Getting the basics right - The Hempcrete Topic: Designing and Building with Hemp-lime - page 104

amples of bad press for hempcrete arising from these issues, we feel it would be useful to
explain why they can happen, and to reinforce the message that a well-trained workforce
which understands the materials is the only sure way to avoid contractor error.
Any problems that do occur are likely to be caused by one of the following factors.
Contractor error
This is the most common cause of problems, especially the problem of slow drying. Most
of the perceived problems with hempcrete are in fact the result of builders being unfamiliar
with lime, and with natural, plant-based materials, and in particular with the procedure for
mixing and placing hempcrete. Issues occur because insufficient thought has been given
to the time of year, weather conditions, drying times and management of drying once the
material has been cast.
Well-publicized examples (including on TV programmes) have included novice hempcrete
builders casting new-build houses on large-scale commercial projects during the winter
months, using a slow-setting binder and applying finishes too early.
Incorrect amount of water
In particular, it is vital to understand the importance of adding exactly the right amount of
water when mixing, as too little can result in the binder not setting properly, and too much
can lead to excessive delays in the hempcrete drying. For more on this, see Chapter 15,
page 192 .
Applying finishes too early
Where this occurs, it is usually due to pressures of the build schedule, combined with inex-
perience of the contractor. The waiting time for the wall to be dry enough to plaster will be
unnecessarily extended if too much water is added at the mixing stage, so contractor error
at that early stage can have knock-on effects at the end of the process.
All finishes for hempcrete should be vapour permeable, so the cast hempcrete will continue
to dry out through the finish. However, by applying finishes we reduce the amount of vent-
ilation to the surface of the wall, especially with plaster/render finishes, which also reduce
the wall's surface area. These two effects together can considerably extend the amount of