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Cavity wall insulation, when fitted properly in appropriate properties, can make the building more energy-efficient.

But if installed incorrectly, or in unsuitable properties, it can lead to damp problems.

Tim Kaye, from near Pontypridd, has spent the last two years working as an installer for cavity wall insulation company, Mark Group. He believes companies like his aren't doing enough to prevent problems.

"It's their responsibility to make sure they follow up every single job. It's people's homes that they're playing around with at the end of the day," he said.

In the last year, dozens of viewers contacted X-Ray about problems in their homes, which started after cavity wall insulation was fitted - from damp, mould, and rotten skirting boards to crumbling plasterwork.

The body which guarantees cavity wall insulation work, CIGA (Cavity Insulation Guarantee Agency), says houses should be surveyed pre-installation, to ensure suitability.

But installer Tim Kaye says the surveys he receives are often of poor quality.

"Anything that could go wrong on a survey sheet, I think has gone wrong," he said.

"We turn up at houses regularly that have damp. If you were to pump the property it would make it worse."

Even when installers are concerned houses are not suitable, he claims company managers put pressure on them to do the work.

"It's very much 'If you don't do it, someone else will'," he said.

Installers are paid piecework, so the more houses they insulate, the more they get paid. Tim claims this leads some installers to rush jobs - also leading to problems.

His fears about the quality of pre-installation surveys are shared by a second industry whistleblower, Richard Newman. He spent two months last year working in the North of England as a subcontracted surveyor for a cavity wall insulation company which also operates in Wales.

With no formal training and after just one day shadowing another employee, he says he was sent to sign people up for cavity wall insulation.

He claims he was never trained to spot factors which might have made a property unsuitable, but was just told to drill into the wall, check for insulation, then note the property's measurements and external features.

Simon Haslett, from Newport, believes he is one of the victims of poor pre-installation surveys.

ÌýHe had cavity wall insulation fitted in 2007, by Mark Group, but two years later he discovered dry rot in his home had caused more than £7,000 damage.

He believes this happened because Mark Group failed to spot problems which made his house unsuitable for insulation - in particular, claiming they sealed up vents below his wooden floor, despite the company website stating such vents "must be safeguarded".

Mark Group sent a chartered surveyor to investigate, who claimed the problem was not caused by the insulation. He said the damp came either from below the floors, or via the property's driveway, which was above the level of the damp proof course.

But Simon is angry this was not spotted in a pre-insulation survey.

"I'm not a builder. I don't know if the drive's too high. They should have picked that up when they surveyed the house."

Chartered surveyor and buildings defect pathologist, Tim Davies, agrees a proper pre-installation survey should have picked up the driveway's height. After surveying the property, he suggested that blocking the vents also contributed to the dry rot outbreak.

Tim Davies says the whistleblowers' accounts fit with his experience of the industry - and with houses he has seen suffering from damp after having cavity wall insulation.

"It doesn't seem to be anybody that sticks their hand up and says, I'll take overall responsibility for assessment of the house accurately and getting it right in the first instance," he said.

In a statement, Mark Group says they successfully install cavity wall insulation or loft insulation in a quarter of a million British homes each year.

They say Tim Kaye (the installer) and Simon Haslett (the homeowner in Newport) are isolated cases, which they have been working to resolve for many months.

The Mark Group says, based on the evidence gathered by its insurers, it accepts no blame for the damage to Simon's property. But they say if evidence is provided which demonstrates they are responsible, they will put things right. As a gesture of goodwill they are willing to pay for a further independent survey.

They add that less than 2% of their customers complain to them - a figure they say they are committed to reducing.

If you've got a comment or story about cavity wall insulation, please email us at x-ray@bbc.co.uk.

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