littlefeller wrote:
woder if its possible to get some kind of moisture counter to switch on the dehumidifier, infact if you really put some thought into it, theres loads you could do with the bag idea

The thing you're talking about is called a hygrometer. Used all the time in vivariums to monitor humidity levels needed for reptiles

There are fancy hygrometers with a switch system in place that will ADD moisture via an appropriate vessel when certain parameters are breached, so as not to create too dry an atmosphere for say, an exotic rainforest species. I'm sure there would be a way to reverse this and link it to a dehumidifier, unless of course there's a dehumidifier with this tech already.