Help! How can I get rid of the mould in my house?

It probably seems innocent at first: a few black spots here, a gentle fuzz there, maybe a bit of a musty scent.

But as many New Zealanders know, it doesn't take much for mould to get a grip on your home. And when that happens, it can have serious repercussions.

Leon Crane, of mould cleaning and restorative services company Restate, says people are becoming more aware that mould in homes can harm their health.

"It is a health issue, so we've seen some horrendous ones where you've just got mould covered everywhere, ceiling, cupboards, walls, doors, and people live there quite happily, but they do have health issues, but they've never put it down to the mould."

New Zealand's mould in homes problem doesn't look set to disappear anytime soon, despite two of the main culprits - moisture and dampness - being targeted by a law change a few years ago.

Some landlords now have an extended deadline, until 1 July 2025, to implement the Healthy Homes measures, which include having at least one main source of heating in the living area and extractor fans in kitchens and bathrooms.

Meanwhile, the prevalence of mould in housing continues, with 2018 Census data showing renters were more likely to be in damp and mouldy houses than homeowners.

So we asked the experts: how do we get rid of mould and prevent it from growing - and who takes responsibility for it when you're renting?

Why do I have mould in my house?

It all comes down to managing your house moisture, according to Crane and two researchers who have investigated mould in housing.

University of Otago's Dr Caroline Halley says the damper a home is, the worse mould growth will be.

"All you really need is like a source of nutrients and something for them to grow on and a supply of moisture and they can actually just absorb nutrients from the dust that settles onto the surfaces... so they don't need much in terms of substrate.

"So, the thing that we can control the most is the moisture."

Even if you have insulated walls, sealed aluminium windows, and double glazing, you can still be at risk of mould growth if you don't let your house 'breathe'. That means ventilating to ensure air flow circulation.

"But then also if it's a rainy day, you don't want to let the moisture in, that's why ventilation systems can be good, because then when your house is sealed up, it's bringing in fresh air that helps push out all the moist air," Crane says.

How should I clean mould - should I use vinegar or bleach?

Massey University's Mikael Boulic - who has investigated mould in homes and classrooms - refers to a diluted vinegar cleaning solution (70 percent white vinegar and 30 percent water) recommended by Australia's leading mould expert, Dr Heike Neumeister-Kemp.

This is one way of cleaning without using harsh chemicals like bleach, which you should be only be using in well-ventilated areas to prevent breathing in the chemicals, Halley says.

Crane says using bleach does not remove the mould's root system, which means it can return. In addition, it can also cause staining on certain surfaces.

Often some elbow grease and hot soapy water will do the trick, depending on the surface and if the mould hasn't eaten into the materials yet, Halley says.

What else can I do?

• Use extractor fans while showering and cooking, possibly keeping them on for a bit longer when you're done, or opening a window.
• Don't use unflued gas heaters. They generate moisture.
• Do you leave your washing inside? All that moisture is evaporating inside your house, which adds to the potential for condensation and mould growth. If you must, ventilate around it.
• Having too many people in a small house could be a problem, because it will mean increased moisture from more showers, cooking and the like. But having adequate extractor fans in those areas might help.
• Even if it's cold, let your rooms air out at least 10 minutes a day, ideally when the sun is shining.
• If you have a room that never gets to see the light of day, having some heating will help prevent mould growth, but ventilating is still important.
• Mould loves to grow on dirt, dust and condensation found on windows. Well-insulated homes should not have cold surfaces, which will favour condensation.

The key to a mould-free home is getting on top of mould fast - and keeping on top of it, Crane says.

"Removing that mould will stop it from exacerbating and growing further and further, because as soon you start getting the patches, they start to release the spores and then they go off and find moisture in other areas of your house and they start growing."

When should I call in the experts?

While mould can be wiped off the surfaces of windows, it's a good idea to get professional help once it starts eating into walls and GIB boards, Crane says.

"When you've got mould in your property on the walls, you've potentially got higher mould spore counts in your air.

"So even through you're wiping off the mould from the surface, there could be a higher mould issue in the property, in the indoor air quality. So we do air treatments to remove the mould spores in the air. So it's a next step further into stopping that mould from coming back."

I'm already doing all of this, and I still have mould...

There could be structural issues at play which are beyond your control such as a leak, poor insulation, not being able to heat your home effectively, or inadequate ventilation.

In these cases, Citizen's Advice Bureau legal and strategic national advisor Sacha Green advises tenants to ask their landlord to look into the issue and explain what they have tried already to remedy the problem.

"If I'm maybe taking a photo of how it is today and then just keeping note of the steps I'm taking to keep the area clean and ventilated, then it might be a matter of saying, 'well, despite doing this there's clearly a problem'.

"But when it's mould in the corners of your bedroom and they're otherwise not being covered with boxes being stacked there and it's just literally mushrooms growing out of your carpet... it's more likely to relate to the condition of the property being an issue that the landlord needs to address."

Who is legally responsible for mould?

While mould is not specifically mentioned in law, it is often something that the Tenancy Tribunal saw as being of "dual responsibility between the landlord and the tenant", Green says.

"If the tenants [are] having long showers without opening the window or turning on the extractor fan, or they're not wiping down surfaces occasionally, those are the types of situations that might be where you would see the tenants' use of the property potentially contributing to mould.

"When it comes to mould in places like bathrooms, that's often one where there can be a bit of mixed views about who's responsible.

"So landlords are sometimes reflecting back to the tenant the steps that they need to be taking to try and reduce the presence of mould. But often the people we are seeing are saying they are already doing all the things they need to do and keeping the property ventilated and there are still problems."

In the past six months, the Citizen's Advice Bureau has had 200 inquiries relating to mould in rental houses. Green says it is consistently hearing about concerns with landlords and property managers failing to take action on mould concerns.

"We help them know their rights around Healthy Homes standards and the landlord's responsibilities for repairs and maintenance, and support them with options to seek action from their landlord, which is often about sending the landlord a 14-day notice to remedy a breach, and if there is still no action, to make an application to the Tenancy Tribunal."

Importantly, Halley says, people should not be ashamed of mould in their homes. It even grows on the space station.

Further support and guidance for tenants can be found here.