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The case of 'Pattern Staining'

Several detached houses built around 11 years ago on a single estate were showing various intensities of discolouration on the decorative finishes marking the outline of lightweight blockwork used for the construction of the inner leaf of the cavity walls. The discolouration was thought to be derived from the mortar which was alleged to be either defective or damp.


One house, that showing the worst discolouration, was investigated thoroughly. All external facing walls were affected on both upper and ground floors. However partition walls were only discoloured for a few feet in from where they abutted external walls.

All walls were shown to be well above the dew point temperature, ie, the temperature at which condensation begins; there was also no visible evidence to suggest that Pattern staining condensation was a problem within the properties.

patstainThe cavity was examined and shown not to be insulated; it was also free from debris.

No moisture meter readings were obtained whatsoever which indicated all materials were dry and not hygrosopic salt contaminated. A sample of plasterwork was removed and extracted in water; there was no soluble coloured extract and the absence of the above salts was confirmed. Furthermore, the wallpaper was shown to be free from 'staining'.

The discolouration followed the line of the mortar beds around the lightweight thermal blockwork of the inner leaf. The discolouration could be readily wiped away and was shown to be the result of extremely fine dust settling on the areas of the mortar beds.

Surface temperature readings showed a marked differential of 0.3º - 0.5ºC between the discoloured lines (areas over the mortar beds) and the adjacent lightweight block; these differentials were very marked between the thin mortar beds and the immediately adjacent blocks.

The discolouration following the lines of the mortar beds around the internal leaf lightweight thermal blocks was solely due to very fine dust collecting on these colder mortar beds. This is a phenomenon known as 'Pattern staining' where very fine dust particles collect on the markedly cooler surfaces, i.e., the mortar acted as a 'thermal bridge' between the cold cavity and the room. (It can sometimes be seen on ceilings where the discolouration clearly shows up the lines of joists)

In these properties the discolouration was most noticeable in those houses which were kept warmer (greater thermal difference between cavity and room temperatures) and were also fully occupied (greater dust production). In those properties that were noticeably cooler and only partly occupied the pattern staining was either not present or at worst only just visible; indeed, there no no readily discernible thermal differences between mortar and blockwork.

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