We discovered and renovated a medieval vaulted cellar under our house and have secured it from collapse, opened up and covered the light well with a glass panel in the garden, and have put down a limecrete screed over foamed glass and have two sumps with pumps. The floor is very much at the water table and any rain sets off the pumps quite regularly, and we get water ingress in the lightwell, and through the back wall. We have been running a dehumidifier 24/7 and it gets the humidity down to about 75%-80% - but runs constantly to do so. It's using a lot of electricity. As the cellar has been there (and must have regularly flooded) for 550yrs at around 100% humidity, I'm thinking that we should just let it be damp, and use the dehumidifier occasionally if we intend to spend any time down there - it functions as our occasional family dining space.

Can anyone see any problems with this approach?

It has some forced ventilation already to try to minimise radon.

I think we'll switch off and monitor.

Posted By: tonylimewash walls, ventilation is needed , i like your approach

Limewashing the walls is a good traditional way to reduce mould, but note that traditionally the walls are washed down every year and a new coat of limewash applied. Was certainly more maintenance than I wanted!