Thermal survery shows damp in wall

[xdave]

I've been carrying out a thermal survery of my home, and identified an area of damp in an external wall. On closer inspection (the old loo roll against the brickwork test) it is indeed damp.

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• xdave
• 7 Apr 2010
• 2

There are actually two walls there, the one to the left is original 1980's and the one to the right is from an extension completed in 2008. The original wall has an open 50mm cavity, but the extension has 50mm of PIR insulation against the inner block skin and then a cavity that I think is about 50mm again. The PIR boards were foil laminated, but were not taped (there is a vapour barrier immediately behind the inner palsterboard, which is attached to battens). The brick work from each 'building' is tied together with wall starters, which start 1/2 course above the external DPC. (DPC is shown as black in the photo). There is a vertical DPC run the height of the building between the wall starter track and the existing wall, which is then bolted through. There is no plumbing or guttering near to this area.

The damp is clearly only affecting the new wall, but I am unsure why. The wall is also south-east facing, and gets good sun light, and there is never standing water on the bottom of the wall. Given the shape of the damp patch it appears to me that it is rising up? Or could it be rain penetrating the brick work?

I would be gratefull if anyone with more experience of this sort of thing could please share any thoughts. Thanks.

 

[JohnD]

what room is it? what is it used for, and how is it ventilated?

have you looked at all the other rooms, and do they have any cold patches?

I can't see the DPC you mention, in the photo

Is there any plumbing in the room above?

Is your photo taken from the outside or the inside?

 

[xdave]

The room is used as a sitting room (both buildings are combined/open internally). It is single storey, and there are no radiators or water pipes anywhere along that wall or in the roof space. As internally the room spans the joint in the brickwork it doesn't sit on the corner of a room.

The room is not used to dry washing (although as mentioned there is a vapour barrier behind the plasterboard internally). There are four windows (1x1780 and 3x1200) and one 1200 french doors, all have trickle vents. Being south-facing there is normally a small window opened in the room during the day if occupied because it does catch the sun and is largely sheltered from wind. There is background underfloor heating (electric not wet) but it is only active for about an hour a day in the early morning. Humidity in the room is approx 45-50% throughout the day.

No other rooms have cold patches visible on the outside walls. There is no corresponding cold patch on the internal face of that wall either, so I don't think it has crossed the cavity.

The photo is from the outside, and the whilst DPC itself is not visible in the photo, you can see the brick turns black towards the bottom two rows - those are the bricks below the DPC, and are black because they are more wet and cooler than those above the DPC.

 

[JohnD]

If the photo is of the outside of the wall, and if the wall has a cavity, it doesn't matter much if it is cold. The cavity prevents damp bridging into the inside of the house.

From your pic, it doesn't look to me like the damp is coming from a faulty DPC (this usually shows as a half-round patch centred on the defect and going upwards). There is a chance that some of the bricks are of different density or absorbency. Have you got a dog or a small boy?

I would have been thinking about a dripping gutter above. Have you been outside to have a look in heavy rain?