28 November 2016

All material contains moisture to greater or lesser degrees. Those that are porous – concrete, mortar, wood – begin their life with their pores filled with water. As they dry, the water content drops and the material contracts. Other materials, like brick, begin life completely dry and water enters its pores over time, causing it to expand. Because of this, most of a building’s problems can be traced to moisture. To prevent ill effects, understanding moisture and how it interacts with and affects building materials and elements is crucial.

What is damp?

Put simply, damp is the localised, excessive build-up of naturally-occurring moisture that has no means of escape. While we have different names for damp, depending upon where and how it presents itself, for the most part, the terms are virtually meaningless; the root of the problem is almost always excess moisture.

What types of damp are there?


The most common type of damp found in buildings is condensation. The causes include cooking, breathing, sweating, bathing, washing clothes and drying them on radiators. What happens is that warm air containing moisture from these activities hits something cool, like a wall or window, and because cold air holds less water than warmed air, droplets form. In the UK, the time when condensation is at its worst is between October and April, when we are less likely to have open windows.

To eliminate condensation, moisture production needs to be reduced. This can be as simple as opening a window; for much of the year, the air outdoors has a lower humidity level than the air indoors. Dehumidifiers can also help with reducing condensation, as can improving ventilation through systems like air bricks, trickle vents, humidistat fans, and creating positive pressure. Also, increasing air temperature (which results in increased surface temperature) can help.

Penetrating rain

When we talk about rain penetration in buildings, we are talking about the above ground admission of rainwater via some point or points in the structure’s envelope, generally as the result of gravity, wind pressure or capillary action. As a rule, penetrating rain typically affects south-westerly facing walls the most. In the UK, one of the primary causes of penetrating damp these days is poorlyinstalled cavity wall insulation. Cavity walls were actually developed to prevent water from penetrating the inner structure, serving as a kind of weather shield. However, when insulation is introduced, a bridge is created between the outer and inner walls, allowing the water access to the interior of the building. Other common points of entry include cracked exterior cladding, roofs, chimneys, roof lights, dormers, poorly fitted vents, and doors and/or windows.

The best way to address penetrating rain is to eliminate the cause. This can be as simple as clearing gutters of debris, repairing a crack, or replacing roof tiles. If the cause cannot be eliminated, there are solutions such as colourless, breathable water repellents that provide protection without changing the appearance of the brick or masonry, and ventilated dry lining systems.

Rising damp

With rising damp, the problem comes from below ground, often with masonry acting like a wick to which the moisture can adhere and enter the building. The most common way to prevent rising damp is with a damp proof course (DPC). However, sometimes rising moisture brings ground salts with it, which can continue to cause problems even after the DPC is applied. There can also be issues caused by the DPC failing, which is a high risk when dealing with physical damp proof coursing.

Today, the most popular damp proofing is chemical. An effective chemical course tries to mimic the performance of a solid physical course as close as possible. For salt problems, the solution is typically replastering. Sand and cement renders, while highly effective, are not suitable for heritage buildings. Plus, they tend to decrease the surface temperature, which increases the risk of condensation. They are also reliant upon the user getting the ratio right. Pre-packaged renovating plasters provide a more consistent mix than mix-yourself versions, and they are less dense, which results in warmer surfaces. For quick turn-around, lightweight plaster can be used in conjunction with plaster membrane; however, damp can rise above it, if an adequate DPC isn’t installed. There are also salt retardant primers and salt-resistant breathable adhesives that can be used.

The other way rising damp can affect a home is through solid floors. However, before any source of damp can be accurately determined, issues like leaks, spills, flooding, hygroscopic salts around chimneys and fireplaces, and any existing construction moisture needs to be addressed.


Although comparatively infrequent, a fourth source of moisture damage is flooding; where a structure is overwhelmed by water in a short period of time. In this case, water can enter the building as a result of extreme weather conditions or via a faulty system (leaks in water supply or heating system, blocked drainage, etc.) Flooding is typically a short-lived issue (hours or days vs weeks or months); however, the contaminants that often accompany the water can sometimes be more difficult to address than the water damage itself.

Other dangers of damp

While damp itself causes problems, there are secondary problems associated with damp that not only cause problems with a building and living environment, but can be detrimental to an occupant’s health.


Mould is the common term used to describe the various types of fungi that thrive in moist areas. Water produced by condensation is ‘pure’ (distilled), making it an ideal medium for mould growth.

Actively growing mould is not only damaging to the material that it lives on – which can compromise the structural integrity of a building – it is also damaging to the inhabitants of that building. People who spend extended time in a damp, mouldy environment can develop problems like respiratory ailments, allergies, infections and other irritations.

