Mystery Damp

[portishead]

hello,
i came home yesterday to a faint streak of damp running along the ceiling of a room and down a wall.
this morning i took the floorboards up and the other side of the plaster is bone dry (the stain is running down the joint in the boards on the ceiling). There are no water pipes anywhere near the stain. i hacked the wet plaster away at the top of the wall (where it was wettest) and there is a blob of what i assume is what is used to hold the plasterboard to the wall, the other side of the wall is outside.
the house is only 6 years old

Can anyone shed any light on what may be causing the damp?

 

[stevethespreader]

Dots of plasterboard Addy on an outside wall can become cold spots which also attracts condensation, its not ideal to d&d outside walls

 

[portishead]

Thanks for the reply Steve, I suspected that might be the case. I'll try to get NHBC on it