To waterproof an interior wall, first fix the moisture source, then treat the wall with an interior damp coating such as Damp Proof over clean, dry, primed plaster. Interior waterproofing manages damp and condensation on the inside face, but it is not a substitute for stopping water at its source — rising damp, a leak or a failed external seal must be corrected first.
Can you waterproof interior walls?
Yes, you can waterproof interior walls, and it is the right fix for condensation and mild penetrating damp. An interior damp coating blocks moisture and salts from pushing through to the paint and stops mould taking hold. It cannot hold back strong hydrostatic pressure from below ground on its own — a basement or retaining wall needs a cementitious system like Megaflex that becomes part of the wall.
The three types of interior damp
- Rising damp. A tide mark up to about 1 metre (m) from the floor with salt staining; the damp-proof course has failed.
- Penetrating damp. Patches after rain, from water crossing the wall from outside through a crack or failed external seal.
- Condensation. Surface moisture and black mould in corners and behind furniture, from warm humid air meeting a cold wall.
What is the best waterproofing for interior walls?
The best interior waterproofing depends on the damp type: use Damp Proof for rising and penetrating damp on the inside face, and Megaflex for below-ground walls. For condensation, fixing ventilation plus a coating is often enough. On the outside face, Shield keeps rain out of the wall in the first place, which is the most reliable long-term fix.
How to damp-proof a wall from the inside
- Find the source. Identify whether the damp is rising, penetrating or condensation, and fix that cause first.
- Strip and clean. Remove blown plaster and flaking paint back to sound substrate, and treat mould with a biocide.
- Let it dry. Allow the wall to dry out fully — coating a wet wall traps the moisture.
- Prime. Prime bare plaster with Solvent-Based Plaster Primer, or stabilise a chalky wall with Clear Bonding Liquid.
- Coat. Apply Damp Proof in two coats, then finish with your interior paint.
Waterproofing a wall before plastering
On a new or stripped wall, a waterproofing slurry applied before the plaster coat blocks moisture at the substrate. Megaflex cementitious waterproofing is designed for this, giving a bonded barrier under the plaster on basements, retaining walls and wet areas.
Condensation and ventilation
Condensation is a ventilation problem before it is a coating problem. Improve airflow, heat cold rooms gently, and vent bathrooms and kitchens, then use a mould-resistant coating. A damp coating alone will not cure condensation if the humid air has nowhere to go.
What interior waterproofing will not fix
Interior coatings manage damp on the inside face but do not cure the cause. Rising damp needs a damp-proof course, penetrating damp needs the external wall sealed with Shield, and a plumbing or roof leak needs the leak repaired.
Frequently asked questions
Can you waterproof a wall from the inside? Yes, for condensation and mild penetrating damp. For rising damp or below-ground water, treat the source and use a cementitious system.
What is the best paint for damp interior walls? A dedicated interior damp coating such as Damp Proof over clean, dry, primed plaster, once the moisture source is fixed.
How do I stop damp coming through an internal wall? Fix the source (rising, penetrating or condensation), let the wall dry, prime, then coat with Damp Proof.
Where to buy
Shop Damp Proof and the Waterproofing range, or read the complete waterproofing guide. Trade pricing: +27 84 985 6141.
