One of the major mistakes that we see in
refurbishment of old solid walled houses is the use of modern
materials over over the original substrates. I have written before
about the use of cement render over these types of walls and what
damage this can cause in terms of damp and potential structural
failure. However, we now need to look at the inside walls.
Gypsum
plasters are too hard and brittle to flex and move with the building,
and most of them will break down in the presence of moisture. Lime of
course allows moisture to pass through it and hence can allow
moisture into the gypsum (in the case of rising damp or rainwater
ingress.) When gypsum has lost its structure it cannot regain it and
so it will then become hygroscopic and just attract more moisture to
it. If then happens then it has to be removed.
Some
gypsum has water repellents added to it, but this then just seals the
surface of a wall and prevents it from ‘breathing’.
So
either way the gypsum and the lime just don’t work together. So
really if you are working on the internal walls of your home it is
best to steer away from combining the two as there is a lot of room
for conflict. So when replastering the walls we would recommend using
lime
plaster.
If
you want a really smooth finish you can still get this by using a
clay plaster skim coat or by pressing in pure
lime putty into the plaster / render when it is still slightly
pliable. Remember though that you will need a breathable paint finish
to maintain the original functionality of the wall. We would
recommend either a
claypaint, a limepaint
or a limewash
for this.
