I have a fibreglass dome and walls containing my 315mm reflector on a Paramount. I’ve got a built in damp proof membrane in the floor and the building is completely watertight. However my dome is not draught proof enough to dehumidify the building, I would be dehumidifying the whole world, so I cover the telescope and mount with a cotton sheet and use two 60 watt tubular heaters to warm the air space and raise the dew point. That works most of the time but occasionally when the ambient temperature warms rapidly I do get condensation on the Paramount and sometimes the telescope mirror, so I should really try to cure that. I leave my PC out in the observatory and have only had one failure in about 10 years.
British Astronomical Association
Supporting amateur astronomers since 1890