What Is Relative Humidity — and Why Does It Matter?
It's one of the most important numbers in your home. Most people have never checked it.
Why this matters: indoor humidity affects whether mould grows, how comfortable your home feels, how well you sleep, and how long your building materials last. It’s controlled almost entirely by ventilation — and most Australian homes don’t manage it well.
What does “relative humidity” actually mean?
Air can hold water vapour — but only up to a certain amount, and that limit changes with temperature. Warm air can hold considerably more moisture than cold air. Relative humidity (RH) expresses how much moisture is actually in the air as a percentage of the maximum it could hold at that temperature.
At 50% RH, the air is holding half its moisture capacity. At 100% RH, it’s saturated — water starts forming on surfaces. This is condensation.
The “relative” part is important. The same amount of water vapour can represent 40% RH in a warm room and 80% RH in a cold one. This is why humidity feels so different between summer and winter — and why cooling a room can actually cause condensation even without adding any moisture.
The sponge model
Think of air as a sponge. A warm sponge can hold a lot of water before it drips. A cold sponge can hold very little. Relative humidity tells you how full the sponge currently is. When it reaches 100%, it drips — onto your windows, your walls, your ceiling.
< 40%
Too Dry
Dry skin and eyes, irritated airways, static electricity, cracking timber and plaster.
40–60%
Healthy Range
Comfortable, mould-inhibiting, good for respiratory health and building materials.
> 60%
Too Humid
Condensation, mould growth, dust mite proliferation, musty smells, building damage.
Why temperature and humidity are inseparable
This relationship explains several things that confuse homeowners.
When you heat a room, you don’t add moisture — you just give the existing air a larger capacity to hold it. So heating a cold, damp room reduces relative humidity without removing any water vapour. The moisture is still there; it’s just a smaller fraction of what the now-warmer air can hold.
The reverse is equally important. When a warm, moist room cools overnight — say, a bedroom after the heating shuts off — the same moisture load now represents a higher relative humidity. If the air cools enough, it reaches 100% RH and condensation forms on the coldest available surface: the windows.
This is why window condensation is almost always worst first thing in the morning, after a night of cooling with closed doors and two people breathing. The moisture was generated during the day; the cold night reveals it.
How much humidity do we actually generate?
The scale surprises most people. Indoor moisture comes from more sources than you’d expect.
| Source | Moisture Generated |
|---|---|
| Breathing (per person, per day) | ~0.3 litres |
| Sleeping (2 adults, overnight) | ~0.5–1 litre |
| Showering | ~0.5 litre per shower |
| Cooking dinner | ~1–3 litres |
| Drying clothes indoors | 2–5 litres per load |
| Mopping floors | ~0.5 litre |
| Houseplants (per plant, per day) | ~0.1–0.5 litres |
| Family of 4 — total daily | 10–15 litres |
A family of four tips the equivalent of a bucket of water into their home’s air every single day. In a draughty older home, much of it drifted out through gaps and cracks. In a modern airtight home, it stays inside unless there is a designed ventilation system to remove it.
What high humidity does to your home
Sustained high indoor humidity doesn’t just feel unpleasant. It causes measurable damage over time.
Mould and mildew
Mould spores are present in all homes. They only germinate and grow when relative humidity exceeds roughly 70% at a surface for a sustained period. Once established, mould produces mycotoxins and allergens that affect respiratory health — particularly in children, the elderly and people with asthma.
Dust mites
Dust mites thrive between 70–80% RH. Below 50%, their reproduction slows dramatically and populations decline. For allergy and asthma sufferers, maintaining indoor humidity below 50% is one of the most evidence-backed environmental interventions available.
Condensation and building damage
Persistent condensation on windows and cold surfaces leads to water pooling on sills, timber swelling and rotting, paint peeling, and eventually structural damage in wall cavities where the condensation is hidden. The visible condensation on your window is the early warning sign.
Timber and material degradation
Timber framing, flooring and joinery expands and contracts with humidity. Sustained high humidity accelerates this cycling, causing gaps, cupping, and squeaking floors. It also degrades adhesives, wallpaper, and composite board materials over time.
Comfort and sleep quality
High humidity impairs the body’s ability to cool itself through perspiration. Even at a comfortable temperature, a humid room feels warmer and more stuffy. Overnight, high CO₂ and humidity in a closed bedroom are both linked to disrupted sleep and morning fatigue.
What low humidity does to your home
While high humidity gets most of the attention, air that is too dry causes its own problems — particularly in Adelaide’s hot, dry summers and in homes with mechanical heating running all day in winter.
Dry skin, eyes and throat: The body loses moisture to dry air continuously, causing irritation, cracked lips and itchy eyes — particularly for contact lens wearers.
Increased susceptibility to respiratory infection: The mucous membranes that trap airborne pathogens function best at 40–60% RH. Below 40%, they dry out and become less effective.
Static electricity: Dry air is a poor conductor, allowing charge to build up on surfaces, carpets and people — a nuisance in homes and potentially damaging to electronics.
