Why old buildings need to breathe

Spending three days talking and lecturing at the National Home Improvement Show made me again realise the huge importance of getting across the message that old buildings need to ‘breathe’. Many people simply don’t understand that using the wrong materials can be an expensive mistake which may wreck their home.

The way old buildings work is incredibly simple and, when I say ‘old’, I’m not just talking about ancient structures but virtually all buildings with solid walls – these include Victorian and Edwardian homes.

What has to be understood is that the modern building techniques that we know today began to be introduced in the second half of the nineteenth century and that cement based mortars, renders and plasters only came into general use by the late 1940s. These techniques rely on impervious outer layers, cavity walls and barriers against moisture – such systems are totally incompatible with traditional solid walls which work in a very different way.

In solid walled, old buildings, the bricks and stones were generally bonded with weak and porous mortars made of lime and sand. Where external walls constructed with these materials, and others such as ‘cob’ (earth), were rendered, lime render was used and this was often limewashed so the structure was able to ‘breathe’. When it rained, moisture was absorbed a few millimetres into the external surface but was able to evaporate when the rain stopped, helped by the drying effects of the sun and wind.

Inside, walls were plastered with lime and finished with simple breathable paints. Any excess internal humidity from washing, cooking and human activity, was dispersed via open flues and draughts, or absorbed by the breathable surfaces. In addition, a kitchen range and open fires burnt from autumn until spring, drawing air through the home and keeping internal surfaces at a steady temperature. Provided the building was maintained, the structure remained essentially dry.

In recent times the understanding of how traditional solid walls work has become confused and builders have tried to apply modern techniques to breathing structures. Cement renders, along with ‘plastic’ paints, waterproof sealants, damp proof membranes and even insulation materials can act as a barrier to the building’s natural ability to breathe. This is where the trouble occurs: the mix of technologies traps water within porous materials and exacerbates the very problems that they are trying to resolve.