'Salt damp' - the problems
Electrical moisture meters will respond to very small quantities of free moisture and certain contaminant salts. In building materials the latter are usually hygroscopic, that is they have the ability to absorb moisture from the atmosphere. The more humid the atmosphere the greater the absorption of atmospheric moisture, and also the greater the concentration of salts the greater the absorption of moisture. Where such contamination exists alone (that is without the presence of free moisture) high electrical moisture meter readings will be obtained even though the surface may appear visually dry. However, where contamination is high, and should it get into decorative finishes there are occasions when a wall can look very damp solely as a result of moisture absorption from the air. Such dampness is regarded as 'salt damp'
However, whether it will visually appear damp will depend on the surrounding humidity and whether the salts actually get into the finish. In the case of a standard paper based wallpaper this can become contaminated quite easily salt damp 1being permeable. Vinyl wallpapers on the other hand do not generally show dampness due to salt contamination since the surfaces are relatively impermeable. Instead they have a tendency to 'bubble' due to the softening of the adhesive and expansion of the paper backing to the vinyl finish.
One way of a quick check to see if 'salt damp' is a possible problem is to peel a small area of wall paper and leave it to dry in the room close to walls, say for about 30 minutes. If the adhesive still feels sticky it is possible that we have a 'salt damp' problem, the hygroscopic nature of the salts contaminating the adhesive and therefore keeping it soft due to continued moisture absorption. In practice the only way to fully determine the possible problem of salt damp is for moisture and soluble salt analyses.
The problem with salt damp is that it is not always visible - it will depend on the humidity. Take the photo below. When the problem was first investigated it appeared considerably more damp than in the current photo. Analysis subsequently revealed that this dampness was purely due to hygrosopic salt contamination, ie, 'salt damp'. At the time of the original visit the internal relative humidity was 80% - it was a very humid late summer's day.
But on the second visit there was nothing to be seen! On this day the relative humidity was 56% and thesalt damp 2 contaminated paper was not absorbing sufficient atmospheric moisture to appear damp although it still recorded very high surface moisture meter readings. So how did we get at the photos? By boiling the kettle for 20 minutes to get the relative humidity sufficiently high to show the problem: at this stage the dampness quickly appeared.
Just one final point - note the new uncontaminated plaster work beneath the decoration up to around 800 mm (not salt contaminated and therefore the decorative finish remains dry). Also note the distribution of hygroscopic salts in lower photo as evidenced by the dampness: these were present up to around 1800 mm in an internal partition wall. How did they get there? The analyses showed that their pattern and distribution was as expected from a long term rising damp complex which had now been partially controlled.
But, of course, rising damp doesn't exist - at least according to some!!!!