Why Should We Use A Silicone Render? – Can I Just Ask? – Ep.9

Silicone render is a polymer-based finish coat applied over a lime basecoat that combines two properties which are difficult to achieve simultaneously in traditional materials: strong water repellency and good vapour permeability. It keeps liquid water from penetrating the surface while still allowing moisture vapour to move outward through the wall, which is exactly the balance needed when insulation is involved — whether externally applied or on the inner face of a solid wall.

The reason this matters so much for external wall insulation, particularly wood fibre, comes down to what happens to the render once insulation is placed behind it. External insulation moves the dew point outward and makes the render surface significantly colder than it would otherwise be. A traditional lime render, which absorbs a considerable amount of water during rainfall, will then hold that moisture against a cold outer face for extended periods — gradually degrading the surface of the insulation over time. Silicone render dramatically reduces the amount of water absorbed in the first place, keeping the outer layers of the system dry and protecting the long-term integrity of the build-up.

The benefit extends beyond external insulation systems. As UK weather patterns shift toward longer and heavier rainfall events, reducing the moisture burden on historic fabric has become increasingly important in its own right. Applying a thin-coat silicone or lime-silicone render over a mesh coat on an existing wall — even without any external insulation — significantly increases the facade’s water repellency, which in turn opens up more options for what can be safely done on the inside. The pragmatic conclusion drawn here is a straightforward one: just because lime renders were used historically doesn’t mean they’re always the right choice in a modern retrofit context. Where a better material exists for the job, it should be used.