I’ve lost count of the amount of cavity walls we have actually cleared of debris. Any building that is cavity wall construction and has low level dampness that isn’t condensation related could possibly have bridging of the damp proof course, by this I mean debris in the cavity.

The below picture shows debris in the cavity along with wet cavity wall insulation.


The below picture shows bridging by the bonding plaster taken past the damp proof course internally.


It is rare for physical damp proof courses to actually break down, so you have to think why is there an issue. I have personally never seen a slate damp proof course broken down, to actually cause rising damp. I have heard people say there can be  rising damp issues with slate damp proof courses if there are structural issues, however if you think about it, there still needs to be mortar to form a capillary for it to transfer…….again I’ve never seen this happen.

How do we survey for possible rising dampness issues on cavity wall. Normally this type of issue is pretty simple, but you have to go looking for it, armed with a damp meter just sticking it into the wall isn’t going to tell you much.


  • Check for a  damp poof course, what height, and also what type
  • Check the external ground levels, are they below the damp proof course, above the damp proof course, level, or even high enough for rainwater to splash over the damp proof course. There needs to be a minimum of 150mm clearance between the ground level and the damp proof course.
  • Check for any external defects, like pointing issues, guttering defects etc
  • Are there any stains on the wall, indicating penetrating damp, possibly from a long-term leak
  • Walls at risk to wind driven rain
  • Has there been retro fit cavity wall insulation

IMG_5706 Here we can see cavity wall insulation in the sub floor vent, this shows the vent is blocked and that the sub floor vent wasn’t sleeved as it should have been.

  • Check drains for any blockage

It is imperative to check the cavity  for bridging of the damp proof course, we can use a boroscope which inserted through a 10mm hole. Sometimes it is possible to insert the boroscope into the sub floor vent to check for bridging and also the insulation type if present, this is most beneficial on a pre-purchase damp survey and timber survey, as this is a non destructive technique.

145642Boroscope inspection showing debris and cavity wall insulation (beads)


  • A visual survey helps identify any obvious areas, like staining to paint, peeling paint, peeling wall paper, decay to skirting, using a damp meter work out the pattern of dampness and follow the trail of suspicion.
  • A condensation survey, ideally carried out early on a winters morning.
  • A ventilation survey, to check if the extraction fans are compliant with Part F of Building Regulations. All ventilation surveys require a calibrated anemometer. A ventilation survey will also help in the risk of condensation and mould issues.
  • A thermal imaging survey, looking for any anomalies. Perhaps a gutter leak, that isn’t visible with the naked eye as the wall could be drying down, but is visible in the infrared spectrum.
  • Missing insulation causing cold bridging,

A damp meter should be profiled up the wall starting at the skirting board (this is a quantitative reading) every 250mm. If it would seem there is rising damp present, the next thing to do is to remove a section of skirting board, and break off a section of plaster beneath this. This allows an inspection of the damp proof course, to see if the plastering has actually been taken down past it and causing a bridging issue. I have been on surveys where people have presumed that the external leaf damp proof course is at the same corresponding height on the internal leaf….not always. This is just presuming, without checking it is impossible to say.

How do we know its not a mains leak?

We can offer a full gravimetric profile of the wall to BRE DIGEST 245, if there are hygroscopic salts present, typically like nitrates and chlorides in sufficient quantities (more than tap water) and the distribution of these up the wall, it’s fair to say it’s possibly a  long term rising damp issue.

Is the damp proof course still working?

Below is the only quantitative methodology you can be 100% certain and differentiate between capillary moisture (rising damp) and hygroscopic salts. Remember hygroscopic salts make a damp meter beep away even if there is no rising dampness.

The below table is the moisture meter readings taken on the plaster surface, 0mm is at floor height profiling with a moisture meter up the wall until no readings of significance are recorded. This is from a property in Wiltshire that was carried out for Wiltshire Council where they instructed us to find out what the actual cause was. We proved that the physical damp proof course was working, there was only rising damp beneath the damp proof course (as to be expected).

Screenshot 2016-08-08 22.43.53

The below table of results show a moisture profile taken using the methodology in BRE Digest 245. This shows the sample taken beneath the damp proof course has capillary moisture from rising damp, but above the damp proof course is dry, this shows the damp proof course is working. So why are there high readings on the damp meter from the floor up? This is because the plaster is bridging the damp proof course ever so slightly, this is enough for a small amount of wicking action allowing the transfer of hygroscopic salts to move from below the damp proof course to above it. Plaster samples were tested and there was evidence of chlorides and nitrate, indicative of a long term rising damp problem up to 500mm off internal floor height.

Screenshot 2016-07-01 15.21.42

What have we learnt?

It is impossible to say there is rising damp and the possible cause without inspecting the cavity with a boroscope camera, and also without removing a small section of plaster to expose the damp proof course internally. Only when you remove the plaster and expose the damp proof course can you see if there is a plaster bridging issue.

If you need a damp investigation please contact Complete Preservation on 01225 769215 or email us on enquiries@completepreservation.co.uk we will be pleased to offer our services to help you find the actual cause of your problem.

Posted by Complete Preservation

One Comment

  1. Nice work! I would love to read articles like this next time.


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