Timber damage: Timber shrinks in very dry conditions, causing gaps in flooring, door frames and joinery. This is the flip side of the high-humidity swelling problem.
Humidity in Adelaide’s climate
Adelaide’s Mediterranean climate creates distinct seasonal humidity challenges that differ from most other Australian capitals.
In summer, outdoor humidity is generally low — some of the lowest of any Australian capital. But inside a closed, inadequately ventilated home, cooking, showering and occupant activity can push indoor humidity well above outdoor levels. The home traps the moisture generated by its occupants.
In winter, homes are closed up for weeks at a time. Cool outdoor air and active indoor heating create exactly the conditions for condensation on windows and cold surfaces. The Adelaide Hills compounds this further — lower temperatures, higher winter rainfall, and homes that are closed for even longer periods.
The result is that many Adelaide homes run at healthy humidity levels for perhaps three or four months of the year — and struggle through the rest without the occupants realising it.
How to measure it
A basic digital hygrometer — the instrument that measures relative humidity — costs $15–40 and can be placed in any room. For a more complete picture, a data-logging hygrometer records humidity over time, allowing you to see the overnight bedroom pattern, the post-shower spike, and the winter baseline.
If you find your home consistently above 60% in winter or below 35% in summer, ventilation strategy is worth reviewing. A reading above 70% is worth acting on promptly.
How ventilation controls humidity
Humidity is controlled by one mechanism: air exchange. Replace moist indoor air with drier outdoor air, and indoor humidity drops. The challenge is doing this without losing the energy you’ve put into heating or cooling the indoor air.
Opening windows works — but it’s weather-dependent, occupant-dependent, and comes with a significant energy penalty in winter. Exhaust fans in bathrooms and kitchens help at the source — but only when running, and only in those rooms.
For modern airtight homes, the solution is MVHR — Mechanical Ventilation with Heat Recovery. An MVHR system runs continuously, extracting moist air from bathrooms, kitchens and utility rooms while supplying fresh filtered air to bedrooms and living spaces. A heat exchanger recovers up to 90% of the energy from the outgoing air, so the ventilation doesn’t come at the cost of comfort.
When properly designed and commissioned, an MVHR system maintains indoor relative humidity in the 40–55% range year-round — not occasionally, but continuously. This is what separates it from every other humidity management approach.
Without designed ventilation
With MVHR
Related reading
Condensation on New Windows
How humidity and window type combine to create — or prevent — condensation
Mould in New & Airtight Homes
Why high indoor humidity is the primary driver of mould in new builds
Damp Bathrooms That Never Dry
The bathroom humidity problem and what NCC 2022 requires
Poor Sleep & CO₂
How overnight humidity and CO₂ affect sleep quality in bedrooms
What Is MVHR?
The ventilation system designed specifically to manage humidity in airtight homes
Airtightness Explained
Why modern airtight homes need designed ventilation to control humidity
Frequently asked questions
What is relative humidity?
Relative humidity (RH) is the amount of water vapour in the air expressed as a percentage of the maximum amount the air could hold at that temperature. At 50% RH, the air is holding half its maximum moisture capacity. When RH reaches 100%, the air is saturated and condensation forms on any surface at or below that temperature.
What is a healthy indoor humidity level?
The widely accepted healthy range for indoor relative humidity is 40–60%. Below 40%, the air is too dry — it irritates airways, dries out skin and eyes, and increases static electricity. Above 60%, conditions become favourable for dust mites and mould growth. The sweet spot for most homes is 45–55%, which balances comfort, health and building durability.
Why does humidity feel worse in Adelaide summers?
Adelaide's climate swings between hot, dry summers and cool, damp winters. In summer, outdoor humidity is generally low — but inside an inadequately ventilated home, cooking, showering and occupant activity can push indoor humidity well above outdoor levels, making it feel stuffy and uncomfortable even when outdoor air is dry. In winter, homes are closed up and indoor humidity climbs as moisture has nowhere to escape.
How do I reduce humidity in my home?
The most reliable approach is continuous mechanical ventilation — specifically MVHR (Mechanical Ventilation with Heat Recovery) for airtight homes. MVHR removes moisture-laden air from kitchens, bathrooms and living spaces continuously, replacing it with fresh filtered outdoor air. Exhaust fans, opening windows and dehumidifiers can help temporarily but don't address the root cause: moisture generation combined with insufficient air exchange.
Humidity is invisible. Its effects aren’t.
Condensation, mould, poor sleep, dust mites, structural damage — most of these trace back to a humidity problem that was never measured and never managed. A hygrometer costs less than $30. Understanding what the number means costs nothing. Fixing the problem, when you know what it is, is almost always more straightforward than people expect.
Not sure what’s driving humidity in your home?
HiPer Haus can assess your home’s ventilation strategy and recommend solutions that bring indoor humidity — and indoor air quality — into the healthy range, year-round.