Lenders are being accused of refusing to lend against properties that have a specific spray foam insulation.

An investigation by the BBC alleges that some of the UK’s biggest mortgage providers will not lend against homes with spray foam in the roof.

Some 250,000 homes in the UK have this type of insulation, typically installed using the Conservative government’s Green Homes Grant scheme.

Spray foam insulation helps stop heat escaping and comes in two forms.

Closed cell spray foam is rigid once set. It is a better thermal insulator and was previously sold to stabilise failing roofs. But if installed poorly, it might put stress on timbers, restrict air circulation and put them at risk of decay.

Open cell spray foam remains soft once set and is more breathable but is often installed where a highly-resistant underlay like bitumen felt is already present, stopping vapour from escaping.

Some lenders refuse to deal with homes with spray foam insulation due to concerns over poor fitting leaving moisture trapped and roof timbers at risk of decay, although the Insulation Manufacturers Association insists spray foam can be beneficial if properly installed.

The BBC says the Health and Safety Executive highlighted situations where using open cell was low-risk but cautioned against applying it directly to roof tiles.

The BBC contacted the 20 largest lenders in the UK. Of those, TSB Bank, Skipton Building Society, Co-operative Bank, Principality and equity release lender Aviva said they did not lend against properties where spray foam is found in the roof space.

Other lenders including. Lloyds, Nationwide, Barclays, the NatWest Group and Santander consider applications on a case-by-case basis.