You could be making the problems worse by boarding over the walls, it sounds as if your walls would have originally been done in a lime render, and should be reinstated with the same method.
墙壁需要呼吸,登机is just hiding the problems and using any sand/cement renders will cause problems.
You need a plasterer who can do lime plastering to look at it.

Hi Joanne, Right first things first unless the walls are modern breeze block or brick the walls should always be sealed mainly because the adhesive or backing plaster will lose its key and because yours is a old farm house and the stonework or brickwork may be loose there may need to be a little strengthening first but that's only if its quite loose. Depending on what style you are after boarding the walls will give you square sharp looking room that is also warmer with the extra cavity the boards give , wet plaster gives a solid fixing but depending on hoe bad the walls are may not be as flat or square both have good sides an cost around the same I hope this information has been helpful to you Joanne.

Brian, Streamline Plasterers

I also agree with N.L.M lime render had its time and now we have new methods and materials for homes and buildings, if the stone work is fine then its just a case of either solid plastering again or drylining .

If it was my house i would hack all the old plaster and replaster all the walls with a sand and cement render with S.B.R and rendo as this will bind all together and the rendo will prevent any damp.
Hope this helps N.L.M

Bjd我认为你应该坚持你的屋顶。
the reason old houses render is cumong away its because of lime render.
Using sand and cement render on old house is FINE we do a lot off damp proofing work for tesco insurance where we chop the old LIME RENDER off and replace with modern sand/cement with the additives.
So there you are you can listern to REAL Plasterers or a ROOFER YOUR CALL