Key concepts in sustainable building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 78

Carbon negative (a positive effect!)
Because many natural building materials start their life as plants, they take up carbon di-
oxide from the atmosphere as they are growing. When they are turned into construction
materials and used in a building, this prevents the re-release of the CO 2 back into the at-
mosphere, as would happen if the plant were left to rot down at the end of its life.
In effect, by using plant materials in building we are sequestering carbon: locking it away
in the building's fabric, where it will stay for the building's lifetime (after which the ma-
terials will either rot down or may be reused in another construction).
Often, and this is the case with hempcrete, 5 natural materials are responsible for negative
net carbon emissions - i.e. they lock up more CO 2 in the building than is emitted during
their production, processing, transportation and any processes involved in their disposal at
the end of the structure's life. This has led to such materials being described as 'carbon
negative': a slightly confusing term which doesn't quite have the positive ring to it that
it deserves. For this reason, other terms have been introduced in an effort to express the
concept more clearly, such as 'better than zero carbon' and 'acting as a carbon sink'.
This last is probably a more accurate description, because if the material is left to decom-
pose at the end of the building's life (instead of being recycled into another building), the
CO 2 absorbed during the growth of the hemp plant would be released back into the atmo-
sphere. If the building lasts for hundreds of years, however, the carbon storage at this cru-
cial point in history, as the effects of climate changes become increasingly apparent, is a
huge advantage.
The traditional breathable wall
Of course, building with local, natural, low-impact materials is nothing new. Before the
industrial revolution, these were the only materials that were available to people.
This led to the construction of simple, functional buildings, which were often built by the
people who would occupy them. The fabric of these buildings, more (we assume) by de-
fault than design, had an interesting quality inherent in the materials from which it was
made. It was, with the exception (usually) of the roof covering, permeable to water vapour.