In our upstairs bathroom, the toilet and cold faucet in the sink stop working every winter when it is cold.
The shower works and the hot water at the sink faucet work.
We have tried heating up the room to 85. Looking for the source line and trying to heat it but can't. Our master bath is on the other side of the shower and we do not have any problems. Our daughter's closet is on the back side of the toilet and sink and her room is usually warm.
We tried turning on the hot water and holding it with rag and forcing water back up the faucet for an airlock and it didn't help either.
There is only one wall to the outside and it has a window.
We don't think there are outside lines but we think the cold line goes up into the attic. We hired someone to look and they said it was insulated where they could see but it might not be when it goes into the wall.