What actually IS rising damp?

It is a source of constant amazement to us that people still about rising damp quite knowledgeably. Surveyors, professionals in the industry, actually speak about it as though it exists.  They used to believe the earth was flat - and the moon made of cheese.  

The term 'rising damp' is not even seen or heard of on the Continent, or the United States.  Professionals in those countries give blank looks when you mention it. The dutch don't even have damp courses in their houses. They don't need them, even though many houses there are built literally in swamps.  Why?  Because water does not rise up walls.  It has been said that water rises by capillary action.  The trouble is that none of us have ever seen this happen in all the years we have been surveying. Not Once!!

Rising Damp is a term coined by the chemical industry to sell damp treatments that are never needed.  That's it, very simply.

Now - the SYMPTOMS that everyone knows about as 'rising damp' - flaky paint, crumbly plaster, dark patches on walls that look like they are damp - so what are these?

It is all about condensation:  

Air containing water vapour is able to diffuse through all building materials - this is why timber swells and dries out again, and why walls and paving dry out so quickly after rain.  Gaseous water constantly passes through the pores of brickwork, stonework and mortar, and causes no damp problems.  Stone, wood, brick and mortar are said to be in equilibrium with the surrounding atmosphere when moisture levels in the air are similar to those within a wall.

Water remains a gas until cooled below its 'Dew Point' when it becomes a liquid.  It is this condensation of water vapour within the pore spacing of walls which gives rise to damp problems.

If a wall is cooled below the dew point for a given set of parameters, water forms in a wall as 'interstitial condensation'. Effectively, fog forms within the pore spacings of the wall.  As conditions change (the sun warms the wall, wind velocity drops, rain stops) the water then quickly evaporates through breathable mortar joints, stone, brickwork and lime plasters, leaving no trace.

Those 'parameters' - well the ideal conditions have been established as this:  About 55% Relative Humidity (RH) at 15 to 18 degrees C.  Not bad eh?  About what most people's homes are.  So it is not hard to avoid problems - but don't be tempted to let things change - at those levels, your walls need to get down to about 4 degrees C for condensation to form - which is unlikely - and why we set them as an ideal.  If you raise the RH to 75% by cooking curries, and not having the extractor on, the Dew Point rises to 8 or 9 degrees - at which point you can see that some of your walls may become vulnerable - for example under the bay window, or on a cold exposed corner.

Damp problems only begin when interstitial condensation is trapped by impervious materials like cement render, cement pointing and gypsum plaster, or damp proofing slurries and tanking.  It cannot evaporate, and builds up within the wall creating the symptoms of 'rising damp'.  This is why as soon as impervious materials are taken off a wall, it dries out.  The photos on this page, of blowing plaster on the wall are a great example.  If this was lime plaster, it would not have blown off, because it's breathable.  Gypsum trapped the condensation, and blew off the wall before the condensation could get away. The wall is now dry. 

So as you can see, the cure for 'rising damp' aka Condensation - is not to inject useless toxic chemicals, it is to manage your environment:

Reduce moisture load in the house that would cause condensation.  

Don't dry clothes on radiators.  

Ventilate the kitchen when cooking.  

Ventilate bathrooms.  Ventilate sub floors.

Keep the heating on, turned down low, so the walls stay warmer - which stops condensation. 

Encourage air flow - don't block every gap.  Unblock old chimneys and let air circulate.

And make sure the walls can breathe.  Replace impermeable gypsum paint, plaster etc as possible.  This is always the difficult bit because it can be a big job. Removing plaster and cement has to be done anyway if it is perished - so the only change you need to make is to commit to using lime plaster rather than gypsum plaster, and breathable clay paints like the Earthborn range rather than plastic emulsion paints.  All these materials are inexpensive, and much more environmentally friendly than modern counterparts - so really, you are doing good for the environment, and good for yourself.  It's a no-brainer.

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