Hi Alan, We have used these for about 3 months now, they seem to work, they use the moisture absorbing cristals that the older type moisture traps use the advantage being that they are a sealed unit so no chance of any spillage, we have noticed since using them that there seems to be less condensation when we use the van now. I'm sure someone will be along soon to tell you what a waste of time it is to use a dehumidifier when there are so many vents in a caravan where moist air can enter. I would rather have the moisture soaked up by these than the furnishings .