Do you have blinds or shades that are often closed on these windows? If so, keep them cracked open a bit to allow air flow behind them. Also, do you have an adjacent master bath that you typically use for showering? Aprroximately a gallon of water from a 5 gallon shower ends up in the air as water vapor through steam and hanging wet towels. This may contribute to increased levels of humidity in that area... run the fan for 15 minutes or so afterwards. Oh, and close the toilet seat too, any sources of standing water will also add to the humidity, especially when they are a few feet away from a heating register. Not such a pleasant thought of breathing toilet water vapor is it. <imagining all the readers running to put the lids down now>
No adjacent bath but the Next Day Blinds are typically drawn down to the sill, but that is the case in all the other rooms-I will monitor this much closer and report as things develope.