The air on my landing is at 69% humidity. Everywhere advises that indoor humidity should be 40-60% to avoid mouldy walls, the kids needing inhalers, and slugs coming in for a wander around. All sources say to increase ventilation and ensure I open the windows to let the damp air out.
The BBC weather app says outdoor humidity is currently 71%. So how does opening a window help? I feel like I’m missing something.
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What you are measuring here is relative humidity. This is a measurement which changes based on temperature. The colder it is the higher the relative humidity even if it is the same air. In general it is usually more humid inside then outside because humans and animals sweat and breathe as well as humidity from cooking. But since it is usually warmer inside then outside the relative humidity is lower. By ventilating you are letting the outside air inn and heating it up making its relative humidity drop.
I know I am going to get some comments from people in warmer climates saying that this is not true. But since you mention BBC I assume you do not own an AC system or at least rarely puts it into cooling mode. When you put an AC in cooling mode it will cool the air through it very low so its relative humidity gets to 100% and condensation forms. When this air is let into the room and heated up as it mixes with the rest of the air the humidity will have dropped due to all the condensed water. So even though it is colder inside then outside and even though you are sweating and breathing and cooking inside the humidity might still get lower inside then outside. You can also buy dehumidifiers which lowers the humidity in similar ways without having to ventilate.
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