The six members of the Aspinall family are living in a cold, damp and mouldy house in Johnsonville. Clockwise from top left, Lamar, EJ, Mel, Ernie, Tyrah and Leila.

A Wellington mother believes her son got sick from living in the cold, damp and mouldy rental house that she thought was insulated.

Mel Aspinall's eldest son, Lamar, was admitted to Wellington Hospital with badly swollen joints in April last year. Tests revealed he had rheumatic fever, which was affecting his heart and blood pressure.

The Johnsonville mother of four believes insufficient heating, and a lack of insulation, were the reasons her son got so sick. He spent two weeks in hospital fighting the illness, and has to have monthly penicillin shots.

Rheumatic fever is often linked to poor housing conditions and overcrowding.

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The Dominion Park St home was fitted with two panel heaters and a heat pump, but the ill-placed pump failed to warm the house or keep it dry, she said.

Every three months, the walls and ceilings had to be wiped down to remove mould.

Aspinall, who lives in the house with her husband and their four children, thought the house was insulated, but after Lamar was released from hospital, Regional Public Health nurse Tineke Snow undertook a health and housing assessment and found otherwise.

"She identified that our house had no insulation, which is why my sons were getting so sick," Aspinall said.

Capital & Coast District Health Board said that, although the landlord had installed a heat pump, Snow's assessment showed it was poorly placed to heat the lounge and bedrooms.

However, Auckland-based landlord Peter Moon said he believed the house was insulated and, when he found out it wasn't, he contacted the family to try to help.

"I'm aware of the problems she's got with it, but I need it empty to do all the work."

"I said to her, 'If you can find somewhere temporary for a few months, you can come back.' They are damn good tenants ... I look after them as best as I can."

The Johnsonville house had been a family home since 1969, Moon said. "I've lived in it. I grew up in it and we never had any problems.

"Nobody has ever been unhealthy in the house. It is the overcrowding. There are too many people in the house."

Snow's assessment showed the heat pump was facing the wrong way, so the heat dissipated quickly before reaching the bedrooms or main living area.

Consumer New Zealand chief executive Sue Chetwin warned heat pumps needed to be installed properly to be efficient.

"No heating will work efficiently if you try and heat the world. There is no use running it if it's going out the walls.

"It [heat pump] needs to be the right size, there needs to be good insulation for it to be most efficient. Also, get an expert to come out and tell you the best way to heat your home."