Windows should be fully sealed on the exterior and they should have an in-built face or bottom drain that allows any water that gets into the frame to escape. If you don't have little weep holes on the front covered by a rectangular cap then yours are probably bottom drained which will allow water to drip out onto the plastic cill. The problem of the damp may be that if you don't have a plastic cill and your windows are sat on brick or stone cills and they are bottom drained then the water is not escaping and runing off but drips onto the cill and because of the silicone sealant it cannot get out and so travels back to your inner walls. The dripping may be connected to this reason.
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