Some Facts & Figures
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) of the Royal Society of London which provided the scientific formulas for the calculation of some of the most important parameters of rising damp. These calculations included:
- The rise height of rising damp in a wall (steady-state height)
- The amount of water inside the walls per linear meter of wall section
- Daily water pass-through rate: how much water is "moving through" a wall every 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)
Being a British research paper, all figures were given for UK climate conditions, and the numbers are really interesting.
Here is an example for a somewhat "average" 500 mm thick wall:
- Rising damp height: 1.12 meter
- Quantity of water stored by each meter of wall section: 111 liters
- Water "flowing" through the wall: 1.1 liters/day /meter wall section. This translates to 408 liters/year for each meter of wall section. No wonder your dehumidifiers get filled quickly.
- Water travel time through the wall fabric (residence time): 100 days to travel through the wall, from the very bottom to the evaporation area (rising damp height).
I have re-done these calculations in Excel and ran a few "typical" scenarios for comparison purposes, so you can see the key figures for different walls and wall thicknesses. The calculations cover:
- Normal residential buildings (0.2 to 0.5 m wall thickness)
- Listed buildings and churches (0.6 to 1.5 m wall thickness)
- Special conservation projects (2.0 to 3.0 m wall thickness)
Here are the calculations:
Based on the above figures, we can conclude the following:
- Thicker walls hold much more water than thinner walls due to their much larger volume. In simple terms, you can think of a thick wall as large bucket, and a thin wall as smaller bucket. Additionally, larger walls also have a larger footprints in contact with the ground, so their water intake from the ground is much more significant.
- Rising damp height increases with wall thickness: rising damp rises until water reaches equilibrium, determined by the balance of water intake (water in) and evaporation (water out). As both thin and thick walls have the same evaporative surface (e.g. 1 sq m), a lot less percentage of the water evaporates from a thick wall, resulting in higher rise height of rising damp in thick walls.
As the figures show, 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 rising higher in thicker walls leads to a larger evaporation surface and thus 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 observed that when their dehumidifier fills up in a day or less.
Several hundreds of liters of water per year can draw up 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.