• I AM HOME Building Biology

Moisture in your home and what you can do to prevent mould growth

With long needed national inquiry into mould-related illnesses (such as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), fibromyalgia, multiple chemical sensitivity, Lyme disease and many more) caused and/or associated with water-damaged buildings launched in Australia a few weeks ago, hopes are high for everyone whose lives have been affected and touched by these afflictions, and who are struggling to get mould out of their homes and their bodies.

When mushrooms are growing on your skirting boards or you have ceiling cracks that leak during heavy rain, you most certainly know that your home is affected by mould and you will probably need to call in a remediator, a builder, and a home ventilation company. The situation is made worse if you are renting your water-damaged home because as we know (and I am writing from personal 15 year renting history) landlords, unless they have high integrity and compassion, are very reluctant to fix anything that costs more than a few hundred dollars.

Last year when Choice, National Shelter and the National Association of Tenants Organisations undertook a national survey of renters they found:

  • Almost a quarter of all renters reported ongoing problems with pests, doors or windows that didn’t close properly, peeling paint and loose tiles.

  • One in seven tenants said they held back from asking for repairs because they were afraid of a rent hike or getting evicted.

  • Around one in ten said they had been evicted for ‘no reason’ at least once since renting​.

Although this is discouraging and I would certainly advise renters, especially those whose immune systems are unable to detox biotoxins (about 24% of population) and who can end up with chronic inflammation and autoimmune diseases as a result, to look for alternative non water-damaged accomodation and get treated to remove mycotoxins from their systems, there are a few tricks you have up your sleeve that can help you deal with moisture and mould growth as well as control air quality in your home, at least to some extent. Once again, these are not ideal solutions, but rather temporary band-aids that can, for example, help you survive through winter if you are stuck in a lease or can't find anywhere else to move at the moment.

The key to treating mould in a home is always, first and foremost, to locate and remove the source of moisture. This can be moisture arising from internal or external sources, or during construction (Bijlsma 2018). Now, while in a rental, you might not be able to affect or fix external sources (leaks through the building envelope that arise from blocked or damaged gutters, damaged roof tiles, damaged flashings, lack of eaves; subfloor moisture and rising damp; landscaping located too close to the house or poor drainage around the site) or moisture during construction, but you can almost most certainly control most internal moisture arising from household activities.

With winter well under way here in Australia, we are trying to retain warmth on our homes by closing doors and windows and turning up the heating. All that moisture from heating, bathing, cooking, laundering, breathing (we generate around 10 litres of water vapour per day) and drying clothes in unvented dryers accumulates in the indoor air and eventually condenses (if not properly vented) on windows, window frames, behind curtains, along skirting boards, on carpet under the favourite couch or bed, under appliances and in corners around the room.

Indoor sources of water vapour in dwellings and indicative rates of release (Source: BRANZ 2012).

Most new homes are also fitted with water impermeable wall wraps, which block the water vapour generated by occupants in the household from leaving the building. Older brick homes that are rendered with conventional cement renders and painted with synthetic-based impervious paints can also have similar issues because that render doesn’t allow the building envelope to breathe. It is like wrapping your house in plastic.

Coupled with dust that’s already on most surfaces around the home, build-up of water vapour inside the building creates ideal conditions for mould growth. The more visible dust the greater the exposure to microbial activity (Bijlsma 2018). Because once there is enough moisture in the air (relative humidity reaches above 70%) - dust on surfaces and contents will become perfect food for mould. This is most likely to occur in those parts of the home that aren’t exposed to sunlight (e.g. the South side of the home in cool and temperate climates, areas where sunlight is blocked by vegetation or neighbouring buildings or due to building design issues that do not allow sufficient sunlight into parts of the house) (Bijlsma 2018).

