How to Manage Persistent Moisture on Exterior Walls
Some exterior walls are permanently exposed to moisture. Retaining walls holding back earth, boundary walls, north-facing walls that never fully dry out, and any masonry that is constantly subjected to excessive water flow, ground moisture, or poor drainage. The water is not going away. Here is how to protect the surface with a breathable barrier that blocks water penetration while letting the wall breathe.
Moisture needs a breathable approach
A non-breathable coating on an exterior wall causes more problems than it solves. Moisture already present in the masonry, or arriving from excessive water flow, ground contact, or condensation, has nowhere to go. It builds up behind the coating and eventually resurfaces as blistering, staining, or mold growth. The repair fails, and the wall looks worse than before.
A breathable barrier works differently. On exterior walls, Anti-Damp Render operates as a coating that blocks water from penetrating through the surface. At the same time, it allows water vapor to pass through from the inside out, so trapped moisture in the masonry can still escape. The wall stays protected on the outside and can dry on the inside.
Step 1: Prep - strip back and prepare the masonry
Anti-Damp Render goes directly onto bare masonry. Remove all paint, coatings, failing or damaged render, or any other finish from the wall. Strip back to a cohesive surface.
Clean off any dirt, moss, algae, or biological growth. Brush out loose material and debris. The surface does not need to be dry - Anti-Damp Render is designed to be applied to damp, porous, and untreated masonry. No priming or sealing is needed beforehand.
If there are cracks or damage in the masonry, repair those first so the render has a sound, even surface to bond to. Read our guide to repairing exterior masonry cracks.
Step 2: Apply Anti-Damp Render
Mix the powder with water and stir until smooth. Apply by brush in a criss-cross pattern, or with a trowel or roller. Work the product into the masonry to get a solid bond. Cover the full affected area and overlap onto sound masonry by at least 100mm on all sides.
Apply to a thickness of between 2mm and 10mm. On walls with heavier or more constant moisture exposure, go toward the thicker end of that range for a more robust barrier.
Allow to dry for 12-24 hours before painting.
Painting over Anti-Damp Render
Once dry, Anti-Damp Render is ready to be painted. Do not apply a skimcoat or finish filler over the top. Adding another layer would compromise the breathability of the system and reduce its ability to manage moisture.
Use a water-based, vapour-permeable paint. This maintains the breathability of the barrier all the way through to the finished surface. A non-breathable paint will trap moisture behind the coating and undo the work the render is doing.
The wall is now protected with a breathable barrier. Water stays out, vapour can escape, and the surface is ready for decoration. Anti-Damp Render is effective when applied to the entire affected area, so it is best to make sure it gets in everywhere that water can penetrate.