Why do damp marks keep coming through?

Article by Noel Larkin Published in Irish Times, Property Clinic Section, Dated 19 March 2015

Five years ago I renovated an old bungalow that included dry walling the interior wall on all the external walls. I am having issues with some of the walls since in that a damp mark from the holding nail (which is about 6cm in diameter) is showing through the plaster that was put on top of the dry wall. I believe they are called “mushroom holders”. I have had to paint over the holes every 18 months or so to cover the mark.

I used stain-proof paint with wall paint over it. Have you any suggestions? I also got the outside of the external walls rendered.

A This sounds like an issue with condensation, leading to staining on wall surfaces. Air contains moisture. The temperature of the air determines how much moisture it can hold, and warm air contains more moisture than cold air.

When warm, moist air comes into contact with a cold surface the warm air is unable to retain the same amount of moisture as it did and the water is released onto the colder surface, causing condensation to form.

This dampness entraps dust particles from the air and can quickly lead to mould. Thermal bridging or cold bridging can happen when lower external temperatures are transferred to the inside of a building through a building material or component by a process known as conduction. The reduced temperature can lead to condensation on internal surfaces.

This is common in older poorly insulated properties such as the one you describe. Thermal bridging can promote dampness and lead to mould growth and staining.

In this case you have completed a renovation project and added insulation to the inside of the external walls and it is therefore disappointing that the problem of cold bridging remains.

The issue here appears to relate to a cold bridge between the other blockwork and the new inner plaster surface.

This bridge is formed by the metal core of the mushroom shaped fixings used to hold the dry lining in place. The metal core can easily transfer or conduct the colder external temperature to the inside.

As with any dampness issue, particularly the issue of condensation, improvement in both heating and ventilation levels can help and should be your first port of call.

You mention that the problem appears to persist on just some of the walls. I suspect that these walls are the colder north- facing ones, or walls in less used rooms where issues of dampness and mould can be more prevalent.

The use of some brush applied treatments can in fact make the issue worse as the building fabric can lose breathability, meaning moisture can become trapped in the structure.

The use of good quality paints can help in some cases. My advice would be to carefully clean and sterilise any staining to reduce the likelihood of regrowth of mould and then repaint.

In tandem with this you should improve ventilation and heating and then monitor the situation. I would be hopeful that these simple steps should improve your situation. If the problem returns it would be worthwhile having the issue looked at by a building surveyor as dampness issues can be tricky to diagnose without site specific investigation.