Design fundamentals - The Hempcrete Topic: Designing and Building with Hemp-lime - page 512

External render (see Figure 18(a) , on the previous page) can be sealed to the wall using
a render stop bead and appropriate (preferably environmentally sustainable) sealant, mak-
ing sure that the stop bead is of the type that has a mesh attached to it which is rendered
into the basecoat to stop the render from shrinking away from the bead itself. Render stop
beads with a mesh are made from stainless steel or PVC.
Cladding (see Figure 18 (b) ) will usually have a ventilated air gap between itself and the
hempcrete, so any join between the cladding and the wall need only be to keep out the
weather. The seal that needs to be made is between the breathable membrane (utilized in
cladding systems) and the brick wall. This can be achieved by wrapping the membrane
around, and stapling it to, a batten the same width as the cavity and fixing the batten within
the cavity to the masonry wall, with expanding tape or a mastic sealant between it and the
wall.
Taking this combination of steps will result in a better junction in terms of weather and
airtightness than any one solution alone would have achieved, and the same multifaceted
approach should be applied to other junctions within the building design.
Alkaline environment
Because of the lime in the binder, hempcrete is highly alkaline, so anything encased, or
partly encased, in a hempcrete wall will have to be able to withstand the corrosive effect of
such an environment. The worst-affected materials are metals. Steel, the metal most com-
monly used for fasteners and fixings in timber-frame construction, may corrode when it
comes into contact with hempcrete.
The process of galvanizing covers steel with a sacrificial layer of zinc oxide, which pro-
tects it from corrosion. Different galvanizing processes exist, with the cheaper method of
electroplating giving a thin layer of zinc oxide, which is shiny in appearance, and the more
expensive hot-dip galvanizing giving a thicker coating with a matt grey appearance. No
matter how thick it is, however, the zinc oxide is a sacrificial layer and will eventually
corrode, exposing the steel beneath. What is more, when screwing fixings (such as straps
or joist hangers) to the timber, the abrasive action of the screw turning against the fixing
can scratch off the protective layer and expose the steel beneath, speeding up the corrosion
process.
Steel is an alloy - a mixture of iron and carbon. In stainless steel, different elements are
added to the iron, depending on the type of stainless steel, but always including chromi-