I've recently purchased an old house (circa 1725), which has a brick constructed outbuilding of about 3x5 meters. The previous owners claim they were using it as an office. There was no evidence of damp, but looking at the state of it when we arrived, I'd be surprised if they'd used it as anything but storage.
The construction seemed solid, so I redecorated and carpeted (removing the previous linoleum). When doing so, I noticed the door didn't quite fit the jam properly - it's an old door for an old building, so air tightness wasn't a priority. However, there were actual sections at the base where you could poke your finger to the outside world. I've sealed it as best I can, and will have it professionally corrected soon. It's probably been that way for about 25 years.
Since finishing the project 5 days ago and beginning to use the room, I've noticed signs of dampness. It's mostly around the door, where the paint has become seriously discoloured already. On heating the room all day, the lower half of the door wall (nearest the worst door hole) was really quite wet. Moreover, the area of paint above 75% skirting board has become similarly discoloured (worse in the corners of the room). There's no evidence of damage higher up the walls or near the roofing (aside from a small area around the upper area of the door wall, near another gap).
Before I panic and think there's some more sinister structural issue, I'd like to ask how likely it is that the gap(s) in the door would have caused this effect? That is, once the door is repaired, and the room dried out, could my problem be fixed, or is the fact that the damp has spread quite a long way from the door a bad sign?