Damp plaster board (dot and dab)

[Overthefence]

Hi all,

 

I’ve been getting damp coming through the dots/dabs behind the plasterboard in a section that was previously an outer wall. 
 

As in the photos, we previously had a 1st floor walkway between our house and garage and we had builders create an indoor space at ground level below, involving putting in a small pitched roof section (with skylight) under which is an exterior door. 
 

However, since the work was done, the plaster is damp to the touch which seems to be transferring from the old exterior wall behind. You can see the damp and mouldy spots in the pictures.

 

The damp only seems to be on one side (not on the old exterior garage wall) . I asked another builder what they thought and they said I should look into whether it is rising damp or coming from above (eg a leak in the flashing). But I’m not really sure how to do this (eg if I cut the plaster away how will I diagnose the issue) or what the solution would be. 
 

Any advice would be gratefully received. I imagine dot and dab was used  instead of batons because there is only just enough width for the external door. 
 

In case it helps, we had a concrete screed poured on the ground with a waterproof membrane beneath.

 

Thanks

 

 

[Mr Punter]

The damp is coming from the stone wall.  The stone is porous and getting wet from rain being blown against it.  You could coat the wall with StormDry, take off the plasterboard, wait a year or 2, then replaster.  If the wall is stone under the plasterboard you could leave it natural stone.

[Overthefence]

Thank you. So you don’t think it’d be rising damp then? 

[Roger440]

19 hours ago, Overthefence said:

Thank you. So you don’t think it’d be rising damp then? 

Could be if the wall doesn't have a dpc. How old is the house?

 

[Conor]

Absolutely is not "rising damp". Moisture migration from the stone wall, or (less likely) condensation. The inside surface of the wall will need to be ventilated. Doesnt look like you've any width to insulate, so I'd leave the wall bare.

Edited May 12 by Conor

[Mike]

9 hours ago, Conor said:

Absolutely is not "rising damp". Moisture migration from the stone wall, or (less likely) condensation. The inside surface of the wall will need to be ventilated. Doesnt look like you've any width to insulate, so I'd leave the wall bare

+1

[Nickfromwales]

There's likely a mix of cold bridging through the dabs, as well as the wall being 'not dry' and the dabs are then wicking that moisture through the PB. A perfect example of where an insulated plasterboard and meticulous use of the dabs plus closed cell foam around the perimeter would have been the way forward.

Defo not 'rising' damp etc.

 

From what I can see there is plenty of room to use 25mm insulated PB on that wall.

[Spinny]

Seems odd if it is still damp now when we have had almost no rain for weeks and weeks. I would take that plasterboard off and that will confirm the damp patches correspond to the dabs. Also give you the chance to see how damp the wall itself is and to try to confirm the damp in the wall is coming from above. I see there is a lead flashing cut into the wall above externally, so once you have the board off you could also put the hose pipe onto that and check it is watertight and to help see if damp is wicking down the wall structure itself. I guess in extremis it might need some kind of damp proof course/membrane in the wall itself to stop water wicking down it, but imagine storm dry would help because the stone looks like it might be a bit porous.

 

Whenever you have the external leaf of a wall running from outside to inside like that then you also have a thermal bridge. When it is cold outside that piece of wall will also get cold on the inside, so if you don't insulate it on the inside, it becomes a prime position for condensation where warm air in the house hits the cold interior stone. Depends a bit how much you heat that part of the house in winter I guess. But if/when you finally re-cover the wall it should be insulated.

 

I would think you would want to know conclusively that you have fixed the damp problem before covering over the wall again, so leaving it uncovered over next winter and checking before reboarding might be a good idea. I wouldn't like to think of long term damp going on behind the wall finish for years without you knowing about it. In that sense the dot & dab has done you a favour and shown the problem hopefully before any mold, rot or other stuff starts growing.

Edited 10 hours ago by Spinny