Rising damp was first mentioned by the Royal Institute of British Architects in 1862.
Rising damp is often mis-diagnosed from salesman trying to flog a damp course of by incompetent builders or even building surveyors. The big question is who do you trust? Do you trust a damp proofing company? A RICS building surveyor? A builder? Ideally all damp diagnosis surveys should follow BRE Digest 245, using this methodology and laboratory analysis you will have factual data determining what the actual dampness is, this will also provide the necessary information on the remedial repairs that will work.
Rising damp can rise higher than most people think.
Rising damp that was caused by a bridging cavity
The stain above the radiator is a hygroscopic salt band, typical of long term rising damp.
Another hygroscopic salt band on an internal wall, typical of rising damp.
Active rising damp on an internal wall at low level.
Rising damp and a hygroscopic salt band on the internal side of an external wall.
Hygroscopic salt band on the external side of the wall, typical of long term rising dampness.
Don’t get me wrong damp meters are great bits of kit for non destructive surveys, but they will not differentiate between moisture and hygroscopic salts.
As you can see in the picture below the damp meter is showing high readings, this is because the wallpaper has been in contact with hygroscopic salts from long-term rising damp. There is obviously no rising damp, I know this as I oven dried it, and the test is after it was dried!
As you can see in this picture the damp meter is showing high readings, this is because the damp meter is in contact with the old lime plaster, again this lime plaster is bone dry, there is no rising damp!
Don’t be fooled by the speedy-carbide meter, these are marketed by many people claiming to be using expensive equipment, these meters are cheaper than decent damp meters and are for sale second hand on Ebay for £50.00, they will not differentiate on site the difference between free moisture and hygroscopic salts, they will only tell you the total moisture content. This is well documented In BRE Digest 245
What is a speedy-carbide meter
The Speedy meter-carbide meter is a portable system comprising a vessel with an integral pressure gauge, a weighing scale and a carry case. A small sample of the material is prepared, weighed and placed into the vessel. A reagent is then added and the vessel is sealed and shaken to mix the reagent with the sample. Free moisture within the sample reacts with the reagent to produce a gas and pressure rise within the vessel that is proportional to the amount of moisture. The moisture content value is then read directly form the calibrated pressure gauge.
What is the speedy meter good for?
Testing moisture of a sample, BRE does approve it but this is destructive of the sample taken and if you need to profile – as the BRE says, it takes a long time to do all of them.
Also the speedy meter will only give total moisture content (TMC %), this is made up of hygroscopic moisture content (HMC %) + free moisture content (FMC %).
This basically means if there has been a rising damp problem there will most probably be chlorides and nitrates in the plaster/wall, these are hygroscopic salts from the ground. So when accessing a rising damp problem we need to know how much free moisture is in the wall (FMC, water) and also how much hygroscopic salts are in the wall (HMC, Nitrates & Chlorides), the amount and the distribution.
I will give you an example, you have a reading of 5% on the speedy meter, what is the free moisture (FMC, water)? What is the hygroscopic salts (HMC, Nitrates-Chlorides)?
It’s impossible to tell, because the speedy meter will only tell you the total moisture content! HMC + FMC.
A carbide can be used for hygroscopic testing but only if you double the size of samples and then subject half the sample to the carbide test and half to the 75%RH and weight test. This means the sample must be well mixed and equal – tricky if it is not homogenous and also the longer you leave it about the more weight it loses or gains before it is analysed.
So why is this important
It’s important because for diagnosis of rising damp quantitatively on site, a full profile to BRE Digest 245 is impossible with a speedy meter. Samples need to be left in a desiccator for a minimum of 24hrs to reach equilibrium @ 75% relative humidity to allow the sample to gain wait if there are hygroscopic salts present. This is not possible on site! A full profile will take to drill and sample a day that is why only one sample is normally taken by the so-called damp specialist who markets them as expensive testing equipment…..
The best place for a speedy meter is in the bin!!!
BRE Digest 245
If your in doubt you should purchase a copy of BRE Digest 245 for £15,I use this document for guidance and is how we carry out our surveys.
This Digest considers the causes of dampness in walls and how to remedy rising damp if found. It is important to diagnose properly the source of any dampness. If a building already has a physical damp-proof course, it is unlikely that it has failed, as most dpc materials have a long life. Various defects, eg mortar droppings in a cavity wall, may make a dpc ineffective by bridging it and allowing moisture to pass up the wall. This and other mechanisms by which an existing dpc might be bridged are illustrated.
Some accumulation of salts can occur in the walls of old buildings even when they do not have a damp problem, so high readings from a moisture meter alone are not conclusive. Appendix A describes how to drill samples from a wall in order to test for moisture content and hygroscopicity. The interpretation of moisture gradient profiles is discussed.
Here is link BRE Digest 245