In her old, cold state house, Sema Taanoa ran the heat pump all day and wrapped herself in blankets to stay warm.

But since moving to one of the houses that have been thermally upgraded as part of a Housing New Zealand pilot, she's barely turned on the heater at all, even through the recent snap of icy weather.

Taanoa and her family live in one of the six Hutt Valley state houses that have already been upgraded as part of the pilot.

HNZ will upgrade 66 houses in the Hutt by the end of September, adding in insulation, double glazing, thermal curtains, ventilation and new heating to create a healthy living environment.

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Taanoa's 14-year-old son, Jason Taanoa, is already feeling the effects.

The teen was hospitalised for two weeks for asthma-related issues while living in the old house, and had to visit the doctor every month or two for checkups.

Now that he is in the warmer home, in Taita, his asthma has been causing little trouble and he only has to visit the doctor when he is feeling unwell.

Sitting in the new home on Tuesday, Sema Taanoa wore only a light cardigan while describing how, in the last place, she would layer up with thick, warm clothes and blankets to ward off the cold.

"This time in the winter I can't stay like this," she said, indicating her lighter clothing.

"That house was very cold."

Taanoa said she would run the heat pump day and night to keep warm.

"Then our bill is coming, it's very high."

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She couldn't say exactly what the bills were, but said they would often pay for a $50 top up on their bill and it would be used up within a week.

They moved into the new house at the beginning of May, and despite Wellington going through several particularly cold periods, Taanoa hasn't felt the need to turn on the heater.

The only times she has turned it on have been when people come to visit.

The HNZ pilot aims to achieve an indoor winter temperature of 20C in the living area, 18C in the bedrooms, and 16C elsewhere in the house, which is in line with the World Health Organization's recommendations.

The timely upgrades are happening at the same time the University of Otago released a report revealing Wellington homes were too cold to be healthy.

READ MORE: Wellington homes are too cold to be healthy, University of Otago research confirms

Researchers looked at 49 of Wellington City Council's homes, before upgrades were carried out on council's social housing, and found the temperatures in the monitored houses were lower than 16C two thirds of the time.

For 9 per cent of the time, dwelling temperatures were below 12C.

HNZ national maintenance and upgrade manager Angela Pearce said they were "keenly interested" in how the upgraded homes perform, particularly over winter.

It would help inform a wider programme of upgrading up to 200 state homes.

The Hutt Valley is home to some of HNZ's oldest stock, with 3766 state houses that are 50 years old on average.

At the moment it costs $133,000 per house to carry out the upgrades, but the figure may shrink as the programme progresses.