Vapour vs Liquid Water

The Importance of Vapour Movement

Table of Content — open

Vapour vs Liquid Water - Differences

Water can be found in three states of existence: gas, liquid and solid. As the solid frozen ice state is a relatively rare occurrence throughout most of the UK due to its mild climate, we are going to explore in more detail the more common liquid and vapour moisture states, their role in in moisture transport and their effects onto old building structures.

The general perception of the vapour vs liquid water question, which ne s more important is that liquid water is the more important because damages caused by liquid water can be very obvious (just think about a water leak), it happens fast, as a result such problems often need to be addressed without delay. So there is high awareness of liquid moisture related damages and there is an urgency to deal with such matters and prevent them.

On the other hand the invisible, light, airy-fairy water vapours are perceived by most as something insignificant and thus non-important, underestimating the role of vapours significantly.

The technical fact is that water vapours play a fundamental role in ALL moisture-related phenomena. Some reasons why water vapours are so important, are:

Vapour movement is the most fundamental moisture transport mechanism because it can take place anywhere. Wherever there is air, there are vapours and thus humidity present. As such, vapour movement affects both the wetting and drying process of a masonry as both rely on vapour movement.
Vapour accumulation can lead to significant wetting: in lack of liquid water the wetting cycle starts with the deposition of tiny water vapours onto solid surfaces. The ongoing accumulation of vapours in multiple layers eventually leads to liquid water accumulation (known as condensation) or to capillary flow. Under the right circumstances vapour accumulation alone can lead to significant wetting.

capillary 6 stages sm — Rising damp and its development stages - accumulating vapors gradually develop into capillary flow

Evaporation and the drying of wet masonry is a vapour-based process. Vast amounts of liquid water can be moved by evaporation, which happens day after day in nature as part of the natural Water Cycle. There are significant amounts of water in the atmosphere, about 12,900 km³ (or 12.9 trillion m³) of water in vapour form.¹

vapour vs liquid water- the water cycle — The Water Cycle in nature

Not being aware of the importance of vapour movement can lead to serious dampness problems, especially in older, damper buildings.

Most general builders are totally ignorant and utterly unaware of the importance of vapour movement and breathability. Hence, the building industry commonly uses non-breathable materials that block vapour movement, which often leads to various moisture problems within a few decades even in modern buildings. The lack of vapour movement is where most dampness problems eventually start. Many problems - condensation, mould, cold and unhealthy buildings, persistent dampness issues - can be traced back to lack of breathability - the use of non-vapour-permeable materials. Once vapour movement is blocked, hidden moisture sources are created that can (and often will) lead to various forms of dampness problems.

Conservation professionals are more aware of the role and importance of vapour movement than general builders, but the technical factors that affect and control breathability are often not well understood. There is more to breathability than just retaining the open-pore nature of the fabric by replacing the cement with a breathable lime plaster to allow the walls to breathe. There are additional variables or technical factors that affect vapour movement and breathability - you can read about these here.

Vapour Transport Phenomena in Damp Masonry

Here are some important moisture-related phenomena that heavily rely on the movement of vapours.

1. Wetting - The Role of Hygroscopic Salts

We have done a number of experiments to better understand the movement of moisture and to clarify a few important technical points, such as:

Can moisture accumulate in a breathable masonry, or moisture will fully evaporate out of a breathable wall fabric?
What is the bonding mechanism of water to masonry - is there anything that makes water stick to capillary surfaces?
Do salts make any difference to wetting at all - is there any difference between fresh water wetting (e.g. from rainwater) or saline water wetting (e.g. from seawater or moisture from rising damp)?

Here are some photos of some of the experiments we have done to get to the bottom of these questions.

We found the answers to all these questions and they were quite an eye-opener. In a nutshell, we learned that:

Moisture can accumulate in a breathable wall fabric, and the open-pore nature of the wall fabric and using lime is not a guarantee that an old wall is going to stay dry long-term. You can read more about the wetting process and the factors influencing it here.
Water bonds to capillary surfaces primarily due to electrostatic surface charges, as a result of the molecular-level interaction between the water molecules and wall surfaces. You can read more about surface charges and the bonding mechanism of moisture here.
The presence of salts impacts significantly the wetting and drying process and the breathability of old masonries. You can read more about the role of masonry salts here.

