Ideal Indoor Temperature to Prevent Mould and Cut Heating Costs

The heating ran in economy mode overnight, and the parquet feels like a thin sheet of ice beneath your feet. The bedroom still holds the damp trace of the night: two people, shut windows and towels left wet from the evening before. There is a dark speck in the corner of the window, wiped away only yesterday. Today, it has returned. Tiny and apparently harmless – at first glance.

Down in the communal hallway, neighbours are debating it: “I won't heat above 18 degrees, or my bill will explode.” Two floors up: “My heating stays on; I want it warm, end of story.” Somewhere between those positions is a temperature that gives both your budget and your lungs some relief.

Often, those one or two degrees determine whether you get through winter in good health – or mould moves in first.

The unseen divide between comfort and danger

The same quiet winter dispute plays out in countless homes: heating bills versus wellbeing, saving money versus worrying about mould. Plain numbers glow on the thermostat, yet behind them lie headaches, coughs and dry mucous membranes – or black marks in the corner. One home feels like a refrigerator at 19 degrees, while another feels like a dry sauna at 21 degrees.

The key question is not simply: how warm is it? It is also: how warm does the air we breathe around the clock actually feel? And how humid is it? The crucial combination of temperature and humidity determines whether mould spores feel as comfortable as we do on the sofa.

We all know the moment: you turn the heating down in the evening because the gas app is flashing red warnings again, while quietly hoping the living room will not become a damp microclimate.

Consider a typical 70-square-metre flat in a German city: third floor, built in 1968, occupied by one person or a couple, with older windows and reasonably modern radiators. The smart meter shows an average temperature of 18 degrees over the previous winter, with humidity frequently above 65 per cent. On paper, that looks frugal, even exemplary. In reality, there is a persistent cough, condensation on the windows each morning, and recurring dark patches in the bedroom and behind the sofa.

Germany's Federal Environment Agency recommends around 20 to 22 degrees for living spaces and 16 to 18 degrees in bedrooms, with humidity ideally between 40 and 60 per cent. It sounds straightforward, almost textbook. In practice, it is different: according to a survey by consumer advice centres, many households now heat well below these levels because they fear additional bills. It is no coincidence that mould problems have become noticeably more common in recent years.

The reasoning is brutally simple: cold air holds less moisture than warm air. If you let your home cool to 17 degrees while cooking, showering, drying laundry or simply breathing, relative humidity can rapidly enter a range where mould thrives. Risks begin to rise at roughly 60 per cent, while above 70 per cent conditions become genuinely critical, particularly on cold external walls and around window reveals. The surfaces of these walls are often several degrees colder than the room air – precisely where warm, damp air meets a cold surface.

The result is that moisture forms as condensation, initially unseen in the pores of plaster or behind furniture. A temperature that still feels “fine” to you may be several degrees lower within the wall. That seemingly minor difference decides whether you merely feel a little chilly or spores become permanent housemates.

The temperature at which mould has no chance

The everyday rule of thumb is surprisingly simple: aim for around 20 to 21 degrees in living areas during the day, closer to 22 degrees in the bathroom, and 17 to 19 degrees in the bedroom – no lower. The key is not to create extreme swings, with sauna-like warmth by day and refrigerator temperatures at night, but to keep conditions reasonably stable. That consistency substantially lowers the risk of mould.

A practical approach is to create two fixed temperature zones in your home. Zone one covers rooms where you spend extended periods: the living room, home office and children's bedroom. Here, 20 to 21 degrees is a sound benchmark. Zone two includes the hallway, kitchen and bedroom. Keep these slightly cooler but never freezing, at 17 to 19 degrees depending on what feels comfortable. Adjusting the thermostat ten times a day is less useful than choosing a sensible, stable average. Consistent warmth is like a quiet bodyguard against condensation.

Let's be honest: hardly anyone manages this every day. Real life means quickly turning radiators up, airing rooms between Zoom calls, using a clothes rack in the living room and leaving windows half open “for a little fresh air”. This combination creates many common mistakes. One classic example is leaving the bedroom ice-cold during the day with the door open, allowing warm, damp air from other rooms to flow in; by night, the room has become a cold condensation chamber. Another is trying to heat one spot with a fan heater while the external walls remain permanently chilled.

Perhaps the most helpful advice is to observe rather than guess. A small room thermometer with a hygrometer costs little more than a takeaway meal, yet it shows mercilessly what is really happening. Once you see how quickly humidity shoots up while showering, cooking or drying laundry, it becomes clear why the black mark in the window corner is not simply “the landlord's fault”, but often a result of everyday habits.

“The problem is not absolute cold, but the combination of too low a temperature, too much humidity and too little air circulation.”

• Ideal living-room temperature: 20–21 °C at 40–60% humidity
• Do not allow the bedroom to become too cold: at least 17 °C, particularly with cold external walls
• Consistency rather than extremes: heat gently and continuously instead of repeatedly switching everything off
• Limit sources of moisture: keep doors shut while cooking or showering, then air the room briefly and thoroughly
• Let walls “breathe”: position furniture away from external walls rather than pressing it directly against cold surfaces

Between heating penalties and cough season – what truly matters

Anyone facing a gas or electricity bill in these months soon feels how intimidating figures can be. Suddenly, 19 degrees seems like a moral duty and 22 degrees like an extravagant offence. Yet the same view emerges repeatedly in discussions with doctors, energy advisers and tenants' advocates: there is no single “correct” number, but there is a narrow range in which health and energy consumption remain reasonably balanced.

It matters less whether your living room is precisely 20.5 or 21 degrees. What counts is whether the air in your home has a chance to shed moisture. It is whether you use heating in a way that prevents walls from constantly cooling down and becoming condensation traps. It is whether you ventilate when necessary, rather than only when it crosses your mind. And it is whether you take the first small dark spots seriously instead of trying to make them disappear with a damp cloth.

Many people only realise how strongly the indoor climate affects them once they deliberately change a few things: place a hygrometer in the room, maintain a steady temperature for a week, and create a through-draught for five minutes each morning and evening. Suddenly, windows mist up less, their nose is no longer constantly blocked, and a child's cough eases. The home does not merely feel warmer; it feels clearer.

Perhaps this winter is the time to learn to view numbers differently: not only those on the annual bill, but also the small readings on the hygrometer, thermostat and window opening. This unassuming combination determines whether mould is sitting at the breakfast table again in a few weeks – or whether your home finally becomes what it should be: somewhere you can breathe freely without worrying about spores, coughs and heating costs.

Key point	Detail	Benefit for readers
Ideal temperature ranges	Living areas 20–21 °C, bedrooms 17–19 °C, bathroom up to 22 °C	Enables targeted heating without lengthy research
The role of humidity	40–60% is ideal; the risk of mould rises from 60%	Helps identify mould risks early and take corrective action
Consistent heating	Heat gently and continuously rather than making sharp temperature changes	Reduces condensation and protects health and the building fabric

FAQ:

• Question 1: What bedroom temperature is genuinely healthy?
• Question 2: How often should I ventilate in winter to prevent mould?
• Question 3: Does it help to switch the heating off completely overnight?
• Question 4: At what humidity level does mould become a concern?
• Question 5: Is it enough simply to wipe away mould spots?