There are significant differences between condensation and rising damp. They are NOT the same.
- Condensation occurs when humidity from the air liquefies on cold surfaces. The water (vapors) from the environment move in TOWARDS the wall fabric . This is an inflow.
- Rising damp is the migration of water from the ground up, through the wall fabric, finally evaporating out into the environment. During this process the water moves AWAY from the wall. It's an outflow.
These are completely opposite flows, rising damp being an outflow from the wall, while condensation being an inflow towards the wall.
Different Amounts of Moisture
Let's look at the amount of moisture being involved in both processes.
- Condensation: air always contains some humidity. A dry room contains about 5 grams/m³ moisture. An area with high humidity (e.g. a basement) contains about twice as much, about 10-12 grams/m³ moisture. This is about a teaspoon of water for every cubic meter, or for a 30 m³ average sized room this translates to about 300 - 400 grams of water in the air.
The air can hold a limited quantity of water - about half a liter or less for an average room, and when there is too much moisture - depending on the ambient temperature - humidity starts condensing on the surface. This the humidity moves from the air to the surface.
- Rising damp: here the source of moisture is the damp soil. The soil, linked to the water table, can source an unlimited quantity of moisture. Thus, walls subject to rising damp can evaporate a virtually unlimited quantity of moisture into the living area.
Based on actual calculations, every linear meter of a 500 mm thick solid wall (e.g. a typical cottage wall) can evaporate about 1 liter of moisture per day. Having a 5 meter long wall with rising damp, this means about 5 liters of water per day. This is not uncommon. Many old building owners have experienced this, their dehumidifier extracting several liters of water per day from that damp room.
Where this moisture is coming from? From the soil! You can't get this much water from the air only.
The amount of water rising damp produces is about 10X more (5,000 ml vs 400 ml) than the amount of humidity air naturally holds. Of course, some of this excess humidity from rising damp will condense on the cold, evaporative surfaces (usually towards the base of the walls, the coolest points in the room), leading to secondary condensation.
But the test is simple: if your dehumidifier extracts several liters of water per day from a damp room on an ongoing basis, you probably have an underlying rising damp problem, not just condensation. If the plaster is also failing or crumbling in certain areas (indicating crystallizing ground salts), and there a musty smell in the room (indicating soil evaporation) - these signs also point in the direction of rising damp.
Here is what we can conclude:
- Condensation is inflow, humidity moves (condenses) from the air towards the solid cold surfaces. In term of abundance, there is relatively small amount of moisture transport involved (tens or hundreds of milliliters).
- Rising damp is outflow. Ground water evaporates through the walls, into the air, filling the living area with moisture. This is an abundant flow, that can result in several liters of moisture per day - involving about 10X or more water quantity than condensation.
- If your dehumidifier extracts form a damp room liters of water per day, on an ongoing basis, you probably have an underlying rising damp problem, not just condensation. Call a competent specialist for diagnosis and advice.
Here is a video where an Italian research scientist explains the differences between condensation and rising damp:
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