Mould on skirting boards but only in 1 room

[willraymond]

I have a new build property with a particularly puzzling problem only in 1 room. I have mould along the base of the skirting boards of 3 of the 4 walls, all similar to the picture attached.

 

It’s been a bit of a problematic room, there was small patches of mould on the plaster before mist coating even after the plaster was 4/5 weeks old, I had plenty of ventilation during this drying time. This mould then showed through the mist coat after a while. I probably left the mist coat 2/3 weeks before top coating with vinyl Matt, no mould has shown since on the walls but now this is showing up on the skirting. I have wiped off once already and the (small) dehumidifier has been running for a few weeks, being emptied whenever full.

 

I have another room and a hallway which the skirtings are still to be decorated and they do not exhibit the same issues. 
 

Could it be water in the cavity coming through? We weren’t roofed and watertight til mid January and it was very wet pre-roof, the cavities weren’t covered during the build (my fault for not checking up on the contractor) or could it be something else entirely?

[ProDave]

Which room, upstairs or down?

Who built the house, you or mass market developer?

 

[JohnMo]

Assuming at least one wall is outside, check the ground level outside - is it at least 150mm below the inside floor level? If not it could be outside ground moisture being pulled in to the house.

[willraymond]

23 hours ago, ProDave said:

Which room, upstairs or down?

Who built the house, you or mass market developer?

 

In the attached images the room concerned is the “bedroom”, the wall with the worst mould is on the wall between the bedroom and the shop. Cavity wall with full fill 100mm dritherm 32 made up of thermalite blocks.

 

It is a self build managed by myself

[willraymond]

23 hours ago, JohnMo said:

Assuming at least one wall is outside, check the ground level outside - is it at least 150mm below the inside floor level? If not it could be outside ground moisture being pulled in to the house.

Ground level is actually well below finished level as the landscaping has come to a halt for the moment. I have got a damp meter but haven’t probed anywhere yet. Will give that a go

[Big Jimbo]

As the two front rooms are offset, I wonder if the water could be getting in at the top. Was a cavity tray fitted ? Either that or could it be something to do with the wet areas on the other side of the wall ? 

[JohnMo]

I would check for a very small leak in the shop WC or under the sink in the lobby first.

[ProDave]

It has to be some form of water ingress either in the wall cavity or from the plumbing in the shop.  The mouldy wall is not exactly an outside wall so I would not expect that wall to be cold.

[Temp]

7 hours ago, JohnMo said:

I would check for a very small leak in the shop WC or under the sink in the lobby first.

 

+1 Lift manhole covers and check water from each waste (sink, basin and WC) reaches a manhole. If that's OK perhaps pressure test them.

 

Where does rainwater go? Soakaway? Surface drain? 

 

Perhaps post photos of the roof in that area showing where roof meets wall (x2) and ideally where the two fascia cross each other.