How does lime plaster work?
Plasterwork in old houses is attached in one of two ways, plaster on lath (strips of riven [split] timber over a timber frame) or plastered on-the-hard (straight onto brick or stone). All these materials are permeable, allowing moisture to pass through them and evaporate to the outside atmosphere. Generally, external walls use lath as this provides physical and thermal separation. The laths are nailed to a timber 'stud' which is generally fixed to the external wall by timber 'dooks' (wedges) which are hammered into joints between stones.
If external walls are prevented from drying out properly, the structure will begin to transfer moisture to the internal surface of the walls affected and can cause decay to these dooks and timber studs. Hair, or man-made fibres nowadays, are added to lime to add strength and allow flexibility and prevent cracking. |
The application of plasterboard and gypsum plaster make walls less permeable therefore keeping more moisture inside and increasing condensation and mould growth. If plasterboard gets damp, moisture can't escape and plasterboard will become soft and weak. Evidence of moisture held in the wall can also be see by flaking paint such as can be seen in kitchens and bathrooms, as most modern paints contain vinyls to make them washable. Again, great for modern construction that is designed to prevent the passage of moisture, and has adequate provision for extraction, but not for evaporation.
More paint producers understand there is now a market for ‘breathable’ paints as the public are becoming better informed, so there are some suitable products, available at most major retailers, that would suit an older property from a breathable point of view and still allow the owner to decorate their house the way they want. That said, it depends on teh level of moisture whether a modern breathable paint is the best or limewash or lime paint should be used. |