The Victorian walls of our holiday cottage a solid and have no air gap, there is no damp course in the brickwork. Previous owners have used chemical injection in the brickwork and also ceramic vent tubes, but this has not been effective in controlling the damp rising on the interior plaster. Last winter by way of experiment we installed an automatic dehumidify machine and it was very effective at reducing the damp. We were keeping the house at 43% Relative Humidity. This makes an enormous difference to the house for relatively low power cost. The low humidity helps to prevent black mould on walls and insect infestations. As we allow guest pets we have to be careful about mites and fleas, this had been a problem in the past requiring fumigation and temporary cottage closure.
We also installed a remote environmental sensor which allows us to monitor temperature/humidity via the internet. We are also alerted by SMS when limits to temperature/humidity are exceeded.
When guests are present in our cottage, we remove the dehumidifier relying on them to open windows as necessary for comfort. It is also one less machine for them to tinker with and break. We soon learned to keep everything as simple as possible for guests, or things get broken. We leave the unobtrusive environmental monitoring running and check the reports occasionally.
We noticed with the last set of guests, during June, that the Relative Humidity had risen from 45% to about 65%. We're not sure why, but it is probably a good indicator that they kept doors and windows firmly shut during their stay. This figure is unusually high for a guest stay. There's a booking gap of one week before the next guests, so I reinstalled the dehumidifier during the gap to reduce humidity. At the moment we are extracting over 5 litres of water per day from the air in the sealed and unoccupied cottage. I'd love to know how they so saturated of holiday home, during the summer rising damp is not normally an issue.