My Hawaii house is a vacation rental and there is a bathroom with no window.  We have a bath fan timer which is selectable from 10-60 minutes.  But after many years of being fine the bathroom is starting to collect moisture after several people have showered.  Since we are a humid climate and the house is completely open, we have a Panasonic Whisperquiet running in the neighboring hall to pull air through the bedrooms which can get stagnant because there’s only windows on one side. 

For this bath I’d like to have another continuous running fan, but with an automatic humidity boost.  I swore this was common but am having trouble finding one.  The Panasonic I found it’s either/or.  All the time or timer or humidity activated.  My concern with being purely humidity activated is that the sensor may fail when we’re not here.  I found an AeroPure, but that was about it.  Any suggestions?

BTW there are no typical issues with humid climates such as needing dehumidification to prevent condensation from air conditioning.  The house is completely un-insulated, with no HVAC system, and the windows are never closed.  I’m simply trying to move moisture out of the bathroom more quickly & consistently.

Thanks!