Passive Purple airtight, breathable membrane used to reduce condensation on cold walls

Condensation on Walls: Causes and How to Stop It

Waking up to water running down the windows or dark patches creeping across a wall? That’s almost always condensation — the most common form of damp in modern UK homes, and the most misdiagnosed.

Here’s what causes condensation on walls, how to tell it apart from other damp, and how to stop it for good.

What causes condensation on walls?

Warm air holds moisture. When that warm, humid indoor air meets a cold wall surface, it cools rapidly and the moisture turns back into water — just like a cold drink “sweating” on a summer day.

Everyday living adds a surprising amount of moisture: cooking, showering, drying clothes indoors, even breathing. In a sealed but poorly ventilated home that moisture has nowhere to go, so it settles on the coldest surfaces — external walls, window reveals and corners. For the full picture, see our guide to what causes condensation in buildings.

Passive Purple internal airtight vapour control membrane Airtight White paintable airtight vapour control membrane

How to spot condensation (vs other damp)

Condensation usually gives itself away with:

  • Black mould in corners, on window sills and behind furniture
  • Walls that feel cold and wet to the touch
  • Streaming windows, especially first thing in the morning
  • A musty smell and peeling paint

Unlike rising or penetrating damp, condensation tends to appear high on walls and in cold spots — not as a tide mark rising from the floor.

How to stop condensation on walls

Three things working together fix it: ventilation, warmth, and a continuous airtight but breathable layer.

  • Ventilate — extract moisture at source in kitchens and bathrooms, and avoid drying clothes on radiators.
  • Remove cold spots — air leakage creates the cold surfaces condensation forms on. A continuous airtight layer like Passive Purple® or paintable Airtight White warms internal surfaces and cuts draughts, while staying vapour-open so the structure can still breathe.
  • Control vapour — a vapour control layer keeps moist air out of the cold structure, where it would otherwise condense.

That combination — airtight but breathable — is exactly what our membranes are designed to deliver.

Battling condensation or black mould? Contact us and we’ll help you choose the right approach.

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