It can be very strange to find that you have damp patches in the middle of your walls downstairs. Paint doesn’t dry and will eventually fall off, even if it is natural paint. Claypaint applied in these areas will be darker than elsewhere and clearly shows the problem. So what has caused this?
Basically what has happened is that cement render has been applied to the old wall when damp proofing work was done. The old lime plaster was hacked off to a level of approx. 1 m and then new waterproofed cement render and gypsum plaster is applied (see picture below).
Sometimes the builders don’t actually damp proof the walls and if they do they commonly use a pressure injection or a cream based silicone. However, this is done when the wall is wet still and this makes it more difficult for the proofing systems to work. Pressure injections are not reliable and the creams, though better, do need to be placed in the lime mortar rather than the bricks.
The installation of a physical damp proof layer is the preferred option for a permanent solution, but this is more costly due to the labour involved.
Many builders, though just hope that the damp will not travel that high and that a quick re-render and plaster will do the job, however, but it does not! The damp from the, normally non-existent, foundations travels up the bricks until it can get out (this is where the new render finishes and the old lime one begins). Hence the damp patch.
The replacement of suspended wooden floors with solid concrete floors can make the situation worse as the associated damp proof membrane under the floor concentrates the rising damp into the walls rather than allowing the damp to be dissipated by the underfloor ventilation.
There is some other bad news as well. The rising water brings with it natural salts and these gradually get deposited in the plaster. When concentrations get too high the paint will react and start to flake off. Once the plaster has been contaminated it has to be replaced.
There are a variety of remedies that are dependent on the work that you are undertaking.
So this can be helped by re-instating a suspended timber floor so that the wall dries out at a lower level.
A physical damp proof course will stop any more water from being drawn up and thus allowing the walls to dry out slowly.
A chemical damp proof course can help if applied correctly. We would recommend using DryZone as it has the highest level of silicone for any treatment that is on the market at present.
You might also hack off all of the cement render and replace with a lime render and plaster. This will allow the damp to be removed from the bricks and dissipated into the internal atmosphere. If this course is taken then you will need to use a breathable paint (preferably a clay or lime paint as these are the most breathable).
Whichever course you take you will need to remove any salt contaminated renders and plasters and replace these. You can seal up the salts in the bricks / stones before re-applying the render, however these finishes are not designed to be breathable.
So there is a set of choices that you can make dependent on your feelings on the use of natural products, breathable structures etc. For more advice please call the Eco Home Centre on 02920373094.
Rounded Developments Enterprises Ltd have endeavoured to ensure that the information contained in this report is accurate. However, Rounded Developments Enterprises Ltd. accepts no liability for the use of this information.
Statement of Vested Interest
Rounded Developments Enterprises Ltd are a well-recognised supplier of a range of sustainable building products and as such have a commercial interest in some of the recommendations contained within the report. In some cases, cost estimates have been given on the basis of current quotations for similar equipment supplied by Rounded Developments Enterprises Ltd, and may not be the only equipment available. However, it is our opinion that the study offers an appropriate level of detail in view of the resources available and information provided. The authors have no expectation of any order being placed with them and would welcome questioning of the choice and costs of any equipment.