Any advice appreciated.
The mould is still there but the dry air stops it from growing and sporing which is what makes you stick. If humidity creeps up the mould will grow again. Be aware that mould is virtually everywhere and is only harmful if there is enough of it around sporing or if you touch it.
The problem is, once the calcium chloride has absorbed the moisture from the air and is saturated, you have to take it out and dry it to make it effective again. That might be cheaper (in terms of energy) than refrigerant phase-change dehumidification, but it's a lot more labor intensive.
Natural airflow with the outside at 90% relative humidity isn’t going to do a thing other than make the interior occupants ornery.
The natural airflow design of Apple Park for example, that works incredibly well, but that same design in Houston or New Orleans would be catastrophically ridiculous.
https://www.cdc.gov/mold/stachy.htm
I'm not really sure about keeping the humidity below 50% though. The UK and most of Northern Europe has rather high humidity in the winter. It commonly reaches 80-90 RH outside, so you'd have to heat your house to 25c/77F to bring the humidity below 50%.
Indoors? No, not at all. In the winter the temperature in my house/office is 21-22C and 17 at the lowest at night, and I have to run a humidifier 24/7 to keep the humidity over 30% (otherwise the wooding floors shrink too much).
Currently in the bedroom (thanks Netatmo) it's 17.8° and 69% humidity (with one of the three windows open about 0.5"). That compares to 14.0° and 78% on the balcony. Looking at the graphs, 50% is the lowest indoor humidity over the last month.
Still - mold in houses is not a problem in houses in Western/Northern Europe, except for the most pathological cases (for example, mold scores you iirc 2 points in Belgium on the 'uninhabitable property' test, where 9 gets your building declared unfit for living in. Having small holes in the roof or small cracks in the walls (still structural defects) gets you 3. That's how unusual mold is around here. OTOH when I lived in New Zealand, mold was met with 'eh, there's bleach in isle 5 at Countdown, just wipe it down.'. I was like 'wut?'.
Alas, it does not, but that's possibly because other windows are open around the place and there's relatively good airflow.
> I'm going to try this same experiment tomorrow, I'm quite interested to know.
I'll see how it goes tomorrow with the window closed and various doors closed to minimise the effect of the other windows.
[0] https://en.wikipedia.org/wiki/File:Relative_Humidity.png
When it gets to the cold part of winter it's obviously easier, but this time of year it's hard to get it below 50% RH inside just by controlling the temperature.
Cold air can simply hold less water overall, at 20C it can easily hold double as much water as it can at 0C.
There being a pressure difference is nonsense, you can get the same air pressure in the winter as in summer (though it tends to go on the Low side more often). What you probably mean is the vapor pressure, ie the natural pressure the liquid wants to be at in a closed container, though while it is largely depdenent on temperature, it does so in a linear/positive way, so at lower temperature, water will evaporate less (otherwise ice would more easily sublimate).
Once you go below zero a water content of a few grams can easily hit 100% RH which would barely manage 5% or less at 30C.
If you can blow dry air over its surface, you can evaporate liquid water (or sublimate water ice) even when the relative humidity is 100%.
You can think of air as a gaseous solution, and its water vapor content like table salt in aqueous solution. Temperature affects the solubility, and if the solution is saturated, no more can dissolve. You can still dissolve salt crystals sitting in a saturated cold saltwater solution by squirting warm freshwater onto the bottom of the container. If that then mixes with the rest of the solution, and everything cools down again, that dissolved salt can then precipitate out somewhere else (like rain), or form a suspension of tiny crystals (like fog or clouds).
The solubility of H2O vapor in atmospheric gas increases with temperature. But you can also do something with a gaseous solvent that you can't easily do with a liquid solvent, which is to change the pressure. Higher pressure lowers the solubility of water in air, but to a far lesser extent than a decrease in temperature. Even though the vapor pressure is dependent only on temperature, evaporation occurs whenever the vapor pressure exceeds the partial pressure of H2O at the interface. Increasing the overall air pressure also increases the partial pressure of H2O vapor by a proportional amount, so inhibits further evaporation. But water vapor is also less dense than N2, O2, and most other atmospheric gases, so the means of measuring pressures gets complicated.
When you involve wind, and stratified airflows, it gets even more complicated. The air blowing across water or water ice could have been previously warmed by the ground just enough to evaporate more water, then get pushed higher by an angled snowbank into colder, saturated air, and it will then dump the excess water as small ice crystals. A solid block of ice can then become "rotten" as a snowdrift forms downwind of it. The overall average temperature may say that ice should stay frozen, but the wind can still carry a tiny bit of water vapor at a time, thanks to local fluctuations, and with enough volume of air to move it, that ice will drift.