Gypsum plasterboards are a quick and cheap way of covering up walls. Combined with a skim of gypsum plaster this is the predominate way of getting a smooth finish on internal walls in the UK.
Gypsum is a common mineral and the boards are fairly energy efficient to produce (only requiring a temperature of 150 degrees centigrade), however they are damaged by water and also support mould growth. So you have to be careful where the boards and plaster are used. In high humidity / damp rooms (bathrooms and kitchens) it is recommended that other boards are used. However by using these water resistant boards one effectively seals up the room and you then have to rely on mechanical extract to remove the warm humid air.
Gypsum therefore suffers a bit from its hygroscopic nature (attractiveness to water) as it will eventually fail and disintegrate from the water absorbed into its structure. By retaining dampness it also encourages mould growth. So whilst gypsum might be an OK plaster (it does give a very fine finish) for more modern buildings when applied in the right areas, it is not an universal product for all building types and locations.
Where breathable solutions are required (in older solid walled homes) the original plasters used were lime based. These naturally inhibit mould growth as they are more alkaline and they are also breathable rather than hygroscopic. So water vapour can pass through them without effecting their inherent structure.
So when choosing plasters it is worth considering the alternatives:
1. Lime putty plaster
2. Clay plaster
3. Cement (hydraulic lime) plaster
4. Insulating plaster
This is the type of plaster used in the older houses. It is a more course finish in the main as it uses aggregates to bind the structure together. However to get a fine finish it is possible to use a final coat of pure lime putty. These plasters are highly breathable and allow excess moisture into the wall structure when required and then release it as internal relative humidity drops. In other words they can act as humidity regulators in the home. This can be seen when using clay paints as a final painted finish. The clay paint darkens as the humidity increases and then lightens as it dries out again. So lime plasters are ideal for old solid walled homes where the breathable nature of the walls is intact.
2. Clay plaster
Clay plasters can only be used a skim coat. They are ideal for going over lime renders / plasters to give a very fine finish. There has historically been an issue with dusting from these plasters as they have no binders in them. Effectively they are dried clay on your walls. However, new innovations have meant that now you can create a non-dusting finish. By using clay you have a plaster that can be dug out of the ground and applied directly to your walls – now that is really low carbon!
3. Cement (hydraulic lime) plaster
These plasters are designed to be more robust and hence are less breathable. They are good where you want a quick set, a harder wearing plaster and one that is getting towards waterproof. These plasters vary in strength depending on the lime / cement used. NHL Lime 2 will give a softer and more breathable finish than a 3.5, which in turn is more breathable than a NHL 5 and so on up to cement. They need an aggregate to work and so the finish is determined by the size of grain in the aggregate.
4. Insulating plaster
The new breed of modern insulating plasters say that you can get a fine finish on them, however having tried them out it would take a lot of effort to achieve this. So they are good as renders, but would require a skim over the top. They are also breathable and so can take a clay or lime plaster to give the final top coat. Obviously these are great in areas where you are removing renders and plasters and you want to achieve an insulating result on an external or party wall.
Overall your choice in plaster will be driven by a range of factors, however we would recommend using lime and clay plasters if you are refurbishing an old solid walled building, or you want to create an eco-build.