Other internal sources of moisture include:

  • open fish tanks and indoor water features

  • humidifiers

  • accidental floods (that are not dried within 48 hours)

  • wet tea towels, dish cloths, clothes, towels and bathmats

  • indoor spas, pools an saunas

  • unflued gas heaters

  • steam cleaning of carpets or furnishings

  • mattresses contaminated with urine

  • pet urine

  • appliances that are plumbed and contain water such as air conditioners, dishwashers and refrigerators that have not been services and maintained in accordance with the manufacturer's instructions

  • overwatering of indoor plants

So, what are your options? Besides, obviously avoiding indoor saunas and humidifiers, and not leaving wet towels in the bathroom - it's not like we can stop cooking, bathing, showering or washing our clothes. We know that heating will assist with drying of damp surfaces. Dehumidification and ventilation will assist in removing the moisture that evaporates from damp surfaces from the air inside your home. Proper and frequent housekeeping will help to reduce dust and mould spores that settle on surfaces. Air purification will assist in removing particulates such as mould spores, dust and allergens, plant pollen, pet dander, smoke, fumes and odours, chemicals outgassing from furniture and building materials and other particulates. It's a final step that can help make the air in your home cleaner and easier to breathe. We need to implement a combination of these techniques to cope with water vapour arising from internal activities and help reduce the chances of mould growth inside our homes as much as possible.

So here are some tips that I found useful:

  • Be alert to condensation forming on the glass and frames of windows as well as in cold closets backing onto exterior walls. These are usually the coldest surfaces in a room and condensation on them is an early warning of high relative humidity that can support dust mite infestations and mould growth. Surface condensation of this sort should always be wiped up to discourage mould growth or decay of timber frames, window sills or architraves. Another frequent place where condensation occurs is behind picture frames and furniture in corners, and other enclosed spaces like built-in wardrobes.

  • Open windows and doors every day especially on warm dry days, for natural ventilation which will reduce the amount of water vapour circulating indoors.

  • Open doors between rooms especially doors to closets which may be colder than the surrounding rooms.

  • In cooler seasons, opening windows in the afternoon, when daytime temperatures are highest, will limit the impact of the cooler outdoor air on the temperature inside.

  • Monitor humidity levels inside your home with a simple hygrometer (see picture below) available from your local hardware store or online. You want to maintain humidity levels between 45 and 55%. If relative humidity levels exceed 60% on a regular basis, place a portable dehumidifier in that room or area. It can be moved around the home and is handy in bathrooms that lack adequate ventilation. Ausclimate has a good range of dehumidifiers. They can help you choose the right one for the size of your dwelling and climate zone.

Thermometer & Hygrometer with Jumbo Display - IC7312 from

  • Use ceiling or small portable fans to increase the capture zone of a dehumidifier and improve airflow, especially in rooms with poor ventilation.

  • Drain and clean dehumidifier collection trays frequently so that they do not become a breeding ground for micro-organisms. Replace filters regularly.

  • Some split system air conditioning units can also act as dehumidifiers. However, you need to confirm that it has the capacity to control both temperature and humidity before investing in one. Air conditioners need to be regularly checked and maintained – once a week is recommended during the cooling season, as these devices provide ideal conditions for mould and bacterial growth with the added disadvantage that they also contribute to the distribution of the spores.

  • Reducing water vapour generated by internal activities is important. Ideally mechanically driven exhaust fans with timer switches or humidity sensors that are ducted directly to the outdoors should be fitted in every wet area like bathroom, kitchen and laundry. To operate effectively, rooms with exhaust fans running need to draw air from other rooms or passageways or through open windows or doors.

  • If you don't have an exhaust fan or it is not vented to the outdoors open a window and/or a door instead while bathing/showering (if possible), and operate a dehumidifier inside that room to help draw moisture out of the air.

  • Clean the exhaust fan and grill with a microfibre cloth on a regular basis.

  • Reduce the amount of water vapour by encouraging shorter or cold showers, not drying wet clothes in small rooms, mopping spills and allowing wet towels and bath mats to dry between use. Clean up immediately after any flood or big spill.

  • Dry clothes outside in as much sunlight as possible or in a condensing dryer.

  • Store clothes that are not used during a season in space bags.

