Damp at extension

[Stevecc]

Hello

Not sure what this is. Looks like damp. It is occuring at the juction between an extension and the original house. It occurs on both sides of the room where the extension meets the original house. It is only where it meets, not in the extension or the original house.

Is this damp. What causes it, and how to resolve it

Thanks

 

[^woody^]

Lack vertical DPC where the previous outside wall is now an inside wall.

Blocked cavity.

 

[Stevecc]

Thanks for your reply

What do you mean by "Blocked cavity"

What can be done to resolve it