Civil Engineering Reference
In-Depth Information
Gypsum plasters and
renders
it to the surface beneath. Also, lime is very
flexible, whereas cement is not. This means
that as long as there are no cracks in the
cement, it will stop water from reaching
the straw. However, due to its rigidity it is
almost impossible for it not to crack after a
short period of time, especially when it is
applied to a flexible backing material such
as straw. This means that when it rains, the
rain passes through tiny cracks and filters
down the inside face of the cement, and
collects at the bottom of the wall, where
it cannot get out. A build-up of trapped
moisture at the base of the wall over a long
period of time causes the rot to set in. This
has been identified as a major cause of
damp in old buildings, and English Heritage
has now banned the use of cement on older
lime-rendered buildings.
Natural gypsum is just starting to be used
again as a render and plaster. It is actually
even more sustainable than lime, because
it needs to be burned at only 200ºC instead
of at 1,200ºC, and it can be recycled very
easily. However, it has an undeservedly bad
reputation owing to the commercial manu-
facture of gypsum boards (plasterboard)
and the practice of applying a skim finish
to them. The glue used to stick the paper
on to the board may make it impermeable
(although this is not always the case), but
the board finish - the skim - does contain
glues and additives to make it stick to the
paper, and these are also impermeable. It
is hoped that natural gypsum, without any
additives, will soon become easier to use as
a render and plaster.
The other consideration with cement
plasters and renders is that because they
are not breathable they have the same
effect as wrapping the straw in plastic. This
lack of breathability creates an unhealthy,
unventilated atmosphere around the straw
which could lead to damp problems in the
long term.
Cement plasters and
renders
There are hundreds of strawbale buildings
in the USA and Canada that have been
cement rendered. Most of these are doing
fine and are not showing any sign of
deterioration. Some of them, however, have
become very damp as a direct consequence
of using a cement render (see Chapter 7).
In practice, there may be many instances
where you can get away with using cement,
or where the life of the building is such that
a bit of rot developing at the base of the
wall does not matter.
There is no doubt,
however, that in terms of best practice, lime
renders are superior to cement
.
Cement and lime behave very differently
from each other, and are used for different
reasons. Whereas lime is a breathable
material that holds water within itself while
it is raining, and releases it once the rain
stops, cement is waterproof and works by
preventing water from penetrating through
