Dampness in buildings explained
June 28th, 2018
Dampness in buildings explained
Buildings and the materials used within their construction are complex. Many buildings will be constructed with a wide variety of different materials ranging from concrete, brick and plaster with a further variety of each such as cement, lime and gypsum.
When buildings become affected by dampness either through rising damp, penetrating damp or water leaks these materials can become contaminated by the water in their possession. Water sources aren’t always clean they often contain a degree of salt, chemical or biological contaminates that can be either natural or purposefully introduced. When water migrates through a building these contaminates can be deposited within the material and the results can often be as damaging as the water itself.
Building materials possess moisture in two ways. Firstly, what’s referred to as hygroscopic moisture. This is a moisture absorbed from atmosphere. Every material has a natural hygroscopic moisture content. Timber for example, a living breathing material which never loses it’s ability to absorb or release moisture from the atmosphere, that’s why the doors in houses sometimes swell. Timber in a considered dry environment will have a natural moisture content between 8 – 16%. For masonry materials however, this will be considerably lower.
The problem occurs when building materials become contaminated. Salts are also hygroscopic meaning they too can absorb moisture from the atmosphere. Therefore if a building material becomes affected by salt, its ability to absorb airborne moisture from the surrounding environment will naturally increase, the material will take on more airborne moisture as a result of its contamination.
So why is this a problem. Well firstly a dry material could be perceived by the inexperienced as wet simply due to hygroscopic contamination. This is often referred to as salt damp. A material not physically affected by water but affected through its hygroscopic uptake ‘airborne moisture’.
The problem often arrives due to the techniques and instruments used to test for moisture in buildings. Often the most favoured and least disruptive is an electronic moisture meter, why because they cause little, if any damage. They’re fantastic pieces of equipment if you know how to use them however, you must be aware of their limitations.
Firstly most electronic moisture meters are designed and calibrated to be used on timber where they will provide accurate moisture content results up to 30% WMC (Wood Moisture Content). So why do we use wood meters on walls?
Because buildings are constructed with a variety of different materials if we were to consider using a calibrated moisture meter for each material we would need individually calibrated devices, not just one for bricks and one for plaster, but one for each type of brick and each type of plaster. Clearly this would be totally impractical. As such we have one meter, it’s calibrated for timber but when used on masonry it provides an equivalent moisture reading, basically if this brick were made of wood it would be this wet, expressed as a percentage. Therefore, on timber materials electronic moisture meters are quantitive meaning exactly how much water is present in the timber and on masonry they provide a quamulative reading i.e. if this we’re wood.
So why? Well using a wood calibrated moisture meter on masonry does allow us to establish patterns and variances in readings taken through the wall which can aid our diagnosis. The pattern and distribution of readings can highlight the path /route of where moisture has travelled, which particularly in the case of water leaks can often lead you to the source. Therefore, irrespective of their potential inaccuracies in related moisture values, these devices do have a valued use.
The other problem with moisture meters is because they work by measuring conductivity or radio waves their results can often be affected by anything within the material which affects conductivity, such as contaminates.
In this scenario contaminates can provide a false reading, an indication that a material is affected by moisture simply through a high reading. In fact, the material cold be dry, it is simply the presence of salt and hygroscopic moisture which provides this illusion.
So how else can you measure moisture in buildings if electronic moisture meters have such limitations?
The most accurate onsite investigation is a calcium carbide test often referred to as a speedy meter. Unlike an electronic meter this test is disruptive meaning samples of the masonry must be removed from the structure for moisture content analysis. The test can however be undertaken onsite and is relatively quick and accurate.
One disadvantage of a calcium carbide test is that it will only measure a samples total moisture content, this includes free moisture and hygroscopic moisture. As such in the circumstance where contaminates are suspected this could provide a false reading again creating the illusion that the wall is wet when the measurement is simply of hygroscopic moisture from contamination.
You can test the for the presence of salts in this circumstance and often the salts present will aid identification too the source of moisture too. A simple salt analysis test will provide information regarding what salts are present and in what quantity.
So how can you tell if a contaminated wall is wet or dry?
In this scenario, most commonly the salts will be present within the surface plaster as this is where ground water salts are left during the process of evaporation. Testing beneath the surface plaster may provide more accurate results dependent upon which measurement of testing you use.
Another way is to differentiate specifically between the hygroscopic moisture and free moisture is gravimetric testing, a process referred to in the BRE document 245 ‘Rising damp in walls: Diagnosis and Treatment’.
Gravimetric analysis however is not a site-based method of testing. This process requires specific laboratory equipment and involves taking masonry samples throughout the subject structure. The samples are weighed and then exposed to a series of different environmental conditions before being weighed again at each stage. This allows a weight measurement of water uptake or loss at each stage which can be converted to a percentage of water.
Gravimetric testing is a precise and complex procedure which isn’t instant, the whole process takes a couple of days to establish however, the results will provide the differential between hygroscopic and free water content within each sample.
If you would like to know more about damp investigations, please feel free to read through our blog or contact us.
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