In addition to mould, damp environments also provide a place where dust mites and bacteria can thrive. The breakdown of the bacteria and/or mould results in volatile organic compounds which cause a musty odour, while the breakdown of the material on which mould and bacteria grow can also result in airborne chemicals, gases and particulate matter.

Measuring moisture content

Types of moisture

There are two types of moisture that can be residing (and therefore measured) in any building material: hygroscopic and capillary. Hygroscopic moisture is inherent to all building materials, and the amount is dependent upon certain factors, such as humidity. This type of moisture is to be expected and isn’t damaging. What is potentially damaging is the capillary moisture. Capillary moisture is the non-inherent, excess moisture left in soil after any gravitational water has drained away. It is the result of surface tension holding moisture in place; creating a moisture film on particle surfaces and filling the spaces in-between particles. Capillary moisture uses surface tension forces to move through the soil, wicking into plants or, in our case, into building structures it comes into contact with, without aid from (and even opposing) external forces such as gravity. This movement is generally referred to as capillary action, capillary motion and/or wicking.

Measuring moisture in walls

There are various ways to measure moisture content, depending upon what type of material you’re working with and the conditions that you are working in.

Electrical moisture meters: while electrical moisture meters serve a purpose, they must be used and understood properly, or else you can come away thinking that you’ve got a moisture problem when you don’t or vice versa. The primary issues with electrical moisture meters are 1) they can be misleading in that they don’t accurately measure dampness in masonry substrates, and 2) the reading is dependent upon the probe’s area of contact with the substrate; that is, your readings can differ according to how well the probes penetrate the material. It is also important to recognise that these types of meters are incredibly sensitive to certain kinds of soluble salt contamination and small amounts of free moisture.

Taking samples: BRE Digest 245: A more accurate means of determining moisture content is by testing samples. Unfortunately, sample-taking is an invasive procedure that many building owners are not inclined to want performed. BRE’s Building Research Digest 245, Rising damp in walls; Diagnosis and treatment recommends that brickwork samples are taken for laboratory analysis using carbide or gravimetric mtesting. While taking brickwork samples does result in 10mm in diameter holes being drilled, it is far better than having plaster removed to install a chemical damp proofing course that might not actually be necessary.

Carbide test: In this test, brick or plaster dust samples are placed in a pressure cylinder along with calcium carbide. Any moisture present in the sample reacts with the calcium carbide and acetylene gas is formed. The amount of gas formed is directly related to the amount of actual moisture found, which can then be read by a pressure gauge. This reading is far more accurate than an electrical moisture reading, as it eliminates salts and other issues that create misleading results. While the carbide test provides overall moisture content, it does not provide information regarding which is hygroscopic moisture and which is capillary.

Gravimetric test: BRE Digest 245 also provides specific instructions on how to determine what portion of the moisture present is capillary moisture. For this test, a wall sample is weighed, put into a humidity chamber, weighed again and then oven dried and weighed a final time. A calculation is performed using those weights so that total moisture content, hygroscopic moisture content and capillary moisture content can be determined.

Measuring moisture in floors

Concrete: According to the Concrete Society, the preferred test for determining concrete floor moisture is the insulated hygrometer test specified in BS 8203:2001+A1:2009, Code of practice for installation of resilient floor coverings. Rather than measuring the moisture within the floor, this test measures the amount of moisture leaving the floor, i.e. the moisture that can adversely affect a building’s interior and environment by increasing interior humidity and heightening the risk of condensation forming.

Wood: Moisture meters designed specifically for wood can be used to determine whether wooden floors and subfloors have the appropriate moisture content. Testing should be performed at several locations, and then averaged out to determine overall content. BS 8201:2011, Code of practice for installation of flooring on wood and wood-based panels provides general guidance for determining moisture in wood flooring; however, it is important to take the environment into account when determining whether the moisture content is at the correct level or not.

Screed: Before applying a floor finish, the screed must be dried to an acceptable level, and it is the contractor’s responsibility to ensure that an accurate moisture assessment has been performed before the final floor finish is put into place. There are a few different methods used for determining moisture levels in screed. This includes the air hygrometer test (time consuming), which is the recommended testing found in BS Standards cited above (8201:2011 and 8203:2001+A:2009), the calcium carbide test (destructive) and the Tramex meter test (non-destructive).

Quantifying measurement methods across materials, applications and countries

The methods being used to measure moisture differ not only according to material and application but also according to country. The international Rilem Group has set up a technical committee (designation: 248-MMB) dedicated to quantifying measuring techniques across materials and across countries. This five year study hopes to result in an exchange of information that provides good practice guidance regarding industry application of measurement, including exploring the possibility of new alternatives to traditional measuring techniques. To read more about the Rilem technical committee and its findings, please visit: Methods of measuring moisture in building materials and structures.

This article has been edited and repurposed from “It's all about the damp”, written for the Construction Information Service.

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