Troubleshooting - Green Growth on Painted Surfaces?

Tŷ-Mawr ‌‌ posted this on 6 Apr 2016

Troubleshooting – Green growth on silicate painted surfaces?

The paint being alkaline and comprising potassium silicate will by its nature resist algae growth. If you experience green growth on silicate painted surfaces the situation is not that the paint that is attracting the algae growth but that the lime render / other porous substrates that it is painted onto could have a high level of water permeability to allow this to occur.

To explain lime renders and vapour permeable paints are very open to moisture movement through the material - this is what enables the buildings to 'breathe' and allow both the absorption and evaporation of moisture off their surface.

Cement and ‘plastic’ masonry paints are impermeable and do not take on moisture - rather it runs down the surface (until inevitably with cement renders you get microcracking and moisture entering the wall. The cement render will not allow its effective release, the moisture, in turn, is then trapped by the masonry paint ultimately leading it to blow and bubble).

If with lime renders and vapour permeable paints you get more moisture absorption than evaporation over an extended period of time then this will allow conditions, together with mild temperatures, for algae growth. This can occur on North facing elevations which will get wetting but get no sun and very little wind drying. It can also occur if the house is shadowed by trees, or a bank etc.

The algae won’t deteriorate the render or paint. It should be gently washed off with a water-based fungicide. The wall should then be allowed to dry. If you have a situation where you are likely to get more wetting than drying then we would suggest you apply an additional clear coating of Beeck’s SP Plus. This provides an ideal situation as it maintains very high vapour permeability allowing walls to release moisture vapour but it is hydrophobic meaning liquid water runs down the surface and does not penetrate the wall.