During my surveys, I will always take time to inspect the outside of the building. In particular I will look at the walls, as the condition of the external façade can affect my recommendations.
Inappropriate render an/or paint on walls often introduce risks around damp ingress over time. Older renders and pebbledash are often hard, cementitious compositions typically finished with a vinyl masonry paint. Cementitious render can usually be quickly identified as no sand particles will come away when you run a finger over. In contrast, lime-based render and mortar is usually a little crumbly and will fizz if you dab a little kettle descaler solution on it.
Cementitious render doesn’t necessarily present an issue whilst the coverage and paint finish remain immaculate. Over time, however, it is inevitable that over time the paint and render will start to develop cracks. Once this starts to happen, water will easily enter through capillary action. Once water is in it will – like a housefly that got in through a small open window – not easily be able to find its way out again.
On uninsulated cavity walls damp ingress through render is not usually a massive issue. Any water that has penetrated will drip harmlessly down the cavity and the wall can dry from the inside. It is, however, possible that salts in the bricks that become dissolved by the water could cause a problem. As and when the wall does eventually dry out as the salts crystallize and potentially spall the brickwork.
For fully insulated cavity walls, the risk of damp ingress presents a significant risk over time. Once mineral fibre (the most common type of cavity insulation) becomes wet it can lose its effectiveness. Henceforth, damp bridges across to the inner brickwork leaf and potentially even collapse insulation into the bottom of the cavity. A damp inner brickwork leaf is then likely to lead to rot in the joist ends.
Similar risks obviously apply to solid rendered walls that are not completely weathertight. Once again there is the potential for issues with floor timbers and damp patches on the internal sides of the walls.
As a general rule we would advise clients not to insulate wall cavities where the render and/or paintwork is not breathable. In this case one option would be to remove the render. A breathable silicone render can be applied in its place, after which the cavity can be insulated. External wall insulation can also be fitted under any new render, however this will still require insulation of the cavity.
The alternative approach is to retain and maintain the existing render finish and fit insulated plasterboard on the internal walls – a depth of 37mm would achieve a similar standard of thermal insulation to an insulated wall cavity.
Contact us if you are interested in our comprehensive Home Energy Survey, looking at render and all issues relating to home energy use.