Plasterwork in old houses is generally attached in one of two ways, plaster on lath (strips of timber nailed to a timber stud frame - either sawn or, more usually riven (split) chestnut or other hardwood) or plaster on hard (straight onto a brick or stone wall. All these materials are breathable allowing moisture to pass through them and evaporate to the outside atmosphere. Generally external walls are 'lath and plastered' and internal walls are 'plastered on hard'. The laths are nailed to a timber 'stud' which is generally fixed to the external wall by timber or iron 'dooks' (wedges) which are hammered into stone joints. If external walls are prevented from drying out properly, this structure will begin to decay and transfer moisture to 'patches' around the internal surface of the walls affected.
Traditionally 'haired lime' is used - hair to allow flexibility in the lime and prevent cracking, lime to allow moisture absorption, flexibility and vapour transfer through the structure.
The application of gypsum-based plasterboard and plasters make walls and ceilings less permeable therefore keeping more moisture inside, increasing condensation and mould growth. If plasterboard gets wet, the moisture cannot escape as readily and staining will occur, plasterboard will also fail, as a material, when wet. Evidence of moisture held in the wall is also visible by flaking paint such as can be seen in kitchens and bathrooms, as most mainstream modern paints contain impermeable vinyl resins. Again, great for modern construction that is designed to prevent the passage of moisture, and has adequate provision for extraction, but not for breathability.
More companies are realising that there is a market for ‘breathable’ paints and the public are becoming better informed and demanding these products, so there are some mainstream products, available at most major retailers, that would suit an older property, from a moisture permeability point of view, and still allow the owner to decorate their house the way they want.