Old building, new paint! Learn how this can lead to dampness.

Here we look at how modern paints on old walls relate to damp.

The above image relates to an inspection i carried out on a marvellous 17th century build property in Surrey. The property suffered from numerous sources of excess moisture including high ground levels and defective rainwater systems. Like many externally decorated homes, the task of painting becomes a cyclical process. One may choose to re-pain the external wall every ten years or so. In this example, it appeared the task had been carried out a couple of times at least. A thick, external coat had been applied to all external walls. I believe the paint used was a modern, plastic/vinyl containing type. It was evident that the external paint had essentially formed a ‘tank’ around the property, or a plastic seal if you like. 

Upon removing a section of paint, saturated brick was discovered with signs of efflorescent salt on its surface. The presence of salt indicated moisture trying its best to release from the brickwork however the paint restricted any form of evaporation. We took a moisture profile within the adjacent inner wall surface & prepared samples for gravimetric analysis. Lab results confirmed a huge free moisture content at the base of the wall (14%) which descended in % all the way up to 2250mm. At the top of the wall, we found nearly 3% hygroscopic moisture which indicated salt presence. Samples confirmed moderate levels of nitrates and chlorides (ground salts) indicating that at its highest point, damp has risen from the ground to 2m+!

A diagnosis was met to resolve the source of moisture which i believe was a sub-ground slate damp proof course. What makes gravimetric results so interesting is that ground water has at some point in the past, achieved a rise of over 2m! I believe there is a direct correlation or influence between the rising damp, the height of the rise and the external paint. It should also be noted that the inner surface was skimmed with a gypsum plaster, again considered to be a modern material. I believe that the exterior paint links to the height in which the damp his risen from ground level. With such a high free moisture content at the base of the wall & no route for unrestricted evaporation, the only way way up. This would also indicate very small pore sizes within the solid masonry. There may be many other technical factors which can influence evaporation such as vapour pressure, diffusivity, temperature and relative humidity however a simplistic view on the issue here points directly at the external paint – it is restricting any form of evaporation.

Modern permeable paints are available on the market however a traditional approach should always be considered if you’re considering painting a solid wall. Paints which do not allow the passage of moisture are likely to fail by blistering as a result of evaporation & salt pushing the surface of the paint outwards.  Traditional methods include the use of limewash. This is derived from limestone or chalk, burnt in a kiln to form quick lime and then ‘slaked’ by adding water. It is used to paint exterior wall surfaces, predominantly on solid walls it seems. It has a by far better vapour permeability of modern resin or plastic paints. This allows for successful evaporation and can minimise surface failure ie blistering.  Not all moisture which wants to evaporate is formed at the base of a wall, such as rising damp issues etc. It is important to have a degree of permeability in a solid wall to manage internally produced atmospheric moisture (think of this as condensation).

There are modern alternatives out there but from my experience, a traditional approach should nearly always (there may be exceptions) adopt a traditional solution. OC