The amount of water that old walls can accumulate from rising damp can be truly staggering.
I recently came across a scientific publication (attached at the end of the page), that provided the scientific formulas for the calculation of some of its most important parameters of rising damp, depending on the wall thickness. These parameters as:
- The height of rising damp in the wall (steady-state height)
- Amount of water inside the walls per linear meter of wall section
- Daily water pass-through rate: how much water is moving through the wall per day, year etc.
- Water travel time (residence time): the average time for the water to move through the wall fabric from the base to its evaporation point (exit point)
The figures in the research paper were calculated for the UK, taking into account the UK climate conditions, and the figures were staggering.
Here is an example for a 500 mm thick wall:
- The height of rising damp: 1.12 meter
- Quantity of water per linear meter of wall section: 111 liters
- Water flow-through: 1.1 liters / linear meter / day. This translates to 408 liters / year / meter. No wonder the dehumidifiers gets filled quickly.
- Water travel time (residence time): 100 days to travel through the wall
I have re-created the calculations in Excel, so you can compare the above values for different wall thicknesses. This includes::
- Normal residential buildings (0.2 to 0.5 m wall thickness)
- Listed buildings and churches (0.6 to 1.5 m wall thickness)
- Large conservation projects (2.0 to 3.0 m wall thickness)
Here are the calculations:
Some Interesting Conclusions
Based on the above figure, we can conclude the following:
- Thicker walls can hold much more water than thinner walls because they have a much larger volume than thinner walls. In simple terms, you can think about them as a large vs small bucket, the larger the volume, the larger the bucket, the more water they can hold. Additionally, larger walls also have a larger contact surface with the ground, so the water intake is much more pronounced.
- Rising damp height increases with wall thickness: rising damp rises until water reaches equilibrium condition, determined by the water intake (water in) and evaporation (water out) ratio. The volume of thicker walls is larger, while the evaporation surface of both thin and thick walls is the same (e.g. 1 sq m). As a result, thicker water rises higher in thicker walls.
The height of rising damp in residential buildings with thinner walls (e.g. 500 mm) is indeed around the proverbial 1 meter, however in thicker walls rising damp can rise significantly higher.
- Thicker water evaporate more water into the living space. rising damp going higher in thicker walls leads to a larger evaporation surface and more humidity into the living space. Several liters of evaporated water per day or several hundreds of water per year is not uncommon - many people have witnessed that their dehumidifier fills up in a day or less.
Several hundreds of liters of water per year can draw in a significant amount of salts into the building fabric, explaining the severe plaster damage in some cases.
- Rising damp is a more severe problem in buildings with thicker walls (considering everything else being equal), leading to more damages and more costly repairs.
For anyone interested in the original article, formulas and calculations, please read the original research paper "Rising damp: capillary rise dynamics in walls" published in 2007 by The Royal Society, London.
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