Breathability

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Modern homes are designed to be watertight, being built with waterproof, non breathable materials – dense bricks, cement mortars and renders, waterproof masonry paints and damp proof courses.  When used correctly, these things work well to keep water out.

Old houses are constructed completely differently.  They are built of stone, brick, timber and earth and were originally held together with earth or lime-based mortars and often covered with earth or lime based plaster, render or paint. These materials allow moisture to penetrate the building and then evaporate away  – they are “breathable”. (This breathability refers to moisture rather than air.)

So, an old building (or a new solid walled structure built of stone, for example) is not waterproof and is not completely dry. However, the combination of breathable materials and ventilation traditionally kept these homes feeling dry and comfortable.  Externally, the porous materials dried in the wind and sun. Internally, air movement – through the roof covering, windows and chimneys – promoted evaporation of moisture from the internal walls. There was often an open fire, which accelerated this process.

Problems arise when old buildings are repaired or renovated using new materials, particularly cement based materials such as mortars and renders and gypsum based plasters.  Also, double glazing, roof insulation and other attempts to improve insulation can reduce ventilation and cause condensation.

To preserve your old house, it is therefore vitally important to use breathable materials and to provide adequate ventilation. For more information, see our other pages on lime and damp in old houses, or get in touch.