Civil Engineering Reference
In-Depth Information
Cement render
When investigating the possible use of cement render to protect a straw bale dwelling
there are a number issues to be taken into consideration. The supporters of cement
render claim that it is more resistant to degradation and consequently is a better
protection of the bales. It can easily be painted, which from their perspective increases the
resistance to moisture penetration into the wall. I agree that cement render is less likely to
break down when exposed to the elements than earthen render and it is easily painted, as
is earthen render. At face value this presents a convincing argument for the use of cement
render, however there are extenuating issues associated with straw bale construction that
are not considered in this simplistic approach. There is always the exception, however,
which I shall detail at the end of this section.
The characteristics of cement render
Cracks in the render are a concern for straw bale construction, as they provide an access
point for water into the bales. In discussing render cracking we are not concerned with
shallow surface cracks, called air checks, but we are concerned about potential cracks in
the render that penetrate through the full depth of the render. On many occasions owner-
builders send me plans and specifications that other draftsmen and engineers have
prepared. Many of these have specified cement render and, without exception, have all
called for expansion joints. While our engineers would not support the use of cement
render on straw bale houses, they hold the position unequivocally that expansion joints
would be essential in such a construction.
It is generally known and acknowledged by all tradespeople that cement is quite
brittle. I agree that the use of expansion joints certainly does reduce the risk of cracking
in the render, but it does not eliminate it. The mere fact that it is necessary to have
expansion joints is confirmation of the fact that this type of render is prone to cracking.
Some time back I viewed a property in Harcourt, Victoria, that had been rendered with a
cement render. The general wall area did not appear to have cracks, however, this was not
the case around the windows. Preventing water entry around windows is a challenge in
itself, an issue covered in depth in Chapter 10, on the installation of windows. To add to
this the problem of render cracking, in my opinion, would be undesirable to say the least.
It is necessary to lay plastic under a concrete slab to prevent moisture from travelling
through the concrete and damaging floor coverings, etc. If this is the case for 100 mm
plus of solid concrete, it is obvious that cement render will have the same moisture
absorbent properties. If then, a cement render were used to eliminate moisture
penetration it would be necessary to seal the surface of the render with something like a
waterproof paint. It is quite easy to apply such a product, but is this desirable?
Moisture and condensation in conventional construction
The simple process of human habitation produces humidity in a house. As humans we
produce a humid environment simply by breathing and giving off body heat, to say
nothing of hot water usage, cooking and heating to name but a few activities.
Conventional construction homes are now wrapped in builder's wrap rather than standard
sizalation. While studying building construction at RMIT (Royal Melbourne Institute of
