Two hundred more state houses in the Hutt Valley will be thermally upgraded by the middle of next year, Housing Minister Phil Twyford says.

Crowded into a cosy living room of a Housing New Zealand home that was recently retrofitted with double glazing, insulation and modern heating, Twyford announced the extra 200 homes would be thermally upgraded on top of the 66 being retrofitted for a pilot programme.

So far 60 homes in the Hutt have been upgraded as part of the pilot, and a further six will be completed next month.

Twyford said it was important to make the announcement in the Hutt Valley, "because this is the great heartland of state housing in the Wellington region".

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The Labour government believed in state housing and wanted to build more homes of a higher quality, he said.

The announcement of 200 more homes getting retrofitted was "overdue", and now was the right time to be doing it.

That was another 200 families that wouldn't be coming home to cold houses, he said.

Cold, damp homes were "terrible" for New Zealand's most vulnerable, including elderly and children, who could be left with a lifetime of lung damage from living in an unhealthy home.

It's not clear yet when or how the retrofitting programme will be rolled out nationally, but changes would need to be made for the rest of the HNZ stock to meet incoming standards for rental properties.

He said it was common for families to only heat the living room, and leave the rest of the house feeling like an "icebox".

This meant more people crowding into one place and sharing winter germs and viruses.

"It becomes a real petrie dish for the usual winter illnesses."

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Twyford joked there should be a temperature recommendation for the toilet, and that Labour could campaign on having heated toilet seats in every state house.

"That would be a new standard of state housing."

Taita residents David and Carol Coffin are tenants in the newly upgraded house Twyford made his announcement in today.

The couple have lived in the house since 1979 and said while they could handle the cold in their younger years, the extra warmth was now welcome.

David said the house was "freezing" before the upgrade, but now they often didn't even need to use the heaters during winter.

They previously had a fireplace in the living room, but even with the fire blazing the cold from the windows cast a chill over their home.

Now the fire is gone, but they don't feel they would need it anyway.

"I was up at four o'clock this morning sitting in a T-shirt," Carol said.

The pair used to layer up in thick dressing gowns to stay warm, but hadn't even worn them since the upgrade, which was finished in June.

According to an online power bill calculator, their electricity costs have dropped 32 per cent. David said it equated to about $50 a month less on their bill.

On top of the double glazing, insulation and heating, HNZ has also improved air-tightness and ventilation for the home.

The thermal upgrades bring indoor winter temperatures up to 20C in the living area, 18C in bedrooms, and 16C elsewhere, in line with World Health Organisation recommendations.

HNZ also plans to build up to 300 new homes in Lower Hutt over the next few years.