How to air your home in winter without losing heat

How do you air a home in winter without losing heat?

The most effective way is short and intense: open windows on opposite sides of the home fully for 5 to 10 minutes, then close everything. With a through-draught, the moist indoor air is replaced by outdoor air within minutes. Walls, floors and furniture barely cool down in that time, and they hold most of the home's heat.

When you close the windows, the fresh air warms up quickly against those surfaces. You do lose some heat, but far less than with a window ajar for hours — and you remove the water that built up overnight.

A family of four puts ~10 litres of water a day into the air through breathing, showers, cooking and drying laundry. Without air change, that water ends up on the coldest surfaces: glass, ceiling corners, walls behind wardrobes. If you already have mould or streaming windows, take the free diagnosis to confirm the cause is condensation rather than water ingress.

But it's 95% humidity outside: won't airing make it worse?

This is the most common question in Lisbon in January and February, when outdoor relative humidity sits between 90% and 100%. The answer is no, because relative humidity depends on temperature. Cold outdoor air holds little water in absolute terms.

An example using the Magnus formula: outdoor air at 10 °C and 95% relative humidity, once it comes in and warms to 20 °C, ends up at about 50%. The air that left, on the other hand, was loaded with overnight vapour. Every air change takes water out of the home, even when it is raining.

The exception is basements and very cold rooms in summer, where warm outdoor air can condense on cold walls. In a Portuguese winter, regular airing almost always lowers humidity.

When to air: morning, after showers and after cooking

Random airing does little. What counts is removing vapour right after it is produced, before it has time to reach cold walls.

• As soon as you wake up: humidity is at its highest. Two sleeping people release ~0.5–1 L of water a night in a closed bedroom. Open the bedroom and the opposite window before making the bed.
• After every shower: keep the bathroom door shut, run the extractor or open the window for 10–15 minutes, and dry the floor and shower screen.
• During and after cooking: lids on pans, extractor on from the start and for a few minutes afterwards, kitchen door closed.
• Before bed: a second quick 5-minute change helps in small bedrooms or when two or more people sleep there.
• When you get home: if the home was shut all day with laundry drying, air it before turning the heating on.

If you dry laundry indoors, that is probably your biggest source of vapour. The guide drying laundry indoors shows how to reduce its impact.

If you work from home, vapour production continues through the day: more people, more cups of tea, more cooking. Add a quick change in the middle of the day, for example at lunchtime, when the outdoor temperature is higher.

Why a window left ajar all day is worse

Leaving a tilt window or a window ajar for hours seems sensible, but it has two problems. First, air change is slow and irregular: with little wind it barely renews the air. Second, cold air flows down the wall around the window and cools it for hours.

A colder wall is closer to the dew point. At 18 °C and 75% humidity in the room, any surface below ~13.5 °C gets wet. That is why mould often appears right around windows that are always left ajar — on top of a higher heating bill.

While airing, turn the heating down or close the radiator valves near the windows. Turn them back on when you close up.

It also helps to keep heavy curtains away from the glass and not leave roller shutters fully down in the morning: the still air between curtain and window cools down, and that is where the glass streams first.

Trickle vents and extractor fans: ventilation that doesn't depend on you

Many modern window frames have trickle vents at the top of the frame or in the roller-shutter box. They let in a small, steady airflow without draughts. Don't seal them with tape or foam, even if you feel some cold near the window: they are what allow you to close the home at night without humidity shooting up.

• Bathroom extractor: it should vent outside or into a duct, not into the false ceiling. Keep it on during the shower and for 15–20 minutes afterwards. Without a window, see the guide windowless bathroom.
• Kitchen extractor: prefer ones that vent outside over recirculating hoods, which filter grease but don't remove vapour.
• Internal doors: keep the bathroom and kitchen closed while they produce vapour, so it doesn't spread into the bedrooms.
• Vents and gaps under doors: they help air move between rooms; don't block them with draught snakes.

What humidity should your home have in winter?

The target is indoor relative humidity between 40% and 60%. Measure at 7 am, before opening windows, with a ~€10 hygrometer placed away from windows and heaters. That morning reading best shows whether the home is accumulating water.

Readings above 70% in the morning for several days in a row greatly raise the risk of mould, because near cold walls humidity exceeds the ~80% mould needs to grow — even without visible droplets. Readings below 35% are rare in a Portuguese winter and mean the air is too dry.

To find the temperature at which your walls start to get wet, use the dew point calculator. Room-by-room values are in the guide ideal humidity at home.

A simple winter routine (and when it isn't enough)

If you follow this routine for two weeks and the hygrometer still reads above 70% at 7 am, airing isn't enough. The most likely causes are laundry drying indoors, gas heaters, many people in a small home or very cold walls.

In that case, a dehumidifier in one room or mechanical ventilation for the whole home are the next steps. The guide VMI or VMC compares the two ventilation options, and dehumidifier or VMI helps you choose between an appliance and an installation.