Unlike modern buildings, which tend to rely on impervious materials or double skins to keep out moisture, those constructed before the mid 19th Century (later in some areas) rely on allowing the moisture, which inevitably enters the fabric of a single permeable skin, to evaporate from the surface.
Lime based mortars and renders are vapour permeable, they allow the building to breathe and so manage moisture transfer naturally.

The walling materials of old buildings in Britain are usually stone, brick, timber and earth. They are all, to a greater or lesser extent, absorbent. Mortars are usually lime/sand for brickwork and lime/sand/ aggregate for laying stone.

Lime based mortars have a number of unique qualities.

They resist the suction of the dry building materials of the construction for longer, after being laid, than other mortars. This reduces shrinkage and maintains a much more intimate contact. The durable bonds thus formed provide permanent, weather-proof, frost-resistant joints. These are now acknowledged to resist rain penetration more effectively than with other types of mortar. Movement and temperature/moisture cycling tends to produce cracking in all kinds of mortar and render but lime mortar by absorbing moisture and air allows small cracks to be closed as

carbonisation occurs in the newly exposed lime. No other mortar has this ability.

As the original mortars were more permeable than the materials they bonded, moisture in the walls was transported out through the mortar. However, where cement rich pointing has been substituted, not only does the whole wall become less able to 'breathe' out moisture, but the mortar is often less permeable than the bonded materials. Moisture transport now occurs through the stone or brick where frost action and salt deposition can cause spalling of the masonry. Thus, while gradual erosion of a building's fabric is inevitable with the passage of time, the use of softer lime based mortars not only minimises moisture build-up but can also act as the sacrificial and most easily replaceable element of the structure.

External rendering was also usually lime based and to further facilitate moisture transport out of the walls a rough texture was used which maximises the surface area for evaporation.

Walls were decorated, internally and externally, almost exclusively with lime wash which can be tinted with natural pigments. Limewash is again highly porous being the final coat in a totally breathable system, as opposed to modern emulsion paints which, in effect, lock up the walls in a plastic film.