I cannot believe that to be correct
Humidity is relative. You take say 1000 litres of air from outside at 10 degrees C, and if it had 9.4 grams of water evaporated in it, it would be at 100% humidity (saturated, can't hold any more)
Warm that up to 20 degrees C and its humidity would be about 55%
Same volume of air, same volume of water evaporated in it, different temperature, rH drops
It's why condensation even forms; your house air at 20 can hold X ml of moisture per unit volume, but put the air next to a cold window, cool it to 10 degrees, the air can't hold the moisture any more so it condenses on the window
I think what aveatry was getting at when they said "outside air is drier" was "outside air, when brought in and warmed up, can hold more moisture than it did when it was outside", and also "
potentially more than the air in the house"
I say potentially because you have to work the numbers
Air at 10 degrees is 100% (saturated) when it has 9.4g of water in it. At 20 degrees it's saturated at 17.2g
If you were to bring a cube of 100% air in, at 10 degrees, with its 9.4g of water (it's just about to get misty, the world air is saturated) and warm it to 20 it would end up with 9.4/17.2 = about 55%
If your house air was only 50% rH you would be bringing more moisture in if you opened those windows, than your house air already had
If your graphing software is tracking outside and inside air, it might also be useful to add a shadowing line of "what would the world air's rH be if brought in and warmed up to the temperature of the house"; if the shadow line is above the house line then there isn't any point opening the windows to reduce the house humidity (but it might have other benefits such as reducing CO2)
as the moisture is drawn out by the bathroom extract fan
There's a consequential element, that the fan is drawing world air in through gaps and cracks, and that air is then heated and is likely to be drier than the house air - it's not necessarily that all the moisture is drawn out by the fan, but also that the fan helps draw in drier air into which the molecules can evaporate and not have a condensing effect elsewhere.
All in you're doing a good job of arranging a stable indoor climate that can buffer and deal with humidity spikes caused by human activity; all in your uncontrolled background ventilation through gaps and cracks, supplemented by negative pressure systems when spiking occurs, well tolerates your personal activities
It's quite likely for a typical UK house, leaky/draughty as they are, that the air will have changed several times per day anyway; your irregular purge may not be doing as much as you think
Not much, should do. Will start opening them on a morning for a bit.
Note that it may amount to needing more than that, and you should really be putting some science behind it, with a few hygrometers around the home or even better, ones that track/graph
Having the windows (or their trickle vents) open slightly on opposite sides of the house, so there is a trickle of air through, having the heating on so the room temp doesn't dip too much (stay above 14)
Not closing curtains and ensuring there hence isn't a compartment of air in the room that is cooling massively, but can then sink and draw warm moist air in at the top of the curtain, cooling it and condensing its moisture like some kind of passive dehumidifier
Changing the heating (and potentially insulation) strategy of the house (do you heat for the morning, have showers, cook, set washing out etc in your nice warm house, then exit, heating off, house cools over the course of the day, the air in it can no longer cope with the amount of moisture laden in it as it cools..) so it remains above condensation territory in internal surfaces
There are a lot of things to consider about how you use your house; ultimately to keep mould at bay you need to understand how you load your house up with moisture, and how you deal with that moisture. Empty, ventilated, unheated houses typically don't go mouldy so you contrast your behaviours with that reference base line and have to keep a balance; if you add moisture, add heat, ventilate it away, balance the heat and the ventilation because essentially you have to throw away paid for heat (unless you mitigate that with technology like an MVHR) - most people are reluctant to do this, so they seal themselves in a box, warm and moisturise it then raise complaint when the inside of the box goes mouldy.
They probably wouldn't raise the same complaint about the kids' school lunch box, left in a warm bag in the lost property of the school for 3 weeks, biohazard as it would become .. but it's the same process..
