Measuring humidity deep underground

One of the problems that I have had with my house outside of Östersund is that the humidity in the basement has been way too high, risking fungi on the wooden beams that support parts of the rest of the house. A humidity above 70% is usually very bad. A humidity around 90% is usually very, very, very, bad. Last summer, the humidity was around 99%. This is so much beyond bad that words can not justly express it.

Some background: there is a basement under half of the house. The basement has stone walls, earth floor and some wooden beams in the ceiling supporting the floor of the kitchen above it. This is typically a rather bad setup: the moisture in the ground is evaporating straight into the air and the air circulation is rather bad, which adds to the problem.

In older times, this was not much of a problem because one of the walls of the basement is actually the foundation for the chimney. When you are using the stoves almost every day for cooking or heating, you heat up the chimney, which heats up the basement, which dries out any excessive moisture in the air.

When you don't use the stoves any more, favoring instead more modern electric stoves, and electric heating, the basement doesn't get heated in the same way, and moisture starts to appear. This is a common problem with many older buildings.

My first attempts at coming to terms with this involved better and more reasonable ventilation of the basement. This failed. My second attempt was a product called Trygghetsvakten, which is essentially just a box of electronics connected to some wires that you run around the foundation. The wires get heated up and should supposedly keep moisture away. This also failed. My third attempt (third time's the charm!) is with a product called Acetec PD-250, which seems to do a rather good job at keeping the moisture down. It works in a two step process. In the first step, it circulates air in the basement through a filter which absorbs the moisture. In the second step, it opens a ventilation hole to the outside, warms the filter and thereby lets it release the moisture to the outside.

In the graph here you can see the result. The PD-250 is set to keep the moisture down below 70%. The times when there is a gap in the graph is when the electricity has been turned off or the computer measuring it has been turned off. I have a problem that sometimes the phone-controlled on/off switch just switches off, so I routinely need to check once a week or so now just to make sure it's still turned on.

It's quite clear that when the electricity has been off, the humidity rises quite quickly but when the PD-250 is working, it brings the humidity down to just below 70%. At present, the humidity is 66%, which I consider to be rather good, and I hope that it will stay this way when the snow starts melting even more!