Why Are the Walls in My Home Wet?

Condensation and a leak look the same at first

Both produce a damp patch. They have completely different causes and completely different fixes, and opening a wall to find out is the expensive way to answer the question.

What condensation is doing

Warm moist air meets a surface cold enough to bring it below its dew point, and water forms on that surface. In a house this happens on exterior walls, in poorly ventilated bathrooms, on cold water pipes running through warm spaces, and in corners where air does not circulate.

The tells are consistent. It affects a broad area rather than a defined spot. It appears and disappears with weather, heating and use. It is worse in rooms that generate moisture, so bathrooms, kitchens and laundry rooms. It often shows first as mould in corners rather than as running water. And it does not get worse when you run a tap.

What a leak is doing

A leak has a source, so it is localised, and it does not care what the weather is doing. The patch has a defined edge, often with staining that spreads downward. It is persistent and gradually worsening rather than cyclical. It may be warm to the touch if the leaking line is a hot one, and it may correlate with use, worsening when a particular fixture runs.

A leak on a supply line will also usually show on the meter.

The test that settles it

Turn off every water-using fixture and appliance in the house, then read the water meter. Leave it an hour with nothing drawing water, then read it again. If it has moved, water is escaping somewhere on the supply side.

That test will not catch a drain leak, which only leaks when something drains, or a roof or flashing problem, which only leaks when it rains. But it separates the most expensive category from the rest for the cost of two meter readings.

When to call

If the meter moves with everything off, call. If the patch is defined, worsening and indifferent to weather, call. If it is broad, seasonal and worst in the bathroom, it is more likely a ventilation and insulation problem than a plumbing one, and we will tell you that rather than sell you a leak investigation.

Where it genuinely is a hidden leak, finding it without opening the whole wall is the skill. The repair itself is usually the easy part.