Damp, Mould and Condensation: Fix It Before You Paint

Mould on an interior wall is one of the most common things we are asked about, and painting over it is one of the most common mistakes. Paint does not kill mould, and it does not stop the moisture that is feeding it. Within a season it usually comes back through the new coat, and you have paid to repaint a wall that is no better than it was.

Wellington homes are prone to this. Many are older, poorly insulated, and sit in a climate that is damp and windy for much of the year. Getting the moisture sorted before the paint goes on is the difference between a repaint that lasts and one that fails.

First, work out which problem you have

There are three common causes and they need different responses.

Condensation is by far the most common. Warm, moist air inside meets a cold surface and the moisture comes out of the air onto the wall. The signs are mould in the corners of rooms, on the ceiling above windows, behind furniture pushed against an exterior wall, in wardrobes on outside walls, and around window reveals. It is worse in bedrooms and bathrooms, worse in winter, and worse where there is no ventilation.

A leak produces a defined stain, often with a tidemark, in one place. It may be worse after rain, and it may be nowhere near a cold surface. Look above it for a plumbing run, a roof penetration, a window head or a junction in the cladding.

Rising or ground moisture shows up low on the wall, sometimes with a horizontal line, and is common where there is a subfloor with poor ventilation or a ground level that has crept up outside.

If you are unsure, watch when it is worst. Condensation is worst on cold mornings. A leak is worst after a southerly.

Deal with the cause

For condensation, the levers are moisture production, ventilation, heating and insulation. In practice, the ones that make the most difference in an older Wellington house are extraction in the bathroom and kitchen that vents outside rather than into the roof space, a lid on the ground under the house if the subfloor is bare earth, drying washing outside or in a vented dryer, and keeping the house warmer more consistently rather than in short bursts.

For a leak, find and fix it. Painting is pointless until it is done and the structure has dried.

If a rental property is involved, note that New Zealand’s healthy homes standards set requirements for heating, insulation, ventilation, moisture ingress and draught stopping. If you are painting a rental to tidy it up, it is worth checking those obligations at the same time, because several of them address the same underlying problem.

Then treat the surface properly

Once the cause is under control, the affected area needs to be cleaned and treated rather than simply painted. Mould has roots in porous surfaces, so wiping the visible growth off is not enough. The surface is treated, allowed to dry, and then sealed if there is staining, because mould and water stains bleed through ordinary topcoats.

Where plasterboard has been wet for a long period it may be soft or degraded, in which case it needs replacing rather than coating. That is a plastering job before it is a painting one, and it is work we do as part of our plastering services.

Choose the right products

Paints formulated for wet areas resist mould growth much better than standard interior paints. They do not make a damp wall dry, but in a bathroom or a laundry with reasonable ventilation they hold up far longer. In a room with a history of condensation, using one on the exterior-facing walls is a sensible precaution.

There is more on choosing finishes in our guide to paint sheen, room by room.

What to tell your painter

Point out every spot that has ever had mould or a stain, even if it is not visible now, and say what you have done about it. It changes the preparation and the products, and it is much cheaper to allow for at quoting stage than to discover halfway through.

A painter who tells you a wall needs the moisture sorted before they will paint it is giving you good advice, not avoiding work.

Getting it looked at

If you have mould coming back on an interior wall and are not sure what is causing it, get in touch and we will take a look before quoting. You can also read more about our interior house painting work.