How can I cool down my house in summer?

Explainer - If you're starting to swelter as temperatures around the country rise, you might be wondering how best to cool your house.

Does using the heat pump mean a big power bill? Is there a trick to using fans for best effect?

RNZ asked the experts what you need to know.

Heat pumps

EECA lead adviser on energy efficient appliances Gareth Gretton said heat pumps working as air conditioners would cost about the same as they did when used to heat. They would be a bit less efficient than when used for heating "but roughly speaking it's pretty similar".

"They will use less energy in summer on a higher temperature setting.

"In the summer you want to be reaching for 23, 24, 25 degrees … ideally, you're using different temperature settings from winter to minimise the cost."

Watch your windows

Gretton said, in most houses, the biggest problem was sun streaming into the windows over a long afternoon.

"A lot of people think north-facing glazing is a problem but actually north-facing glazing is really good because in the middle of the day, the sun is really high in the sky and not so much of it is beaming into your house.

"The problem you have is with more east, and especially west, facing glazing the sun will be coming straight in. For those houses where you've got a lot of sun coming in you've basically got a lot of heat you're trying to get rid of."

He said on a sunny afternoon that heat could be equal to half-a-kilowatt per square metre.

"So if you think of having quite a big ranch slider or something, that could easily be four square metres - easily more. So that means you're going to have at least two kilowatts of heat coming in and more in some situations. That's heat you're just constantly trying to get rid of.

"The only way you can really solve that problem is by having external shading which is basically going to block the sun coming in. That means shutters, essentially, at the design stage … it's quite common in other parts of the world and I think with our changing climate it's actually the solution we ought to be reaching for in most situations."

But if that is not something you can do, there are still options.

"Curtains and blinds of different types can block out varying amounts of heat so it does help. The best thing is you can actually have the windows slightly open behind the blinds or curtains because what's happening then is that the sunshine is coming through the glass, it's going to be warming up the curtains or the blinds and they could potentially get quite warm and if you've got your windows open at that point the heat is able to escape before it starts to diffuse into the room."

Phaedra Applin, head of architecture at WSP, said people could add shade sails or plants to shield the light.

"Planting is also really great for that feeling of kind of wellbeing as well. So it's sort of a double bonus if you like."

Consumer product test leader James le Page said another option was to apply film to windows.

"A film's solar heat gain efficient (SHGC) tells you how much solar radiation will pass through it - so 0.8 means 80 percent will pass through. Look for the film with the lowest SHGC. The downside to films is they can distort the colours you see outside, making things look darker than they are."

Fans

Gretton said fans were all fairly similar in performance.

"They're not going to make the air temperature any colder. They are just going to make you feel more comfortable."

Le Page said they generally worked well. He said pedestal fans were generally better at moving air around a room but they were not as attractive as the tower variety.

"Most plug-in fans have a small electric motor (50-60W), so power usage is miniscule. For example, a 60W fan running eight hours a day costs about $4 a month. However, if you start chucking fans in every room and running them non-stop, you could be in for a shocking power bill. They don't cool you down when you aren't in the room, so make sure they're switched off when you leave."

He said pointing a fan at a window at night would help.

"The idea is to push the hot air from your room, leaving space to draw in the cooler night air from outside."

Good design

Applin said the best way to stop houses overheating was to consult an architect at the outset.

If you are building your house, you have options to keep it cool.

"It's really about sort of thinking about those design decisions. At the outset, smart passive design such as orientation, shading, ventilation, insulation, window size, that sort of thing. So you're making those correct design decisions at the outset, which can really help to prevent overheating in homes."

She said problems arose when standardised designs were used without thought for the conditions of a site.

"Even from the outset, it's thinking about the orientation of the building and where you're placing those windows.

"And also making sure that you've got windows on either side so that you can have cross ventilation. And essentially then, if you're thinking about those sorts of design considerations and the depth of the roof, eaves and overhangs, if you can have some external shading, then it means that you've actually got the building doing most of the work for itself, rather than having to put in air conditioning afterwards."

She said the problem was becoming more well known as more townhouses and apartments were built.

"Think about the distance that you've got between them and make sure that you've got enough room for planting and enough shade ... enough space so that you can get air flow through. Again, design quality is really the key to keeping those high density homes cool ... compact doesn't have to mean overheated, but it does require deliberate attention to sunlight, cross ventilation, privacy and planting to make the homes liveable and and that people can enjoy."

Other quick wins

If you can open windows on opposite sides of the house, you might be able to create a cross breeze.

"It will flush all that warm, sticky air from the house. And even if it just drops the temperature by half a degree, that will actually make a big difference to how you feel," le Page said.

Avoid using the oven or stove if you can, and don't turn on the dishwasher until later in the evening.

"These appliances will heat up your kitchen. If you want to cook, you won't heat the kitchen as much if you use an air fryer."

Sign up for Ngā Pitopito Kōrero, a daily newsletter curated by our editors and delivered straight to your inbox every weekday.