Like dealing with solid walls you have a couple of choices here when dealing with rooms that are below ground.
Choice 1 – Work with nature
Choice 2 – Try and work against nature
I am a Choice 1 type of guy, as nature will always find a way of getting through our defences. Ask King Canute!
So we are looking at solid walls that are porous and a subterranean location that may well be permanently damp. How to keep the cellar dry?
Well, this is a difficult aim to achieve with 100% success rate and I think really depends on each households sensibilities and access to a range of resources.
If you find that the walls are not very damp and that you are using the space as a typical cellar function (i.e. there is no pressure on keeping the room totally damp free) then the walls may well be easily treated by using a breathable finish on the walls and ensuring a good air flow. So leaving the walls bare (or painted with a lime / clay / chalk paint) and having a good draught will allow any moisture coming through the walls to be vented away before presenting any problem. Note, though that furniture etc should not be placed too close to the wall as this will restrict air flow and could lead to mould formation.
If you want to use the room as a more conventional living space then keeping it dry gets more important. Having lots of ventilation in this type of room then gets more difficult as we are not so fond on a keen breeze around our ears! So how can we allow the walls to breathe whilst keeping damp at bay?
The way the English Heritage recommend is to use a dimpled membrane. Basically this is a plastic sheet that has dimples in that create a vented space around the wall. See below:
So the plastic keeps the damp away from your new wall, but what happens to the water?
Well this is where individual factors come into play. If the walls are really wet and literally running with water then you will need to install an internal drain rather than relying on ventilation only. This drain can be directed straight into the main drains, or it might need to be fed into a sump and then pumped out (via an automatic pumping system).
So the ‘Heritage’ system is a bit of a hybrid that wants to work with nature, but has to provide some very clear guidelines in which elements are not acceptable! Purists will no doubt say that it is case of using very breathable materials, drains and ventilation, however I feel that it is down (as ever) to the individual site conditions, usage patterns and sensibilities of the client.