Damp and black mould behind 50 mm insulated plasterboard around window. Condensation or water ingress?

Joined
7 Jul 2026
Messages
1
Reaction score
0
Country
Ireland
Hi everyone,


I’m looking for some advice from people with experience in damp diagnosis.


I removed some 50 mm insulated plasterboard from around one of my windows in the bedroom, after noticing damp around the window. The board was mechanically fixed directly to the wall with mushroom fixings.


When I removed it I found:


The concrete block wall behind was damp.
There was black mould on both the back of the insulated board and the block wall.
The left-hand side of the window is noticeably wetter than the right.
The right side is not as damp, but still has damp.
I haven’t yet removed the insulation above the window, so I don’t know the condition there, I only removed small section, but it looks dry.
There was no standing water or dripping, just damp masonry and mould.

Some additional details that may be important:


The foil facing on the insulated board was intact overall, but I cut it in a few places to run electrical cables.
There is one socket cut-out in the insulated board.
Around the outside of the window there was a gap between the frame and the render which I never sealed with silicone or another external sealant.
The house has hollow concrete block walls (Ireland).

I’m trying to determine whether this is most likely:

Condensation caused by warm air getting behind the insulation board.
Rainwater entering through the unsealed window perimeter.
A combination of both?

Before I reinstall the wall, I’d really like to identify the actual cause rather than just covering it up again.


I’ve attached photos of:
The exposed wall.
The removed insulation board.

Any opinions or suggestions on what to inspect next would be greatly appreciated.
 

Attachments

  • IMG_1022.jpeg
    IMG_1022.jpeg
    870 KB · Views: 2
  • IMG_1017.jpeg
    IMG_1017.jpeg
    1 MB · Views: 2
  • IMG_1018.jpeg
    IMG_1018.jpeg
    1.3 MB · Views: 2
  • IMG_1020.jpeg
    IMG_1020.jpeg
    1.1 MB · Views: 2
  • IMG_1021.jpeg
    IMG_1021.jpeg
    1.1 MB · Views: 2

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Back
Top