Timber-framed double-glazed windows that are assembled on site are failing after only five years, defects experts have warned.

In a report due out this month, insurer Housing Association Property Mutual says incorrect use of materials and mounting methods are leading to water penetration and causing window seals to fail.

Paul Wornell, head of the insurer's technical audit unit, said failures were a growing problem.

The company, which insures components against premature failure, favours factory-glazed windows, but most contractors prefer to glaze on site as this reduces damage.

Where site glazing does take place, Housing Association Property Mutual recommends dual-sealed units and a drained glazing system.

  • Greenpeace is stepping up its campaign against the use of PVCu in construction. It is planning a series of briefing notes targeting specific materials, starting with PVCu window frames, which Greenpeace says should be dropped in favour of timber. Greenpeace's Mark Strutt said timber windows "last longer, offer better value for money and are better for the environment".

David Strang, product director of PVC window manufacturer LB Plastics, said Greenpeace's claims are "flawed and misleading".

A British Plastics Federation spokesperson added: "The claims are misconceived and mislead specifiers. PVC has distinct environmental, technical and commercial benefits."