Odd "line" of damp on bedroom wall

techguy81

Recently purchased a 1930's house with a modern upper extension built 11 years ago. The extension contains a bedroom with three external walls.

This morning I noticed a thin line, approximately an inch thick, extending horizontally across the full length of one of the external walls in the bedroom. At first glance I thought it was a shadow, but upon closer inspection it looks damp. It's located about 50cm above the skirting board, and runs in a very neat, uniform line. I own a cheapo moisture metre, and the reading on the line is off the scale. Immediately above and below it's zero. As the day has worn on the line has faded, but there are little marks where it appears the moisture has wept a little.

Can anybody suggest what might be going on here? I've had damp problems in previous houses, but nothing like this. It strikes me as odd that it hasn't manifested itself when it's been raining heavily outside. But the temperature wasn't far off freezing last night, and we had the central heating on overnight, so possibly some kind of condensation issue?

Norman_Castle

To get condensation just on this line, this part of the wall needs to be colder than the surrounding wall. Any reason for this?. Could it be a leak which only opens in cold temperatures?.

ariba10

We had experience of the same thing in a neighbours house with a damp line showing through wallpaper.

When stripped there was a gloss painted line allong the wall. When sanded off the problem dissappeard.

techguy81

Norman_Castle wrote: »

To get condensation just on this line, this part of the wall needs to be colder than the surrounding wall. Any reason for this?. Could it be a leak which only opens in cold temperatures?.

That's what is puzzling me, I can see no reason why just this thin line of wall is colder than the rest. On the external side of the wall, the upper half is exposed brick. A sloping roof starts about half way down, covering a ground floor extension. The internal damp line looks to be covered by this roof externally, I would say it's roughly half way down from where the roof starts outside. Hope this description makes sense!

Actually, thinking out aloud here, I wonder if the line corresponds with where the insulation lies in the external roof? Could it be that the insulation is wet?

techguy81

ariba10 wrote: »

We had experience of the same thing in a neighbours house with a damp line showing through wallpaper.

When stripped there was a gloss painted line allong the wall. When sanded off the problem dissappeard.

Thanks for the suggestion, but this wall is painted plaster, no wallpaper. Nothing obvious hiding behind the paint.

new_owner

Is the moisture just in the plaster? can you knock some out to see if its coming through or moisture running down or an old pipe or something in the wall.