One of the fundamental points of confusion on the subject of dampness is the confusion between rising damp and condensation. Recent research on condensation has finally clarified the differences between these two phenomena.
Rising Damp vs. Condensation – Simple, Non-Technical Explanation
Rising damp is caused by soil evaporation, the evaporation of vapors from the soil. The detailed explanation of the phenomenon along with accompanying research data can be found here.
In nature, there is the natural Water Cycle that describes the large-scale movement of water in nature. Liquid water falls as rainwater, goes into the ground, then evaporates and rises up in form of vapors. As part of this cycle the soil constantly evaporates moisture into the air.
When a wall is built onto the soil – blocking the free evaporation of moisture – the moisture from under the wall will now first evaporate INTO the porous wall fabric (into the wall capillaries), then into the air. In other words the evaporation path will become: Soil > Wall fabric > Air.
Some part of this moisture gets trapped by the building fabric due to surface attraction and it starts accumulating, progressing through several stages: from vapors, to a thin liquid film, to partial capillary flow, eventually developing into full capillary flow.
Capillary action is the last stage of this cycle, however the primary mechanism of rising damp is not capillary action but vapor movement driven by natural soil evaporation.
Vapors rise and so does humidity. The rise of vapors from the soil under the building results in a gradual slow accumulation of moisture that over time can become so severe that it needs attention - a problem known as rising damp.
Condensation involves the phase transformation of water between vapour and liquid as a result of temperature changes.
It occurs when moist air (usually warm moist air found indoors) comes in direct contact with colder surfaces (e.g. walls, window panes). It is more prevalent at the bottom of external walls and cold corners or in places where moisture stagnates - in areas with little or no ventilation (e.g. behind furniture).
A full description of condensation and its scientific background can be found here.
Differences Between Them
Although there are some similarities between condensation and soil evaporation as both phenomena are linked to water vapor movement, there are key differences between them, as summarized below.
Caused by excess humidity from the soil
Caused by temperature differences
Vertical moisture gradient
Horizontal moisture gradient
Seasonal (during cold months)
- Driving force: condensation is driven by temperature differences (hot-cold), typically external walls being cooled down by cold weather. As a result below a certain temperature (dew point temperature or condensation temperature) vapors liquefy or condense.
Soil evaporation is the vertical movement of vapors from a humidity saturated area (mostly the soil) into the air, the driving force being the presence of excess humidity.
- Vapor movement direction: condensation on vertical walls is commonly a horizontal gradient (from cold to hot) driven by temperature differences. Soil evaporation in vertical walls is vertical humidity gradient (from the soil upwards).
- Frequency: condensation is seasonal, mostly occurring during the damp season between October and April.
Soil evaporation is ongoing 24/7 but it displays seasonal variations. During warm summers its intensity decreases, but during the rainy season - due to rain and higher water tables driving more vapors into the building fabric - it increases.
Rising Damp vs. Condensation - Research Data
We have taken detailed measurements on several buildings including a 150-year-old brick/stone cottage, during the months of November and December, a period of intense rain, winds and cold temperatures nearing freezing conditions.
The soil in the area was predominantly clayey, resulting in high water table during winter time and lots of surface water around the building.
Detailed measurements have been taken during the experiment using an array of embedded micro-sensors, collecting humidity and temperature readings from various areas of the wall, from both depth and surface as well as from the ambient environment. Various electrical parameters of the fabric such as the spontaneous voltages and currents present in the brickwork have also been recorded.
The readings have been taken by a Keithley / Tektronix DAQ-6510 professional datalogger with 0.0025% accuracy.
Here are the most important findings:
The Depth of the Wall is Warmer than the Surface
One of the surprising findings was that the temperature in the depth of the wall (blue) during wintertime is warmer than its surface (red); the depth being about 2°C warmer than the surface. This at first might sound counter-intuitive, but further investigation revealed that the surface of the wall is cooled down by surface evaporation.
The same phenomenon can be observed during summer months, when during the night the surface of the wall cools down more than the depth, the fabric storing and retaining the heat better than the surface.
This is in line with some of our earlier measurements about the soil, where we found the soil in wintertime to be warmer than the surrounding air near it. Ground floor heat pumps utilize the heat stored in the soil to heat houses; the deeper you go the warmer the soil becomes. The surface, on the other hand, due to constant evaporation it cools down and stays cold. Radiant cold from the surface of damp walls also confirms this idea.
The whole phenomenon of evaporation is probably triggered by temperature differences between the warm core and colder surfaces. Inside the wall warm air expands, creating an airflow towards the surface, resulting in evaporation.
The Depth of the Wall is a Non-Condensing Environment
Another interesting finding was that in the depth of the wall there is no condensation. Despite RH in depth being high (between 85% - 90%), the wall’s internal temperature (red) stays consistently by about 2°C above the dew point or condensation temperature (yellow), indicating no condensation taking place inside the wall.
We can also see below the variation of the relative humidity (dark blue) as well as of the absolute humidity (light blue), which is the real moisture content of the wall, independent of temperature changes.
Comparing Various Wall Areas
Finally, here is the overall picture of the wall we tested, showing the relative comparative values of several key parameters of the wall (surface vs. depth, bricks vs. mortar).
Comparing several parameters from various parts of a wall , we can conclude the following:
- The wall is damper in depth than on the surface (light blue): absolute humidity values in depth (7.5 – 9.0 g/m³) are about 40% higher than absolute humidity on the surface (5.0 – 7.5 g/m³). The closer to the ground, the damper walls are.
- The wall is warmer in depth than on the surface (red): on average, the wall’s temperature in depth (8.5 to 12.9°C) is about 2°C warmer than the surface (6.5 – 10.5°C); evaporation cooling the surface down.
- Both the surface and depth are non-condensing environments (yellow): dew point temperatures (or condensation temperatures) mirror the wall temperatures closely (yellow vs red lines), the wall temperatures being by about 2-3°C warmer than condensation temperatures in both surface and depth. This means there is no condensation, and the wall needs to cool down another 2-3°C for condensation to start occurring.
- Surface humidity varies significantly more than depth humidity (dark blue): surface heating and cooling creates a 14% change (68% to 82%) on the surface, and only a 3% (85 to 88%) change in depth. This indicates that changes on the surface are driven by the heating and cooling cycles of the environment, but these have very little effect in depth.
Based on the research data we can conclude the following:
- Despite of a very wet and cold December wall temperatures in depth were always about 2°C higher than the dew point (condensation temperatures), indicating that no condensation has been taking place inside the walls.
- Near ground the coldest part of the wall is the surface. This indicates that while the wall fabric retains the heat, the ongoing moisture evaporation cools wall surfaces down. (As a side note: higher up the wall, outside the ground evaporation zone, the wall surface can be warmed up by heating.)
- Despite no condensation taking place, the humidity in depth of the wall was high (RH: 85-90%, absolute humidity: up to 9.0 g/m³), the humidity increasing closer to the ground. This indicates that the moisture source is not condensation but the soil itself.
- Rising damp is not condensation, nor driven by condensation. The primary moisture source and driver of rising damp is soil evaporation.
- We do not exclude the possibility of condensation and soil evaporation to co-exist in walls under certain circumstances, however that does not change the fact that the primary cause of rising damp is soil evaporation. This has been demonstrated in tightly controlled lab experiments.
- Further experiments are being done to clarify additional aspects of moisture movement and vapor-liquid moisture transformation.