy home for her and her family.”

The Ministry of Energy Security and Net Zero declined to comment but cited a ministerial statement from February which said: “The government is committed to protecting all consumers undertaking home improvement work as well as improving overall consumer behaviour.”