Damp, mould and condensation has been caused in Welsh social housing by inappropriate cavity wall insulation, a report has found.
More than 280 homes have had insulation removed in Rhondda Cynon Taff alone, while 900 properties require remedial action in Neath Port Talbot.
The problems have been revealed in a draft report for the Welsh government, seen by BBC Wales.
The Welsh government said it could not comment.
The report, by the Building Research Establishment (BRE), was commissioned by the Wales Low and Zero Carbon Hub, which advises the Welsh government.
It was obtained under a Freedom of Information request by campaign group Cavity Insulation Victims Alliance.