When Australians think about humidity problems, the sticky, uncomfortable feeling of a humid summer usually comes to mind. Yet for many parts of Australia, winter presents the opposite challenge: indoor air becomes uncomfortably dry, leading to a range of health and comfort issues. Understanding why this happens and how to address it can significantly improve your quality of life during the colder months.

The Winter Humidity Problem

It seems counterintuitive. Winter in Melbourne, Adelaide, or Hobart often brings rain and cool, damp conditions outdoors. So why does the air inside our homes become so dry? The answer lies in how heating systems work and the relationship between temperature and relative humidity.

When cold outdoor air enters your home, whether through ventilation, gaps in insulation, or simply opening doors, it brings relatively little moisture. Cold air cannot hold much water vapour—this is basic physics. When you then heat this air to comfortable indoor temperatures, its capacity to hold moisture dramatically increases, but no additional moisture is added. The same amount of water vapour is now spread across a much larger potential capacity.

This is why relative humidity—the percentage of moisture in the air compared to its maximum capacity—drops so dramatically indoors during winter. Air that was at 60 percent relative humidity at 10 degrees Celsius might drop to only 20 percent relative humidity when heated to 22 degrees. The absolute amount of water has not changed, but relative to what the warmer air can hold, it represents a much smaller percentage.

Understanding the Numbers

Air at 10°C with 60% relative humidity contains about 5.6 grams of water per cubic metre. When heated to 22°C, that same air now has the capacity to hold about 19.4 grams per cubic metre. The original 5.6 grams now represents only about 29% relative humidity—well below comfortable levels.

How Different Heating Systems Affect Humidity

Not all heating systems have the same impact on indoor humidity. Understanding how your heating method affects air quality helps you develop appropriate strategies for maintaining comfort.

Ducted Gas and Reverse Cycle Systems

Ducted heating systems, whether gas or reverse cycle, tend to create the driest indoor conditions. These systems move large volumes of air, heating it and distributing it throughout the home. The constant air circulation accelerates moisture loss from surfaces and from the air itself. Reverse cycle systems in heating mode can be particularly drying because they extract moisture from outdoor air as part of the heat exchange process.

Radiant Heaters

Radiant heaters, including panel heaters and some types of gas heaters, warm objects and people directly rather than heating the air. While they still cause the ambient air temperature to rise, they typically create less air movement and may be somewhat gentler on humidity levels compared to forced-air systems.

Wood Fires

Open wood fires and even many enclosed wood heaters can significantly dry indoor air. The combustion process consumes oxygen and creates a draft that draws air from the room and up the chimney. This air movement accelerates the replacement of indoor air with drier outdoor air. Additionally, the high temperatures near the fire create very low local humidity.

Signs of Winter Dry Air

Dry winter air manifests in numerous ways, some obvious and others subtle. Recognising these signs helps you identify when action is needed.

Physical Symptoms

  • Dry, itchy, or cracked skin, especially on hands and face
  • Chapped lips that persist despite using lip balm
  • Frequent nosebleeds or dry, crusty nasal passages
  • Scratchy throat, particularly upon waking
  • Dry, irritated eyes
  • Increased severity of eczema or psoriasis
  • Static electricity shocks when touching objects or people

Household Indicators

  • Wooden furniture, floors, or musical instruments developing cracks
  • Gaps appearing in wooden floorboards
  • Paint or wallpaper beginning to peel or crack
  • Indoor plants with brown, crispy leaf edges
  • Books and artwork showing signs of drying damage

Quick Test

If you are experiencing frequent static shocks, indoor humidity is almost certainly below 30 percent. Static electricity builds up much more readily in dry conditions and is a reliable indicator that you need to add moisture to your indoor environment.

Health Implications of Dry Winter Air

Beyond comfort issues, low humidity has genuine health implications that deserve attention. The respiratory system is particularly affected.

Your nasal passages are lined with mucous membranes that trap pathogens and particles before they can reach your lungs. When these membranes dry out, they become less effective at this protective function. This is one reason why respiratory infections tend to spread more readily during winter—dry indoor air compromises one of your body's first lines of defence.

Research has shown that influenza viruses and other respiratory pathogens survive longer and spread more easily in low-humidity environments. Studies have found that maintaining indoor humidity between 40 and 60 percent can reduce the viability of airborne viruses and may help limit disease transmission within households.

People with asthma or other respiratory conditions often find their symptoms worsen in dry air. Dry airways are more reactive and more easily triggered by other irritants like dust or pollution.

Strategies for Maintaining Winter Humidity

Addressing winter dry air typically requires multiple approaches working together. A humidifier is often the most effective solution, but other strategies can help as well.

Using a Humidifier

A quality humidifier is the most direct and controllable way to add moisture to dry winter air. For best results, choose a unit appropriately sized for your space and use a hygrometer to monitor humidity levels. Aim to maintain 40 to 50 percent relative humidity.

During winter, you may need to run your humidifier more continuously than at other times of year. If your home is particularly dry or your heating system runs frequently, consider using humidifiers in multiple rooms rather than trying to humidify your entire home with a single unit.

Reducing Moisture Loss

Several practices help retain existing moisture in your home. Running exhaust fans in bathrooms and kitchens removes humid air, so use them judiciously during winter—just long enough to remove excess steam without over-ventilating. If your home allows, leave the bathroom door open after showers to let that moisture disperse through the house.

Sealing air leaks around windows and doors prevents the continuous exchange of indoor and outdoor air. While some ventilation is necessary and healthy, excessive air exchange in winter constantly replaces moist indoor air with dry outdoor air.

Houseplants and Evaporation

Indoor plants release moisture through transpiration and can contribute to indoor humidity. While plants alone cannot substitute for a humidifier in a very dry environment, they do provide a modest humidity boost while also improving air quality in other ways.

Similarly, placing bowls of water near heat sources allows passive evaporation that adds some moisture to the air. This is a very low-tech approach with limited effectiveness, but it can supplement other humidity strategies at no cost.

Regional Considerations Across Australia

Different parts of Australia experience winter dry air to varying degrees. Understanding your regional conditions helps you calibrate your humidity management approach.

Melbourne, Adelaide, Hobart, and inland cities like Canberra typically experience the most significant winter dry air problems. Cold temperatures, frequent heating use, and lower outdoor humidity combine to create very dry indoor conditions from May through September.

Sydney and Brisbane generally have milder winters with higher outdoor humidity. While heating systems still dry indoor air, the effect is typically less severe. However, heavily air-conditioned buildings in any climate can experience similar dry air issues.

Tropical and subtropical areas rarely need humidification at any time of year. The challenge in these regions is more often managing excess humidity rather than insufficient moisture.

Winter does not have to mean months of dry air discomfort. By understanding why indoor humidity drops, recognising the signs of overly dry conditions, and implementing appropriate solutions, you can maintain a comfortable and healthy indoor environment throughout the cooler months. Your skin, sinuses, and respiratory system will thank you for the attention to indoor air quality.