OLD HOUSES DEAL WITH DAMP BY 'BREATHING'

This article explains how an old house might have dealt with rain and dampness when it was first built, what could have happened since to change that and  ways of reinstating the original balance.
If the fabric of an old building allows water vapour to enter but dry out again, this is called 'breathing' and means that damp can dry out harmlessly.
If water is trapped inside the walls of an old house by impervious materials or finishes it can cause decay.
A genuinely old building usually relies upon this 'breathing' but when the pores and surfaces that permit this to take place have been sealed up then problems can occur.

1  How old houses cope with rain
This section explains how traditional roofing and external wall materials worked.
The original builders of old houses had little in the way of genuinely waterproof materials which were both cheap and readily available. The challenge was to provide a good roof and walls out of the more accessible materials.
Thatch, for roofs and mud for walls are perhaps at the more extreme end of the scale but they illustrate the principles:-
A thick layer of thatch laid at the correct angle will absorb some rain but deflect the majority. If the thatch also overhangs the walls sufficiently then it will help keep them dry.
Following rain the air can circulate between the thatch stems and allow it to dry out ready for the next shower.
A wall of compressed earth would easily dissolve if allowed to get too wet, so it is built on a stone or brick base to keep out as much ground dampness as possible. But some damp will still percolate upwards so it is important that the wall is not sealed higher up so it can dry out.
The walls of earth buildings were often coated with a thin skin of earth plaster and covered with limewash, both are breathable materials and help smooth the surface to deflect rain. [For information about limewash see our article:Lime & old house repairs.]
After rain, any water that has been absorbed by the walls and roof can dry out. The same principle also applies to some extent to other traditional constructions, clay roof tiles and brick walls for example.
Traditional roof coverings such as tiles, wooden shingles and slates, had to be interlaced, like a fish's scales, to be able to cover a large area. This  allowed for air to circulate between each tile or slate and also allowed for movement.
Old buildings are often prone to move slightly, this was allowed for in their construction and decoration. [A separate article available now at this site looks at this subject: Movement in old houses.]

2  Condensation and old houses
This section looks at the causes of damp from within a building; how it was dealt with traditionally and the consequences of modern occupancy of old houses.
Condensation has been a problem in some twentieth century homes because the amount of washing, bathing, showering and cooking in modern households produces a lot of water vapour in the air.
This vapour turns back into water when it hits a cold surface, such as a window pane. If it finds a cold surface deep inside the construction of a wall then it deposits the water there, this can give rise to hidden decay.
The way to remove steam or water vapour is by keeping rooms well-ventilated and surfaces just warm enough to prevent the vapour condensing into drops of water.
When most old houses were built solid fuel fires would have been commonplace. These needed plenty of fresh air to help the fuel burn; air was drawn from the gaps around doors and windows. This circulation of air helped to draw out any damp in the structure. 
Nowadays the balance has changed and central heating has often been installed in old houses, which provides heat but not ventilation. The gaps around doors and windows are usually sealed up and chimneys blocked up to prevent cold draughts. Kitchens, bathrooms and showers are generating water vapour in the heart of the house. 
Consequently there is a lot of water vapour in the house looking for somewhere cold to go and turn back into liquid. Water lodged in bricks and stones can gradually destroy them by the repeated action of freezing and expanding. Water in the timbers of an old house can mean decay because it makes the wood attractive to fungi and beetles. [For more on dry and wet rots and the commonest UK wood-boring beetles, see the books Maintaining and Repairing Old Houses + Old House Care and Repair + Country Cottage Conservation displayed on this site's home page.]

3  How old houses dealt with damp in walls and floors
This section looks at the way that 'rising' damp was accommodated in old construction; how some twentieth century finishes proved incompatible with this and how remedial measures attempted to solve the problem.
Moisture can enter from the ground around and underneath an old house.Traditional materials were not able to banish damp from entering in the first place (as modern construction aims to do). So some dampness  was accepted and this could be dispersed by ventilation.
The building materials and finishes available to builders of old houses were usually 'breathable', allowing water to evaporate out of them. Dampness was able to travel through the fabric and decorations until it reached a place where it could dry out, either in the sun and wind on an outside wall or drawn into internal ventilation and out through chimney flues.
Traditional finishes such as clay floor tiles or lime-washed or distempered walls are examples of materials that can allow damp to pass through on its way to dry, without necessarily spoiling the surfaces.
A present day problem is that during the twentieth century new paints and floor and wall finishes were developed that were suitable for modern buildings but these were also used on old houses.
Some characteristics of some of the new finishes, for example being glossy and washable, often also meant that they were less vapour-permeable than traditional finishes. Consequently they might stop the building drying out adequately and 'bottle-up' dampness, resulting in blistering and staining. [To find out more see our article: Paints & paint removal.]
Instead of acknowledging that perhaps old houses needed to behave differently and be furnished differently to modern houses, there was a demand for applying various damp-proofing systems to old houses to try to make them behave like new ones.
When building a new house it is relatively easy to see that all the damp-proofing measures are in the right place. However it is less easy to see what is going on when the potential pathways for damp are hidden inside an unknown old house construction.
Sometimes extra layers of relatively impervious renders were added as an extra barrier. This further decreased the vapour permeability of the structure, making 'breathing' more difficult.
A simpler solution can sometimes be possible and effective.  That is to return to the way the house was intended to deal with damp. This means allowing maximum acceptable ventilation, together with appropriate heating and using only suitable 'breathable' finishes inside and outside over a totally 'breathable' fabric. 

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