Here in the UK, we’re a bit like bears or, for people of a certain age, the Blue Peter Tortoise: as soon as it gets cold, we lock up our homes, whack the heating up and hibernate until we have to go out or it gets warm enough to do so.
That would seem to make sense. With energy prices still incredibly high, wasting any heat feels like throwing money out of the window. Yet, there’s a real danger of closing up our homes, and opening windows for a short time each day, even in the middle of winter is not only a brilliant idea, it can reduce condensation and mould.
That may fly in the face of convention, as we’re often told to seal all gaps in winter, particularly in old, draughty homes. But opening a window to let cold air in has a big impact on our overall health. Here’s why.
Why do we get condensation?
Air holds water. The amount of water in the air is usually noted down as the relative humidity (RH, but normally just referred to as humidity), and is quoted as a percentage. The issue with this is that the percentage is used to describe the amount of moisture in the air versus how much moisture the air is capable of holding. As the air temperature rises, the amount of moisture it holds also increases.
Condensation in the home occurs when warm air in your home meets a cold surface, such as an external wall or window (particularly single-glazed). As the air cools, the amount of water it can hold decreases, and the dew point is reached. The dew point is the temperature that air has to be cooled to for its contained moisture to turn into condensation (or fog, mist or dew on the grass). The exact dew point changes based on the relative humidity and current temperature.
The simple thing you need to know is that if your home is humid, as it cools overnight, the air cools and, as it makes contact with a cold surface, condensation is formed. As mentioned, you’re most likely to meet condensation on a window pane or external wall, but you can find it forms on clothes, or even on furniture touching a wall.
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With a moist environment, you’re more likely to get black mould appearing in your home, on walls, clothes, books, furniture and more. And, condensation is the biggest cause of black mould, according to the Centre for Sustainable Energy.
The issue in the colder months is that we shut our homes up, so moisture is trapped in the air. That excess water comes from drying clothes indoors, showering, cooking and, horribly, breathing.
With no escape, the moisture builds up and our homes become more humid. Condensation should be removed when spotted, either by using a cloth or, more easily, with a window vacuum. However, proper ventilation and opening windows can have a bigger impact in the long term.
Stoßlüften or shock ventilation
In Germany, they have a word (of course) for the process of opening all of the windows on a cold day: Stoßlüften, which translates to shock ventilation. Even on the coldest of days, Germans will open all the windows for a short while, flooding the house with cold air. It sounds quite mad, but it makes a lot of sense.
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Let’s go back to the humidity issue. Remember that the warmer the air, the more moisture it holds. That also means that if you heat up air without adding moisture, the relative humidity drops: you’re increasing the air’s ability to hold moisture without adding moisture.
This is an important factor to note. Opening windows lets the warm, humid air escape outside, and the cold air to flow back inside. Even though the outdoor relative humidity might be higher than the indoor reading, the total amount of moisture held is lower and, as the cool air warms, the relative humidity drops.
If you use Richard Stanton’s excellent humidity calculator, you can see the effect. Enter the outside temperature and humidity, and the indoor temperature, and it will show you what your home’s humidity will be as the air warms up.
As I write this, it’s 2.7°C outside at 87% humidity, and 18°C inside. Cold air coming in would warm up, and the relative humidity would drop to 33%,
That’s too low, and humidity between 40% and 60% is better. However, opening the windows for 15 to 20 minutes won’t expel all the hot air, and the cool air that comes in helps reduce indoor humidity and provide fresh air. In turn, that helps reduce condensation, which reduces the chances of black mould.
And, lower humidity means that your heating has less work to do in order to heat your home, which will mean some energy savings.
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More ventilation
Ventilation is very important, particularly in older houses with solid walls. Opening windows every day is a simple way to achieve this, but there are ways to help prevent heat loss or wasting energy.
A smart heating system that has open window detection is a good idea, such as on the Tado X system. When these systems detect a sudden drop in temperature, they turn the heating off to prevent wastage.
If you really don’t want to open windows all of the time, then a Positive Input Ventilation (PIV) system is a good idea. These are typically installed in a loft, and use a small fan to take cold air from the loft space and push it into the home, reducing humidity. In flats, PIV systems can be installed through a wall.
Using a dehumidifier, such as the MeacoDry Arete Two, is also a good option, particularly when drying clothes indoors. Sucking out moisture, a dehumidifier can keep indoor humidity at optimum levels.
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