Why do old houses need to breathe?
Because they were built without a damp-proof course or a cavity, and are designed to absorb moisture and release it again. Sealing them with modern materials traps that moisture inside the fabric, which is how well-intentioned improvements cause damp, decay and the destruction of the very walls they were meant to protect.
The two philosophies:
Modern construction keeps water out. A cavity, a damp-proof course, a damp-proof membrane and impermeable finishes form barriers; moisture is excluded and ventilation is controlled mechanically.
Traditional solid-wall construction manages water. Rain penetrates the outer surface, and lime mortar, lime plaster, permeable paint and generous ventilation let it evaporate back out. The wall is wet and dry in cycles, and that is normal.
Why lime matters specifically. Lime mortar is softer and more permeable than the bricks or stone around it, so moisture and salts move out through the joints, and the mortar takes the damage rather than the masonry. Cement is harder and less permeable, so water is forced through the bricks instead — which spall and crumble, while the pointing survives perfectly. Repointing an old wall in cement is one of the most damaging things routinely done to historic buildings.
Lime is also flexible, accommodating the movement old buildings make, where cement cracks.
What else traps moisture: impermeable masonry paint and cement render on solid walls; vinyl wallpaper; gypsum plaster on damp walls; modern waterproof renders; sealed chimneys with no ventilation; and internal insulation fitted without understanding the resulting dew point.
Why blocking ventilation backfires. Old houses ventilate through chimneys, air bricks, suspended floors and loose joinery. Removing all of it — draught-proofing every gap, blocking airbricks, sealing chimneys — while adding showers, tumble dryers and lower heating produces condensation, mould and rotting floor timbers.
Doing it properly: use breathable materials throughout; insulate with vapour-open systems like wood fibre rather than sealed plastics; keep airbricks and underfloor ventilation clear; cap chimneys but leave them vented; and get advice from someone experienced with traditional construction.