There are several methods to establish the cause of dampness within a building structure when undertaking a survey.
Listening to what the occupier has to say is often overlooked but can be one of the best sources for gathering historic evidence. After all, they live in the property, so are likely to be the one most aware of issues with the dampness, or in some cases undertake work to cover it up.
Undertaking a good visual inspection of the property, both external and internal is paramount - I always recommend an inspection of the whole of the property, not just the area directly affected, as this can often identify reasons related to the dampness or other potential defects.
The environmental conditions both within and outside the property will have an impact on the meter readings taken. Building materials have different rates of Hygroscopicity, so the surrounding atmospheric conditions will influence their moisture content. Their uptake or loss of moisture content will be balanced with the environment they are located in. This is referred to as the state of equilibrium.
Whilst it is essential to establish the environmental conditions, it should be remembered this is only a snapshot in time and will constantly change.
The relative humidity, air/ surface temperates along with the internal and external vapour pressures must be recorded to establish how much moisture is held within the atmosphere of the area being inspected and if there is a risk of condensation forming on the surface.
A conductance meter, (commonly incorrectly referred to as a moisture meter), is a useful instrument to establish the extent of a moisture problem, it works by measuring the electrical resistance between the two electrodes or contact pads.
Moisture is a good conductor of electrical current. The higher the moisture content, the higher the reading shown on the meter. The meter is measuring electrical resistance, not moisture content, hence the 'WME scale' (Wood moisture equivalent - this is the theoretical %mc value that would be attained by a piece of wood in contact with and in moisture equilibrium with the material under test).
Readings can be affected by other influences such as metallic surfaces and hygroscopic salts, so it is essential the surveyor fully understands how to use the meter and its limitations, to be able to interrupt the readings correctly. A conductance meter cannot conclusively prove rising dampness. It can only inform the operator of the moisture profile and area affected, and further investigation will be required to establish the actual moisture level and source.
Calcium carbide meter
A calcium carbide meter is a moisture testing device used to establish the total moisture content of a sample of masonry. The readings can again, be affected by hygroscopic salts within the sample, so do not confirm where the moisture is derived from. This means it cannot prove or disprove rising dampness.
The calcium carbide test also has limitations; the test is disruptive, requiring samples to be removed from the wall. For this reason, carbide testing is not generally undertaken on pre-purchase inspections, unless permission is granted before the inspection.
The readings recorded are limited to the location of the samples taken, where a conductance meter can be used to profile large areas of a wall surface, so providing a moisture profile, with no damage to the wall surface.
So how does the carbide meter work? The calcium carbide meter is a sealed vessel in which measured samples of masonry and calcium carbide are mixed. The calcium carbide reacts with moisture present within the sample and produces acetylene gas.
The amount of gas released by the reaction is directly proportionate to the level of moisture present in the material tested, by measuring the amount of gas produced on a calibrated gauge contained within the unit we can derive the total Moisture Content (MC) of the material tested.
Gravimetric testing is accepted as the most accurate process for establishing the actual moisture content of a masonry sample and determining its moisture content and composition being described in BRE 245 'Rising damp in walls: Diagnosis and Treatment'.
Test samples are removed from the wall using destructive testing, the same as the calcium carbide test. The total moisture content of the sample is the sum of its capillary moisture and free moisture introduced from and external source rising damp, penetrating damp, water leaks and the atmosphere.
The hygroscopic moisture content, being from the materials’ ability to absorb moisture from the atmosphere, through contamination from hygroscopic salts. This indicates the moisture came from the ground. 'Rising dampness' is the burning of fossil fuels, 'Salt dampness’ is in and around a chimney or contaminated building materials, introduced as part of the building products along with 'Contaminated sand'.
No other testing process can differentiate between a samples hygroscopic and free moisture content, this method of testing is accepted as the professional standard. It must still be remembered the results are limited to the samples taken. To establish the source and the extent of the dampness, and the elements of the building at risk i.e. timbers, it is down to the surveyors’ knowledge and understanding of the subject, which requires training and experience.
Your home is the most expensive investment you are likely to ever make, it will need maintaining to stay in good working order and allow your investment to grow in value whilst providing you with a safe, dry and comfortable place for you and your family to live.
When you home develops a problem, think of it along the same lines of you becoming ill. You would seek the advice from a qualified Doctor, hopefully ‘not some guy from down the pub’.
Olympic Construction cover the North West and Cheshire. Should your home develop a dampness/mould/ decay related problem, please contact us for qualified processional advice. We also offer a ‘Home health check’ inspection service, to undertake an inspection of your home and provide you with a written report of existing potential dampness, mould or fungal decay related problems.