On 24 Sep, 02:22, "Jim" wrote:
I have a house (victorian terrace) that has several wet patches on the
walls downstairs (kitchen and through dining room)

These patches come and go, but are always in the same place.

1) In the kitchen around the edge of the tiled splashback, and under the
work surface.
2) On the chimney breast
3) On the wall next to the chimney breast (not wet but can seen a change in
colour on the walls that comes and goes)

The patches appear 2 metres or higher on the walls. I have previously
removed the plaster and replastered the area on the chimney breast (2), and
fitted an airbrick to the breast.

The wall next to the chimney breast backs onto the shared passage, therefore
the wall should be quite dry, and unlikey to be penetrating damp. I also
understand that it cannot be rising damp, and there is no dampness at floor
level, but higher than 1m.

The wall in the kitchen is similar not an outside wall, and I can see no
reason for moisture penetration.

The house is fully double glazed, so the walls may be the cold spots. Could
this be caused by condensation? particulary as it directly adjoins the
kitchen. What would be the recommended methods to resolve this? Would a
dehumidifier be useful in these circumstances. Any other suggestions?

Answer ?
In a word -hygroscopicity

Hygroscopic salts are contaminating the plaster and it should be
removed and replaced with a non-gypsum preferably sand cement with
salt retardant additive and SBR and a water proofer.
If it is not TOO bad and you want to be a cheapskate you could try
painting it with a metallic OS paint but I dont recommend it

Cause on the chimney is SO2 from fossil fuels going to sulphur
trioxide to sulphous acid and then sulphates

elsewhere dont know but could be water entry over the years bringing
in salts

Could be defective sand in original mix!

IF condensation on Cold spots that would only be a contributory issue
as hygroscopic salts reduce themal insulation

Would not dehumidify
suggest getting rid of water in the first place as better
Chris