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Five percent moisture in walls - is it dry?

"There's less than it 5% moisture content in the wall - the damp-proof course is working perfectly"

How many times have we heard this statement? The client has called someone back because they still have a damp problem. The 'Test' - usually involving a Carbide meter - shows less than 5% in the wall; this, the client is told, is 'acceptable' even though there may be spoiling due to dampness.

Where did this 5% 'acceptability' level come from?


Building Research Establishment Digest 245
tmcIt actually originates from Building Research Establishment Digest 245, "Rising damp in walls: diagnosis and treatment". Building Research Establishment Digest 245 is effectively derived from BRE TIL 29. Both documents are quite old, the TIL being published originally in the early seventies and the last revision of Digest 245 in 1986. The TIL, "Diagnosis of rising damp" deals with the distribution of moisture in masonry, i.e., the oven dry and Carbide meter methods for determining total and hygroscopic moisture content of materials. Digest 245, "Risingtmc=hmc+fmc damp in walls: diagnosis and treatment", covers the same ground as the TIL but adds extra explanation relating to rising damp, distribution of moisture, and interpretation of results with special emphasis on determining the difference between total moisture content and hygroscopic moisture content; it also discusses treatment of rising damp and performance of injection damp-proof courses. It is in this Digest that the '5%' figure appears.

But does it actually says this level of moisture is 'acceptable'? What are the relevant parts of the Digest which relate to 5% in masonry? The figure is referred to in a couple of paragraphs in the section on page 4, "Interpretation of results":-

"If the found moisture content (MC) is less than five per cent at the base, it is unlikely that the wall is severely affected by rising damp." The Digest also goes on to state, "Although only a rough indicator, the 5 per cent threshold does represent a reasonable general guide to whether or not some kind of remedial treatment is needed. This emphasises the importance of the difference between the HMC and MC measured on samples"

Nowhere in that Digest is it written or do they maintain, especially after damp-proofing works:

* A moisture content of less than 5% is perfectly acceptable.
* For a wall to have rising damp and it must have a moisture content in excess of 5%.
* A moisture content of less than 5% is unlikely to be rising damp.
* A wall must have in excess of 5% hygroscopic moisture content before it has rising damp.
* A moisture content of less than 5% after a wall has had a chance to dry it shows the rising damp has been effectively controlled.

All the above have appeared in reports and said to be part of the Building Research Establishment Digest; some of these are even stated in consultants reports! Clearly, such statements do not form part of that Digest. Indeed, on the latter point above, taking a moisture content at the base of the wall would certainly put it beneath any injected dpc and so any figure for moisture content obtained would be totally irrelevant to the performance of that damp-proof course! And certainly, BRE do not make comment on the expected performance of an injected damp-proof course by maintaining that rising damp has been effectively controlled if a free moisture content of 5% is reached after drying!

And one very dangerous and misleading statement from a report, "Rising damp is not normally considered to require intervention if the hygroscopic moisture content (HMC) is less than about 5%". It is the free water content that is really important - this indicates a source of water ingress. Effectively in the above quote the hygroscopic moisture content is irrelevant to water ingress arising from rising damp - the importance is the difference between the MC and HMC (ie, free moisture) as described in the BRE Digest above! And just in passing, hygrosopic salt contamination sufficient to cause HMC's in the order of 5% is pretty heavy and under some conditions probably sufficient to cause the material to appear damp solely as the result of their contamination!

It becomes very clear that those 'quoting' the Digest have not actually read it properly: they report only what they believe it contains, or more likely what they have heard it contains.


Interpretation of '5%'
There is nothing wrong with the contents of the Digest as it is actually written. Less than 5% total moisture content at the base of a wall is unlikely to represent severe rising dampness, and around 5% at the base of a wall (the base of a wall means the base, i.e., at the floor/wall junction - not 100mm, 250 mm, etc, above!) is a broad guideline to suggest some action should be taken.

