The subject of dampness in old buildings can be a complex and intimidating topic, however with the understanding of the basics the topic becomes actually quite simple.
Main Moisture Sources
According to the industry's current understanding, the main moisture sources affecting old buildings are:
- Rainwater ingress (Liquid moisture): which can take many forms: rainwater ingress, wind driven rain, chimney leaks, roof leaks, water splashback at the base of walls, sideways penetrating moisture in underground areas etc. The moisture source, however, is always rainwater.
- Condensation (Vapor to Liquid transformation): involves the phase transformation of water between vapour and liquid as a result of temperature changes. It occurs when moist air (usually warm air found indoors) comes in direct contact with colder surfaces (e.g. cold walls, window panes).
Solutions to Dampness
Based on the above, keeping old walls dry mostly involves solving the following problems:
- Rainwater ingress (Liquid moisture): the primary solutions to handle rainwater include drainage (to decrease the amount of liquid moisture at the base of the walls) and making the building envelope and walls watertight by various means (e.g. repointing, rendering, flashings, roofing etc.).
- Condensation (Vapors to liquid moisture transformation): condensation, which primarily occurs due to temperature differences, is solved by heating and ventilation. Heating to keep wall surfaces warmer, ventilation to move excess humidity from the living space into the outer environment.
After all these measures applied we should have a nice dry wall fabric long-term. Or not?
Not quite. Often not.
The Hidden Moisture Source – Soil Evaporation
Latest technical research on old buildings has uncovered that in addition to rainwater and condensation, there is a third important moisture source: SOIL EVAPORATION.
Liquid water falls as rainwater, goes into the ground, then evaporates and rises up in form of vapors. This is known as the natural Water Cycle that describes the large-scale movement of water in nature. 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 now first evaporates INTO the porous wall fabric (into the wall capillaries), then into the air. In other words the moisture evaporation path becomes: soil > wall fabric > air.
Under-floor evaporation: in case of suspended floors, the evaporation of the solum is just another form of soil evaporation which has been known to exist.
Underground areas: partially of fully underground walls can act as large evaporative surfaces (depending on the breathability of the plaster used), evaporating moisture into the living environment.
Broken water pipes or other natural moisture sources (such as underground water streams etc.) can keep the soil damp, generating significant water vapors on the surface, or even excess liquid moisture resulting in capillary action, although this is not very common.
Soil evaporation, as a moisture source has not been recognized in the past, being considered something unimportant. This was an oversight.
Recent research has shown that soil evaporation is an important and powerful phenomenon that can markedly affect the building fabric long-term.
Soil evaporation is what causes rising damp which is probably the most confusing and least understood phenomenon in the building sector. It took years of research to understand the mechanism of rising damp and demystify this very confusing subject. You can read a simple, non-technical explanation here, and the underlying research data here. If your building has issues that affect the base of the walls, make sure you read and understand this subject.
Why Did it Stay Hidden?
There are several reasons why soil evaporation, as a dampness source, has remained hidden for so long. Unlike liquid moisture which is:
- Visible, easy to spot
- Fast: it creates obvious damages relatively fast (sometimes within hours or days)
- Difficult to conceal
soil evaporation is:
- Invisible: vapors or moisture evaporation can’t be seen with naked eyes (although it can be measured in other ways).
- Slow to build-up: damages created by vapor accumulation are slow, can take years. Breathable surfaces are often dry due to heating and surface evaporation, giving no obvious clues about any moisture accumulation in the depth of the masonry for quite some time. Obviously if surfaces are covered with non-breathable materials (moisture barriers) then moisture accumulation under the surface can speed up significantly, resulting in damages quite fast.
- Easily overridden or made hidden: most other moisture sources (liquid moisture sources such as rainwater) or even replastering can easily override it, often temporarily hiding the problem for years, giving the false impression that the problem has been permanently solved. This is not the case.
False Assumptions and Confusions
Not recognizing the existence and importance of soil evaporation led to many false past assumptions (never really proven by in-depth research) in order to explain certain building problems that have been observed to exist. Quite interestingly, many such problems seem to affect lower wall areas in the proximity of the soil, close to soil evaporation, rather than higher wall areas.
Here are the most common assumptions and explanations voiced by various parties that modern research has proven to be incorrect. Several of these assumptions are about rising damp, one of the most complex and misunderstood phenomena. The data readily available on the internet is confusing, inaccurate, not backed by research, various parties presenting dead opposite opinions (e.g. it exists vs. it doesn’t exist) about the subject. This in itself leads to confusion as BOTH viewpoints, black vs white, CAN’T be right.
- Moisture barriers (such as cement plasters) are the primary cause of dampness problems: this is only partially true. Moisture barriers often create dampness problems because they prevent the evaporation of moisture. However in order to make moisture accumulate in a wall a primary moisture source must be present! This often is the invisible soil evaporation which in the presence of cement plaster leads to a fast moisture accumulation in the building fabric.
- Rising damp is capillary action: research has shown that the primary mechanism of rising damp is not capillary action, for the simple reason that buildings are built on moist soil and not on liquid water (except maybe Venice). Hence old walls are primarily subject to evaporation from the damp soil and not to liquid capillary action. The mechanism of rising damp explains how soil evaporation can eventually lead to capillary action.
The assumption that rising damp is capillary action led to incorrect remedies, assuming that the elimination of liquid moisture sources (e.g. through drainage, pointing etc.) leads to dry walls. This is not the case. Vapor migration from the soil also must be sorted to have dry walls.
- Rising damp does not exist: this is incorrect. The validity of rising damp can be easily verified from countless period documents, independent research papers and as well as modern research data. There is also a political angle to the question, read more about this here.
- Rising damp is condensation: it is not. Modern research has shown that the depth of the walls is a non-condensing environment. interestingly, the core of a wall is also warmer than its surface, even in winter time, for the simple reason that evaporation is what cools the surface. Read more about this here.
- Rising damp can be solved by drainage or by making the fabric watertight. This is not the case. Although drainage can help to reduce liquid moisture while also reducing soil evaporation, modern research has conclusively shown that even a very well drained soil contains more than enough moisture to make walls damp by soil evaporation alone. Read more about this here.
- Slow, steady heating can solve all or most dampness problems: this again is an assumption that does not withstand the scrutiny of modern research. In the presence of soil evaporation the effect of heating can be quite insignificant. Read more about this here.
Many of these statements are originating from and are the opinion of one person. You can read more about this here.