Internal cork coating for cold, damp-feeling walls.

Why does my wall keep getting condensation and black mould?

Condensation forms when warm, moist air meets a surface that is cold enough to turn that moisture back into water. If a wall stays cold and damp for long enough, black mould follows.

Most people I meet have already tried the usual fixes: bleach, anti-mould paint, a dehumidifier, opening windows. The mould comes back because none of those change the two things driving it, which are how cold the wall surface is and how much moisture is in the air. It is often worst in a child's bedroom, behind furniture, in the corners of an outside wall or around a bay window reveal.

Before anything else I check it really is condensation. Leaks, broken gutters, failed pointing and plumbing faults can all look like condensation mould, and they need fixing at source, not covering up.

What is the dew point, and why does wall temperature matter?

The dew point is the temperature at which moisture in the air starts to turn into water on a surface. Keep a wall above it, and droplets don't form.

A simple example: in a room at 20°C with 60% humidity, the dew point is roughly 12°C. Any part of the wall colder than that will run with condensation. But mould doesn't need visible droplets. It can grow where a surface stays at around 80% humidity or more for long periods, which in that same room means a wall colder than roughly 15°C.

So the aim is not just to scrape above the dew point. It is to keep the surface comfortably warm and keep indoor moisture under control, both at the same time. That is why ventilation always comes into the conversation. Cooking, showers and drying clothes indoors add a lot of moisture, and a warm wall still needs that moisture taken away.

How does internal cork coating help?

Cork slows heat moving through it, so a layer of cork plaster on the inside of a cold wall lifts the surface temperature a little. On a wall that sits just on the edge of the problem, that small lift can be the difference.

• Thin build-up. The layer is measured in millimetres, so you lose almost no room space and skirting, sockets and window boards usually need little change.
• Vapour-open. Cork plaster lets water vapour pass through, so it suits older solid walls that need to dry.
• Warmer to the touch. Cork feels less cold than bare plaster, which helps comfort near the wall.
• A normal-looking room. The supplier's internal system can take a smooth skim finish, so once decorated it looks like any other wall.

I have used cork plaster inside homes as part of my work on cold, mouldy rooms, and I apply it as a SprayCork UK approved applicator, following the supplier's specification for thickness and finishing layers.

Who is internal cork coating for?

It suits homeowners and landlords with a wall that is cold enough to cause trouble but not bitterly cold throughout. Typical examples are a chilly corner of a bedroom, a reveal around a window, or a room in an older solid-wall house where losing space to thick boards is not an option.

It is one of several cork uses I cover; the spray cork overview explains the others. If mould is your main worry, my page on condensation and black mould covers the wider fixes.

What happens when you coat a wall inside?

Most rooms take a few days, including preparation and drying between coats. Here is what to expect.

1. Furniture is moved or covered, floors protected, and the room sealed off as far as possible.
2. Mould is cleaned off properly and affected areas treated. Loose or blown plaster is cut out and patched.
3. The wall is primed to suit the surface, then cork plaster is applied in coats to the specified thickness.
4. If you want a smooth wall, a skim finish goes on top. Otherwise the natural cork texture is left.
5. We tidy up, walk round together, and you get aftercare notes, including the right type of paint to keep the wall breathable.

I hand the room back ready to decorate. If ventilation needs improving, such as trickle vents, an extractor fan or door undercuts, I organise it with trusted trades rather than fitting it myself.