Wall insulation causes 'green slimy' marks on Arfon homes
"Green slimy" marks which have appeared on the outsides of homes fitted with external wall insulation are being investigated.
The problem has been spotted in two Gwynedd villages where work was done as part of a Welsh Government scheme called Arbed, aimed at saving people money on their heating bills.
Some properties have also experienced damp getting through to the inside.
The government said it has sent inspectors to look at the problem.
Work was carried out between 2014-15 in Deiniolen, Carmel, Y Fron and Dinorwig and included new windows, boilers and sealing exterior walls for free.
Resident Kerry Roberts owns two adjoining properties that both had external wall cladding fitted.
He said: "It makes the houses look tatty. There's a noticeable green slimy line next to one of the downpipes even though the work was carried out less than four years ago.
"Next door has black marks on the front of the house.
"People have suggested it might be because we're in an exposed location.
"But I suspect that the job has not been done properly - even an extra coat of paint might have been all it needed."
The side wall of Margaret Roberts' house now has a prominent green streak along it.
She said: "It makes you cross really... the first thing people see is that [the mark]. I had visitors on Friday and they said 'what's happened to your house?'
"It makes the house look untidy."
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Sian Gwenllian, assembly member for Arfon, said her office has received over 20 complaints from residents: "These people have ended up with houses in a worse condition than before they entered the scheme.
"It's a matter of accountability. This was a Welsh Government scheme and the Welsh Government needs to take responsibility for the shoddy workmanship that's happened and they need to sort it out."
A Welsh Government spokesman said the problems in Deiniolen and Carmel seem to be isolated cases.
"We have received isolated reports of issues in Arfon related to work undertaken under Arbed which ran until 2015.
"An independent review, including surveys of properties is due to report its findings to the Welsh Government by the end of the year."