Hi Jamuk,
I aren't an expert on guineas by any means but since moving here I feel I am with condensation. We suffered with alot here so I googled loads on it.
We have condensation in our room where the guineas are especially if it's rained, I think they maybe even being small addto the problem I think they have in our room (they are in our bedroom).
Do you have curtains closed on the windows or nets as it stops the air circulating, we got rid and put vinyl and blinds up so air can circulate it really helped. It doesn't necessarily mean the whole room is damp there could be a few factors why your windows are steamed up.
Do you have any washing drying around the house especially at the moment with the rain?
It isn't always as option but I have found soon as the temp goes before a certain temp you get it. If its a small room could you maybe have a small heater to come on? I no it isn't ideal though, on Saturday morning I looked out and thought god not that again, my guineas have been fine though and their hay.
a dehumidifier is a good idea though.
S x