Unfriendly fungi: Dealing with a mouldy house

By Kate Cranney, Anthony Wright

26 August 2020

6 minute read

Mould on the ceiling of a house.

Mouldy house? We’re not a fan.

You’re holed up at home. Perhaps you’re living under lockdown. You enter your bathroom and notice an unwelcome guest. It’s here. It’s… mould.

It has been a wet winter and, with many of us spending our days at home, you might have noticed a build-up of this unfriendly fungus.

Anthony Wright, scientist and former building designer, shares his tips for dealing with a mouldy house.

What is mould and where does it live?

Mould is a fungal growth that thrives on moisture. Mould spores are always present in the air, but they only germinate when they encounter moisture and organic material.

In your home, mould generally grows where moist warm air meets a cold surface. The air moisture condenses, wetting the surface and creating the ideal environment for mould to thrive.

Mould thrives in damp, dark, poorly ventilated environments, like your bathroom or laundry. It can grow on carpets, curtains, walls, ceiling tiles, insulation material, behind furniture, and in cluttered storage areas. Mould also grows on food.

Apart from adding an unpleasant dinginess to your home, mould can damage building materials costing you money in maintenance. It can also cause health problems: mould releases toxic chemicals, called mycotoxins, which can cause allergic reactions for some people.

Black mould spots in the corner of a roof.

It thrives in damp, dark, poorly ventilated environments, like your bathroom or laundry.

Out with the mould, in with the new!

So, what can you do when mould takes hold? And how can you prevent it in the first place?

Ventilate

Make sure you vent wet rooms to the outside air. This means making sure your exhaust fans are ducted (connected to) outside the roof space. Avoid dumping the moisture into the ceiling cavity (unless the cavity is ventilated).

Insulate

If the walls and ceiling of your bathroom have insulation then their surfaces may not be cold enough for water to condense.

Double glaze

You are much less likely to have water vapour condense on a double-glazed window because they don’t get as cold as single glazing. Just make sure the window frames are ‘thermally broken’. This means that you have some sort of insulating material that breaks the pathway of heat energy being transferred from inside and outside your home (that would happen with a purely metal frame).

Treat mould as soon as you see it

Diluted vinegar, bleach or mould killers can work to reduce the spread. But vinegar is actually a food source for some moulds and bleach discolours mould without necessary removing it. The best thing to do is find out the underlying moisture problem causing the mould as soon as you see it. A mould problem is always a moisture problem, so removing mould is only temporary until the moisture problem is fixed.

Who you gonna call?

If there is mould on your walls, ceilings or floors outside wet rooms, call in a building inspector. This can be a sign that your house has faulty construction, a sign of much larger problems on the other side of the plaster.

You’ve got a mouldy house – what do you need to do?

If you only have a few local patches, wipe them away with bleach, vinegar or mould killer and thoroughly dry the area. This may prevent further problems. Keep the room dry afterwards to prevent recurrence.

“If you are certain the mould is not from plumbing failures, water ingress or condensation, then it might be an isolated event.” explains Dr Tim Law, an architectural scientist at Victoria University.

“You can remove the localised patch. You can use a damp microfiber cloth for non-porous surfaces, or HEPA (high-efficiency particulate air) vacuum for porous materials. If the cleaning makes you feel unwell, stop immediately. The agitation of spores, hyphal fragments and mycotoxins might affect some individuals. If you suspect you have an environmental sensitivity you might prefer to leave the cleaning to a professional.”

If you have mould on walls, ceiling or floors, seek further help. Building inspectors can tell you the likely source of the mould. It might be the faulty installation of insulation or building wraps. It might be rising damp. Or your exhaust fans could be dumping moisture into your ceiling cavity and it is ‘raining’ inside the roof space.

Black mold growing in a hotel room shower ceiling

Mould is a fungal growth that thrives on moisture.

What if your new house gets wet inside?

It depends on the cause of the problem. In colder climates, the problem is often incorrect installation or use of insulation and building wraps. This can cause moisture to condense inside the wall, floor or ceiling construction. In tropical climates mould is more likely to be due to poor ventilation and lack of air movement.

The National Construction Code sets out the standards required for condensation management and fresh air. If your house was not compliant then the builder, designer or certifier may be at fault.

Can energy-efficient homes cause moisture issues?

Energy-efficient houses allow the interiors to be heated well during winter. It is also important that these houses are built well, and all insulation, wraps and construction layers are designed and installed with condensation management principles in mind.

Energy-efficient houses can be very well sealed and nice and warm inside. But if the insulation isn’t done correctly, the air movement might be low and moisture levels high. This is the perfect recipe for mould. The Passive House standard requires heat recovery ventilation to be used. This removes moist, stale air and replaces it with fresh outside air, all without losing much heat.

The inside of an energy-efficient house

Energy-efficient homes are often well-sealed to increase comfort, so they also need to be properly ventilated. Image: CSIRO.

How can I design a house that’s warm and cosy, but not prone to mould?

Ask your builder or architect for a home that is energy-efficient, and check if they have experience in moisture management. This might mean using a standard like Passive House.

Ask how the house will be ventilated. Will exhaust fans have ventilation to outside air? Has all the insulation been designed and installed in accordance with the manufacturer’s directions? What guarantees will the builder or architect provide?

With a bit of planning, you can create a comfortable and un-musty home!