Condensation

Air containing water vapour as a gas 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.  This sort of water is all around us - we never notice it because it is a gas. 

Water remains a gas until cooled below its 'Dew Point' when it becomes a liquid.  This is what condensation is.

If a wall is cooled below the dew point, water appears within the wall as 'interstitial condensation' - effectively fog forms within the pore spacings of the wall.  It then quickly evaporates through breathable mortar joints, stone, brickwork and lime plasters, leaving no trace.

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.

Condensation IS 'rising damp'.  It is what the damp industry don't want you to know. They will tell you that you have the mythical rising damp, claim to cure it, and then when a problem appears, blame it on condensation.  This is how the Property Care Association members get away with never paying out on GPI (Guarantee Protection Insurance) because they simply claim they cured the rising damp and the new problem, even though it looks like the same, smells the same, and IS the same - is now condensation, and no... that's not covered.

Condensation in walls is also covered by other authorities - Historic England, Historic Scotland, National Trust, CADW - all of these organisations are well aware and publish good technical information on, condensation in buildings.  Check their websites, and you will see that the damp industry has been using this to make millions for too many years!

Why does condensation form?

If water vapour is cooled down, it condenses.  The walls of your home are full of temperature variations.  It is why we use thermal imaging cameras.  They show temperature variations - and that tells us where we may get condensation.

For condensation to take place, we need somewhere cold.  Below dew point.

Lets look at an average house - say, about 18 degrees C inside, and at 55% relative humidity.  Nice and warm, and dry.  For condensation to form anywhere - for example on a window, that window needs to be at, or below, about 8 degrees.  So if in the middle of winter, it gets down to 5 degrees, your windows may get condensation.  Now - the walls are warmer - if the house is solid walled, built with solid brick or stone, they are probably somewhere between 18 degrees and about 14 degrees close to the skirtings.  They can't get condensation forming in them either.  So no damp problems.

Now lets take a very cold wind, biting cold, blowing on a corner of the house.  It might JUST manage to cool the corner of the building enough that it drops below 8 degrees - and often does - and that means you will get a bit of condensation.  No problems though - as soon as the weather improves, the condensation evaporates - AS LONG AS THE HOUSE IS BREATHABLE!!! That means NO gypsum plaster, and NO cement render of pointing - so the condensation can get away again - it just evaporates.  Problem gone before it started. The photographs above show just this scenario - a cold corner on a Welsh cottage, with mould growth on the inside of the wall.

Damp and Condensation
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