What is 'damp'?

This question is not as simple as it seems. The problem arises with the word 'damp' as it implies your house should be totally 'dry'. Your house must contain a certain amount of moisture for you to remain healthy and your house to remain sound.

All houses, whatever their age, contain moisture. The amount of moisture varies continually with time but usually stays within a range that depends on the building, its occupants and its environment. Moisture is present in the materials of the building itself, and the air within the building. In this country, moisture also totally surrounds the building - it's in the air, in the ground and regularly falls out of the sky.

Houses built before 1940 are designed to stop damp problems by managing natural moisture flows to maintain a healthy balance. Houses built since then try to actively stop all moisture flows – good or bad.

'Damp' really refers to the amount of moisture being higher than the desirable maximum.

The excess is always caused by one of two things:

  • a sudden increase in the amount of moisture entering the building, or
  • a build-up of moisture caused by its inability to escape.

From the early 1800s onwards, some houses had experimental types of cavity wall (they were not perfected until after 1945). Also, from the mid 1870s onwards, all houses should have some form of built-in damp proof course to help protect the ground floor joists. These features supplement the natural moisture control mechanisms of the walls. All correctly maintained houses built before 1940 are naturally resistant to excess moisture entering from outside the building and are extremely resistant to condensation (airborne excess moisture that normally arises inside the building). Houses from this period with early types of cavity wall and damp proof courses are fantastically resistant to all forms of damp.

For houses built after 1945, the use of different building materials and techniques mean the old natural moisture movements that regulated moisture levels no longer work. This makes houses from this period less resistant to excess moisture entering from outside the building than many earlier houses. It also makes them extremely prone to condensation.

The natural tendency of moisture, is to spread out from wet to dry areas, and also to move downwards under the influence of gravity. Moisture is not restricted to one type of material as it moves. It will just as happily move to and from air - the air is just another dry or wet area. It is this natural tendency for moisture to move from wet to dry, and its ability to pass into the air, that pre-1940 houses use to prevent dampness. By contrast, houses built since then attempt to stop moisture from moving – causing it to build up. As moisture builds up, it becomes more difficult to contain so the slightest imperfection can enable it to suddenly escape, creating a major damp problem.

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