However, perhaps we should look at the 5 per cent figure a little further to appreciate what 5% really means:

saturationBuilding materials vary in their permeability/porosity, i.e., they vary in their moisture carrying capacity. For saturationexample, bonding plasters can hold well in excess of 20% moisture before they become saturated whereas a dense brick may only hold around 6 - 7% before it too is saturated; neither can hold any more water because they are both saturated. Put these two materials side by side and subject them to an unlimited amount of water then it becomes quite clear that the bonding plaster could hold in excess of 20% yet the brick could only reach a maximum of 6 - 7%. In practice, it is quite possible to drill a sample from brickwork and get, say, 3% yet because it is more permeable and porous the mortar in direct contact with that brick could be 7 - 8% (see left figure). relmcontThis makes it quite clear that one cannot directly compare the moisture contents of Relative moisture contentsdifferent materials: this is a common error! And 5% in a material that can only hold say 8% (63% saturated) is probably not acceptable. Therefore, the moisture content of adjacent materials will depend on pore structure and potential saturation moisture contents.

So what Digest 245 is actually looking at is 5% in the most permeable material at the base of the wall - if it is less than 5% then it is unlikely to have a severe rising damp problem! Similarly, if the moisture content in such a material is in excess of 5% then some action may be considered. Fair comment.

It therefore becomes extremely important to determine how the moisture is distributed because hygroscopic moisture content can be in excess of 5%: this is emphasised in the Digest and discusses the interpretation of the results.



Other 'additions' to the Digest
Another 'spin-off' from the 5% figure has just become evident. In one particular report it was stated that if the moisture content is not brought down to below 5% after damp-proof course injection then the damp-proof course has failed. Nothing of this kind is even suggested in the Digest in relation to the performance of a chemical damp-proof course! Indeed the Digest explains the limitations of chemical injection damp-proof courses, and it is not surprising therefore to find levels in excess of 5% just above the injection position.

A further piece of fiction attributed to the Digest is, "The use of a Carbide meter as recommended in Digest 245." This, again, is frequently quoted in reports. The Digest doesn't "recommend" Carbide meters. However, it does show how they may be used to determine total moisture content and hygroscopic moisture content of materials, i.e., a full moisture analysis. And nowhere does it even suggest they should be used for determining a single total moisture content on a material, another practice commonly reported to be "recommended" in the Digest. Again the Digest clearly shows how to use a Carbide meter to determine total and hygroscopic moisture, and it emphasises the importance of determining the difference between the total and hygroscopic moisture contents (something that certainly cannot be done with a carbide meter on site!).

What the Digest does is to emphasise the importance of proper moisture measurement and distribution; its basis is to show the importance between total moisture and hygroscopic moisture because this actually shows that there could be some form of water ingress. What the Digest certainly does not state, or even imply, is that anything below 5% moisture content is perfectly acceptable, or that a wall must have more than 5% moisture content for it to have rising damp, etc. Neither does it recommend the use of Carbide meters, especially to determine the total moisture content of single samples. It does however describe two methods of determining full moisture distributions, i.e., oven dry method and the use of a carbide meter.

So if you receive a report which contains references to Digest 245, then you should check the authenticity of the reference/quote - most, in our experience are wrong!

A cautionary tale:
A survey identifies rising damp in a wall by means of an electrical moisture meter, and a dpc is installed a replastering carried out. Perfectly acceptable practice.

Say 4 years later during a house purchase survey a surveyor identifies with the aid of a moisture meter that the injected wall is 'damp'. The original contractor is called back and he too gets high moisture meter readings. However, this time he uses a carbide meter and shows that the wall has 'less than 5% in the brick'; this, he declares is acceptable.

Question. If the high meter readings now represent an 'acceptable' level of moisture in the wall, why then didn't the high meter readings obtained at the time of the initial survey 4 years earlier also represent and 'acceptable' level of moisture???????

Finally, it has been stated (incorrectly) that a figure of less than 5% free moisture in a wall shows that rising damp has been effectively controlled. The BRE Digest only refers to '5%' at the base of a wall, i.e., this would be beneath any injection position. So if such a figure was obtained then it would not have been through the insertion of the dpc - it would have been there before the injection work! Therefore, it must be concluded that, based on the actual BRE comment, "Although only a rough indicator, the 5 per cent threshold does represent a reasonable general guide to whether or not some kind of remedial treatment is needed", the wall may not have needed treatment in the first place!


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