How to cool your house down without turning on the heat pump

Feeling the heat? You're not alone. Many parts of New Zealand are set to hit 30 degrees this week as a warm wave hits the country.

Keeping the air conditioning on full blast might feel tempting, but your bank account won't thank you for it. Luckily, ventilation experts Stephen McNeil and Dr Manfred Plagmann have a few tips on how to cool your house naturally - without the sky-high energy bill.

"The fundamental issue is really solar gain, it's the sun coming in through your windows that's the main reason your building's getting too hot," explained McNeil, a senior building scientist at the Building Research Association of New Zealand (BRANZ).

"It's really about managing or mitigating that excess heat, and the way you operate a home throughout the day is quite important."

Does opening the windows help?

Absolutely. Opening up your windows is an effective form of flush ventilation (flushing the warm air out of your house) - so long as the air outside isn't warmer than the air inside your house.

"Opening up as many windows as you can, particularly when you get home from work, can be an effective way to get on top of the heat," McNeil says.

BRANZ senior scientist Dr Plagmann adds that the method is particularly effective at night.

"Most of the time, particularly in the evenings and at night, the air temperature outside is usually cooler than what's indoors, so yes it's an effective one, and it's more or less free."

Are some areas of the house harder to cool than others?

The kitchen is an obvious one, with extra heat sources in that space making it more difficult to manage. But according to McNeil, the bedroom is where people will feel it the most.

"Bedrooms overnight would be the thing people are most cognisant of, that's the thing people notice is their poor sleep during the summer. That's where leaving a window open overnight is probably quite useful for a lot of cases, but then you're managing security, bugs and all of the other things that come with that time of year."

Will putting a wet towel on the fan help?

It can work - so long is the air in your house is dry and not humid.

"You're basically making an evaporative cooler, where the air is flowing through that and evaporating that moisture. You're actually giving up a whole lot of heat for that to occur, so that cools the air stream that's coming out," McNeil says.

"The challenge, of course, is you're adding a whole bunch of moisture to your house, so you still need to keep on top of ventilation... sometimes mould can be quite an issue... and once you've saturated the air and you're not able to evaporate anymore, that's gonna be a problem.

"So when it's really, really humid, it's not gonna do a hell of a lot."

Dr Plagmann says it's a good idea to ventilate with open windows while trying this method.

"That way the moist air you're generating will escape and be replaced with dry air, the evaporation will come quickly and the cooling effect will be much higher."

Does where you place your fan make a difference?

Some people like to face a fan out the window, with the idea that it'll flush hot air out of the room and allow cold, fresh air to circulate. McNeil says this can work, but the difference will be marginal.

"It takes advantage of shifting the more dense air or cooler air, rather than the warmer air ... the difference is probably not humongous but there'll be a benefit, but how quantifiable that'll be will vary from situation to situation... it's basically assisted window ventilation."

Dr Plagmann says ceiling fans can be a more effective way of cooling down, as cold air travels down and helps evaporate your sweat better.

"Ceiling fans sort of slowly move the air, and because of the sweat on your skin, it evaporates and cools you down - not necessarily the house - but it cools you down. The air moves over your skin and takes the moisture layer away basically."

Creating some shade outside can help

Dr Plagmann says creating some shade outside your windows, particularly in the northern and western parts of the house, can help keep some of the heat out.

"What you see in the Meditteranean is external shutters on your windows, so you're not letting the sun get into the house. We unfortunately don't have that often here in New Zealand... other means of shading like drawing the curtains, the heat is already in your house so it just heats up the curtains, and that radiates the heat inside.

"Shading is the most effective when it's outside the building, so if you have large pot plants, you can place them outside the windows ... shading with an awning would help a lot."

A cold compress can go a long way

A strategically placed wet towel can do wonders for cooling down your body temperature, Dr Plagmann says.

"On your neck, your wrist, the blood circulates close to the skin there, so if you put that under cold water or put a wet towel there, you will instantly feel cooler."

It's all about the evaporation of sweat, McNeil says. That's why sleeping with one leg out the covers feels so good.

"Your leg's quite a big muscle group, so that's quite a lot of mass that's exposed to the cool air... really it should be both legs out of the covers."

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