How to Manage Persistent Moisture on Interior Walls
Some walls are always going to be damp. Basement walls under ground level, garage walls with no cavity and walls affected by rising damp - the moisture is not going away because the source cannot be removed. You cannot fix it, but you can manage it. Here is how to apply a breathable damp barrier that keeps the internal face dry and paintable without trapping moisture inside the wall.
Why persistent moisture needs a different approach
Standard damp treatments are designed for situations where the source has been fixed and you are dealing with what is left behind. Persistent moisture is different. The water is still coming and it always will be. Sealing the wall with a non-breathable barrier does not work here. It traps moisture inside the masonry, and eventually it builds up, pushes through, and the finish fails - blistering, staining, or mould growth behind the repair.
What you need is a barrier that blocks liquid water from reaching the internal surface but lets water vapour escape through it. The wall can still breathe and dry toward the inside, but the moisture never reaches the finished surface. That is what Anti-Damp Render does.
Step 1: Strip back to bare masonry
Anti-Damp Render goes directly onto a bare substrate.
Brush off loose material, dust, and debris. The masonry does not need to be dry - Anti-Damp Render is designed to be applied directly to damp, porous, and untreated substrates. It does not need priming or sealing beforehand. The surface needs to be cohesive to facilitate good adhesion.
If there are cracks or damage in the masonry, repair those before applying the render so it has a sound, even surface to sit on. Read our advice on how to repair cracks in masonry.
Step 2: Apply Anti-Damp Render
Anti-Damp Render is a breathable barrier that controls damp on interior and exterior masonry. It blocks liquid water from passing through the wall while remaining vapour-permeable, so trapped moisture can still escape as vapour. This is what makes it suitable for permanent moisture - it works with the wall's ongoing moisture cycle rather than fighting against it.
Mix the powder with water and stir until smooth. Working time is 3-4 hours, so you have plenty of time to cover a large area before the product starts to set.
Apply by brush in a criss-cross pattern, or with a trowel or roller. Work it into the masonry to get a good 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. For walls with heavier or more constant moisture pressure, apply toward the thicker end of that range for a more effective barrier. On walls with lighter damp, a thinner coat is enough. The thicker the application, the more robust the barrier.
Allow to dry for 12-24 hours before painting.
Painting over Anti-Damp Render
Once dry, Anti-Damp Render is ready to paint directly. There is no need for a skimcoat or finish filler over the top. Applying a standard skimcoat would compromise the breathability of the system, which is what makes it work in the first place.
Use a vapour-permeable (breathable) paint. This maintains the breathability of the barrier all the way through to the finished surface. Standard non-breathable paints will trap moisture behind the coating and undo the work the render is doing.