Hello,
vacancy is always a problem. And once the walls are damp, it takes time for them to release the moisture again. That is completely normal. To believe that ventilating twice a day is enough is unfortunately completely wrong. This misconception has persisted for years but is completely inadequate in houses built nowadays (after 1990). The tight windows alone prevent air exchange. In the past, there were drafts everywhere (my oldest house dates from 1692; you should be inside during the wind, all the candles wobble). When there were drafts, more heating was used, which in turn led to dry air. That is no longer the case today, and then there is the "stinginess is cool" mentality. We are literally saving ourselves sick.
In principle, two permanently tilted windows facing each other would be ideal for air exchange. The problem is that the areas around the windows cool down due to the continuously inflowing air, and this is the real reason why tilt ventilation is bad. Because moisture condenses on the 'cool wall' and mold occurs. That is why mold prefers to form on window sills and outer corners.
If the apartment is vacant for a longer period, the walls cool down. This in turn leads to condensation of the air moisture in the wall and permanently to the moisture saturation of the walls. If the tenant has not ventilated properly beforehand, the problem is perfect. To believe that you can get the problem under control in two weeks with ventilating twice a day is completely naïve.
You must not forget that you are constantly introducing new moisture with your life/cooking/showering. But to remove 'old' moisture and keep the apartment dry, you must remove more than you bring in. And ventilating twice is already not enough for normal living.
You can see that yourselves because if I read that during cooking the humidity rises to 70%, that shows that you have too little air exchange.
So once again, make the apartment warmer. Every degree more allows the air to hold more moisture. And ventilate more often.
I once did this with a resistant tenant. He had managed to have the entire west wall moldy. The apartment had been rented without problems for 10 years before him. When I arrived there, I almost fell over. Water on the windows and an air you could cut with a knife. WITHOUT wiping, I managed to get the windows dry in about 5 hours. Heatings all ON (25 degrees in the living room) and every 20 minutes 2 minutes of shock ventilation. You could see the first successes after just one hour. Wallpaper was completely removed and mold treated. But it took quite a while before the wall could be papered again. However, and that was the most important thing for me, the tenant understood. There were no more problems after the action.
And for measuring moisture. I myself use a Trotec T660, which is quite good. There is also a smaller one that costs about 60€ (BM31). For a rough analysis, that should be sufficient. If you then find out that the walls are damp (and I strongly assume that), it will take a long time until the 'stored' water is released again into the room air. And only if there is a moisture gradient toward the room air. If you have 40% for a few minutes, the water migrates toward the room air. At 60%, probably nothing happens at first. So the longer (or more often) you have 40% humidity, the more you get out. Half an hour in the morning and half an hour in the evening is by far not enough.
You can calculate yourself how much water comes out of the wall....
So measure, ventilate, measure, ventilate... The more and the more often, the better. And make the place properly warm. The warmer, the better.
And one more thing. The energy loss due to ventilation is the lower the less moisture you have!!
And to put some numbers in play: 0 degree cold air can hold 5g/m³ water, 20 degrees warm air 17g/m³, and 30 degrees warm air 30g/m³. A room with 20sqm has a volume of 50m³. At 20 degrees, there can be 850g of water in it. If you replace this air (ventilate) with 0 degree cold air, then you only have 250g of water in the air. You have therefore removed 600g outside. This amount of water can, if the air now warms to 20 degrees, be absorbed again.
If you now also know that a human emits about 900g of water per day into the environment through breathing/sweating, you can calculate how well ventilating twice helps. Cooking, showering, plants, pets etc. you can gladly add on top.
That is also one of the reasons why I nowadays only build with forced ventilation for my objects. With today's tight construction, you no longer ventilate adequately, no matter what people claim!!!