I haven't dried clothes in the house for a few days now.
That alone, will make a vast difference, to the cause of the mould. By all means, keep an eye on your humidity levels, but those levels, are only part of the story which can cause mould to spread. Any surplus moisture, in the air, will want to condense out. If your walls are colder than the air, it will choose the walls to condense out on, but a humidity meter, cannot/will not show that extra moisture, because it is no longer in the air.
We never open windows in the cold weather, but my rules and the measures I have taken, keep the condensing moisture levels low. We have two fans, which both extract outdoors. One in the bathroom, which is PIR and humidity triggered, and once triggered, it stays on for at least 20 minutes. Then the second in the cooker hood. This latter is always turned on, when ever anyone is doing any cooking, and extracts through the wall. A recirculating hood, is completely pointless. We have cavity wall insulation, and decent double glazing, which helps keep the walls warm, prevents moisture condensing out.
Indoor drying, is completely banned, apart from in the separate utility room, which is fitted with a fan, to stir the air, an effective dehumidifier, and washing lines, for over-night drying - when the room would otherwise be unneeded. Other than that, we have a large garden, with a very long line, plus more outdoor lines, on the sheltered verandah, of the summerhouse.
Below, is my own recent record of humidity. The blue line is outdoors, the green the indoor measurement. The hump in the green, around 19:00 is when I took a bath, the bathroom fan, then rapidly, and very effectively, extracts the moist air. The indoor humidity sensor, is adjacent to the bathroom, on the landing.
