Getting the basics right - The Hempcrete Topic: Designing and Building with Hemp-lime - page 96

building, if not significantly longer. However, even in the event of a failure, since such
buildings always contain a load-bearing structural frame, the hempcrete could be removed
and re-cast without catastrophic effects on the building as a whole.
Because the lime binder has a lower embodied energy than cement, and because the hemp
plant takes up carbon dioxide while growing, which is then locked up in the building,
hempcrete acts as a 'carbon sink'. In other words, even accounting for the embodied en-
ergy in its production, transport and construction, a hempcrete wall represents negative
carbon emissions: it is responsible for a net reduction rather than a net increase in atmo-
spheric CO 2 . This process is what is referred to as carbon sequestration. Timber products
can also claim to be sequesters of carbon; however, hemp is superior to wood in this re-
spect, since it absorbs CO 2 much more quickly, creating a very hard woody stem (2-4m in
height) in only 4-5 months. The time taken for the production of timber is much longer,
even for fast-growing trees.
A range of figures on the carbon sequestration of hempcrete is available, although further
research is needed into this area to bring clarity. For example, one 2003 study suggests
that a total of 325kg of CO 2 is stored in 1 tonne of dried hemp. 1 Lime Technology cite
the following net carbon sequestration figures for their Tradical ® shiv and binder system:
sprayed hempcrete sequesters 110kg CO 2 per m 3 of hempcrete construction, and shuttered
and hand-placed hempcrete sequesters 165kg CO 2 per m 3 of hempcrete, depending on the
level of compaction during construction. 2
Both the lime in the binder and the structure of the hemp plant itself contribute to hemp-
crete being a 'breathable', or vapour-permeable, material. This means that any water that
gets into the wall is able to get out again, rather than being held inside it, where over time
it is likely to cause damage to the building's structure and also reduce the thermal perform-
ance of the wall. Furthermore, both the lime and the hemp are hygroscopic, which means
that the surfaces of the wall are able to absorb moisture from the air during humid times,
and release it again when the air dries out. This has the great advantage of helping to main-
tain the quality of the indoor air, and is thereby beneficial to the health of the building's
occupants. For more on this see Chapter 4, pages 58 and 65 .
The breathability of the finished material must be maintained over time through the use of
vapour-permeable finishes such lime plasters and render, and vapour-permeable paints or
limewash, so it is important that the occupants of the building are aware of this.