All these phenomena related to various aspects of the wetting process, moisture accumulation and retention process heavily depends on various vapour phenomena.

2. Drying and Evaporation

Vapour movement plays a very important role in the drying and the breathability of the wall fabric. While a masonry commonly gets damp by rainwater ingress which is the most abundant moisture source, the drying-out mechanism is evaporation.

In order to understand how a damp masonry dries out and which variables control the dehydration process, we have done several technical experiments. We have saturated some bricks with water, then placed them on high-precision custom-built scales and measured the natural drying and subsequent weight loss of the test bricks under carefully controlled lab conditions.

Here are some photos of the technical setup.

The findings led to very interesting results. We have observed that the drying-out of a water saturated masonry occurs in two distinct phases:

Capillary (liquid) phase: this first phase is a relatively fast and linear drying, assisted by capillary action. During this phase the masonry gives up about 2/3-rds of its moisture content during 1/3-rd or less of the total drying time.
Vapour phase: the second phase is the much slower vapour drying phase, during which the masonry gives up the rest of its moisture content, becoming completely dry. This takes about twice as long as the faster capillary drying phase.

dehydration-phases-core-conservation — Drying phases: how a damp masonry loses its water content

At the end of the fast capillary phase the moisture content of the masonry is still significant. Most most old-style bricks with a porosity of 25-30% still retain a significant (about 7-10%) moisture content at the end of the capillary drying phase (phase 1), the masonry being far from dry. The vapour drying phase (phase 2) is that brings the moisture content of the drying masonry down to (close to) dry values, however this is a much slower, time-consuming process. In order for the masonry to reach dry state, it must go through both drying phases and there is no shortcut around the much slower 2^nd vapour phase. Without the slower vapour phase the masonry would not be able to dry out.

3. Rising Damp

Rising damp is a complex phenomenon, involving multiple moisture transport mechanisms where vapour movement plays an essential role.

Technical research has shown that there are two different mechanisms that can cause rising damp: liquid water or water vapours. Depending on the height of the water table and the amount of moisture available in the soil one or the other mechanism dominates:

At normal water table [most common scenario]: rising damp can be caused by indirect capillary action as a result of excess vapour accumulation.
Commonly, the water table is situated much lower than the base of the walls. This makes direct capillary action or suction impossible due to the absence of liquid water. Above the water table we have a layer of damp but drained soil (depicted in orange on the sketch below). which evaporates its moisture content slowly upwards in form of vapours. The moisture content of the soil, even if well drained, is always vapour-saturated, being at 100% RH. This makes the wall capillaries near ground also vapour saturated, leading to moisture accumulation.
At high water table [in special cases]: rising damp is caused by direct capillary action or capillary suction, the known "wicking effect". This only occurs during flooding or very high water table scenarios, when the water table is touching the base of the masonry, the wall being in contact with the water table. Although this can happen, it is a relatively rare occurrence, and should be regarded as a special-case scenario rather than being the everyday norm.

soil-zones-2a-core-conservation — Indirect capillary action, caused by vapour deposition and accumulation in the masonry

Water vapours evaporate into the wall fabric, where they start accumulating layer by layer, leading through a number of mechanisms to non-saturated or saturated capillary flow. This indeed checks out, as in in real life, there is very often not enough liquid moisture in a drained topsoil to sustain a capillary flow, so capillary action in the walls can only occur by vapour accumulation through a liquid (water table) -> vapour (soil) -> liquid (masonry) transformation.

Not understanding the role of vapours in the development cycle of rising damp leads to incorrect remedies. The false assumption that rising damp is always caused by liquid moisture driven capillary action - which only happens in special cases - can make some believe that by channelling away liquid moisture from the building (e.g. by drainage) will automatically prevent and resolve rising damp problems.

Good drainage is important and it can decrease the intensity of rising damp. However, because drainage can only resolve liquid moisture related problems, it won't be able to fully solve rising damp which can also be caused by vapour accumulation.

Vapour movement and vapour-transport phenomena have a VERY important role in all moisture-related processes and their effects should not be underestimated.