  • Check for leaking pipes and dripping taps and moisture under appliances (dishwasher, fridge, washing machine). Address these as soon as you find them. Braided water hoses frequently burst and are a common cause of water leaks in a home. These hoses should be replaced every 5-7 years.

Braided supply lines connect to valves beneath sink fixtures. Image sourced from Jupiterimages/ Images

  • De-clutter to reduce dust load (dust is a food source for mould). At least once a year do a thorough ‘Spring Clean’ and clean under and behind appliances and furniture, especially beds and those items that are standing up against external walls.

  • When positioning freestanding furniture, avoid placing large items against outside or south-facing walls as fas as possible and always leave space for air to circulate on all sides. This applies especially to wardrobes and cupboards used to store leather shoes or garments which are particularly susceptible to mould growth.

  • Leave doors in wardrobes and cupboards open to encourage air flow in enclosed spaces and use damp chaser rods inside built-in wardrobes to warm, dry and circulate air through a continuous process. Zeolite packs are also handy in small confined spaces. They help reduce mould, mildew, dampness and condensation odours naturally.

  • When heating is needed, it will have the greatest effect on lowering relative humidity and condensation risk if it is applied continuously and through all interior spaces at an even temperature. Rooms left unheated, right accumulate enough water vapour to reach 70% relative humidity level that makes mould viable.

  • Use slightly damp microfibre cloth followed by a dry, clean tea towel to dust your home every week, especially in places where dust accumulates such as grooves of windows, skirting boards, on top of cupboards, behind and under furniture and appliances. Vacuum carpeted rooms regularly with an asthma friendly vacuum cleaner fitted with a (HEPA) filter, motorised head and an electrostatic disposable bag. Replace the bag regularly as house dust mites thrive in it. Replace the HEPA filter at least every 6 months. The most effective way to reduce dust is to vacuum against the carpet’s fibres taking around 1 minute per square metre.

  • Vacuum mattresses at least once a month. Air the mattress outdoors as often as you have time for, but aim for once or twice a year.

  • If you have ducted heating and it hasn't been serviced recently, disconnect it and close/seal off all ducts and the return air vent.

  • Avoid drapes and fabric curtains as they harbour dust. Replace curtains with non-fabric timber or flat blinds.

  • Frequently air bedding, rugs, curtains, furniture and all fabric furnishings in the sun. Also hang them over a clothesline or railing and beat the dust and dirt out of them (wearing a P2 mask) on a regular basis.

Image sourced from from

  • Leave your bed unmade during the day so that moisture from from the sheets, pillows and mattress can evaporate. This will also help to control dust mites.

  • Avoid indoor plants in bedrooms, as they are a source of moisture and microbes.

  • Ideally, carpets should be avoided because they act as reservoirs of dust!

  • Air purifiers can be used to improve air quality. However, dehumidification should be a priority. When selecting an air purifier, make sure it has the following technology: MERV-rated HEPA (High Efficiency Particulate Arrestance) filter (for particulates and biologicals) and Granular Activated Carbon filter (for gases, toxicants and large micro-organisms such as mites). Innovair has a good range of portable air purifiers, most of which have medical-grade HEPA filters. The unit can be moved between the rooms as needed. To increase the capture zone of the unit, operate a ceiling or portable fan in the room.

  • If you want to use an oil diffuser, choose a waterless one as water-based diffusers can contribute to significant amount of moisture in the air.

That's it! I know it's a long list. But as they say, start with one or two things that you can do daily and one or two things that you can do weekly, and then see what else is doable on a regular basis. Ventilation, dehumidification and regular cleaning will help significantly to cope with water vapour arising from internal activities and reduce the chances of condensation and subsequent mould growth.

A healthier home one step at a time!


Australian Government and States and Territories of Australia 2016, Australian Building Codes Board, Condensation in Buildings Handbook 2014, 2nd ed., Canberra ACT.

Bijlsma, N. 2018, Healthy Home, Healthy Family, 3rd ed., Australian College of Environmental Studies, Melbourne, Australia.

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