We are renovating an old stone farmhouse and have built a wrap around extension around most of it so that the majority of the original walls are internal. However one side elevation will be behind a lean to glass conservatory to act as thermal mass heat collector and will not be rendered but repointed.Obviously this side will breathe, so do we have to be particular about how we finish the internal side of this wall i.e. use a breathable finish e.g lime plaster? The stone wall is 700mm thick in very good structural condition except that the inside has been hacked about over the years and would not make an attractive bare wall. As the house was very damp when we bought it is now very dry after we hacked off the external pepple dash cement render the internal cement render with plaster finish.and dug up the floor and laid concrete over a DPM.

  • Mark Brinkley

    It’s an interesting point and the answer isn’t clear cut, because your "external wall" will be inside a conservatory. This means that the most important source of wall moisture – the rain – will not be a factor, but it is possible that the wall will suffer from ground water (i.e. rising damp). The other problem you can get from stone walls is when you insulate inside, but I imagine you are not going to do this if you want to benefit from the heat collector effect.

    In short, it probably isn’t critical on a well protected wall like this just how breathable the internal plaster is, but why take the chance?

  • Tim Pullen

    A breathing wall woks by air moving from the warmer side to the cooler side. Generally that is inside to outside and generally that pushes any moisutre in the wall to the external surface. The wall covered by the conservatory may try to work in the other direction which could be a problem if there is any rising damp. If you use gypsum plaster on the internal surface any damp may tend to get blown off.

    As Mark says, why take the chance? Treat them all as breathing walls and all will be well.

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