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 analyzed rising damp in detail, providing some of the scientific formulas for the calculation of some of its most important parameters, such as:
- The steady-state height: the stable-height of rising damp in a wall
- Quantity of water per linear meter of wall section
- Water flow-through: 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 or rise through the wall fabric from the base of the wall (entry point) to its evaporation point (exit point)
The figures were calculated for the UK, taking into account the average yearly evaporation rate for the UK, and the figures were striking. Here is an example for a 500 mm thick wall:
- The steady-state 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., Or 408 liters / year / meter.
- Water travel time (residence time): 100 days to travel through the wall
I have re-created the formulas and calculations given by the paper in an excel file and calculated the above parameters for various wall thicknesses between 0.2 to 3.0 meters, covering:
- 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)
Just for fun. Please see the summary chart below:
Some Interesting Conclusions
Based on the above calculations, here are some interesting observations about rising damp.
The thicker the walls...
- The higher rising damp rises. This is normal, as a thick walls have much larger volume than thinner wall, so they can hold a lot more water than thinner walls. For residential buildings with thinner walls the average rise height of rising damp is indeed around 1m, however it can rise significantly higher for thicker walls. The sideways evaporation surface being equal for both, the impact of evaporation for a thick wall is less than for a thin one.
- The more water is stored/trapped inside the walls. for residential buildings of up to 500 mm wall thickness this can be up to 111 liters / linear meter of wall. for a 3m thick wall this increases to a staggering amount of 1600+ liters / meter.
- The more water evaporates into the living space. as a consequence, many liters of water are evaporated each day for every linear meter of wall section. This again is not uncommon as many people have confirmed, that their dehumidifier bucket can fill up in a day or less.
This amounts to hundreds of liters of water / year. No wonder that the plaster gets damaged to such an extent.
- This just shows that rising damp is a much more serious problem in large (and older) buildings, where the water rises higher, retaining much more water inside the building fabric due to its size and the reduced impact of sideways evaporation.
Rising Damp vs Condensation
Rising damp is the migration of water from the ground up, through the wall fabric due to capillary suction, then evaporating into the living environment. The water moves AWAY from the wall as it evaporates into the living environment.
Condensation occurs when humidity from the air liquefies and gets trapped by cold surfaces. The water (vapors) moves INTO the wall fabric from the environment.
These are completely opposite flows - different phenomena.
Let's look at some figures for condensation. An area with high humidity contains about 10-12 grams/m³ absolute humidity in form of water vapors.
This, for a 30 m³ room translates to about 300 - 400 grams of water, which is about 10X less water volume then the amount supplied by the forced evaporation regime of rising damp.
Evaporation as a result of rising damp in an abundant flow, which can amount to several liters per day. Condensation on the other hand is a much less significant flow, resulting in much less water flow (over 10X less) than rising damp.
Again, they are very different phenomena with different outcomes.
For anyone interested in more details, please review the original research paper "Rising damp: capillary rise dynamics in walls" published in 2007 by The Royal Society.