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 is the indoor humidity? Allowing airflow from an area of greater humidity to an area of lower humidity will decrease the humidity in the more humid area. An outside humidity of 71% is fine to help alleviate an indoor humidity of 85%.
The amount of water that can exist as a gas in the air depends on the temperature. The warmer the air the more water it can contain.
The number you use is relative humidity so a percentage of max at that temperature. If you change the temperature of air without adding or removing water the relative humidity will change. So if the 71% humidity is for cooler air then you have inside then the relative humidity in it will drop when you let it in and heat it up.
You can ensure absolute humidity too that is in units like grams per cubic meter. It is independent of temperature.
If we look at https://en.wikipedia.org/wiki/Humidity#Relationship_between_absolute-,_relative-humidity,_and_temperature and look up the absolute humidity. Assume 20C indoot and 15C outdoor and round the humidity to multiple of 10.
20C indoor at 70% relative humidity you have an absolute humidity of 12.1g/m^3
15C outdoor at 70% relative humidity the absolute humidity is 9g/m^3
If we now look at 20C air 50% relative humidity is 8.7g/m^3 and 60% is 10.4g/m^3 so if you let in the 15C outdoor air and heat it up you get somewhere around 55% relative humidity.
We can also look at 10C at 70% relative humidity and we get only 6.6g/m^3 which will be in the 30-40% range if you heat it up to 20C
So you can only compare relative humidity directly if the temperature is the same. If you let in air with another temperature and heat or cool it you need to go via absolute humidity and to get the relative humidity at the new temperature.
The table on Wikipedia have quite large steps so you can use https://www.omnicalculator.com/physics/absolute-humidity and input any values you like You can enter relative or absolute humidity and then change the temperature to so see what the result is
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.
What’s the temperature outside and what’s the temperature inside?
Relative humidity (expressed as a %) will change based on the temperature of the air. If it’s cool outside and warm inside then your 69% humidity inside probably has more water in it than the 71% outside.
Edit: Swapped hot and cool because I had them backwards.
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