Key concepts in sustainable building - The Hempcrete Topic: Designing and Building with Hemp-lime - page 89

the amount of energy wasted in 'leaky' buildings, through heat that escapes through holes
that allow the passage of air. In conventional construction, certain materials, for example
membranes, are used for the specific purpose of adding an airtight perimeter to the interior
heated space of the building. Due to the complexity and multi-material nature of modern
construction, however, this airtight perimeter is vulnerable where sections of this mem-
brane overlap and are joined (usually by a tape), and at junctions with other materials.
The fact that hempcrete forms a continuous mass within the building's walls makes it com-
paratively easy to ensure airtightness. Although hempcrete is naturally porous, with an
open surface, it achieves good levels of airtightness when plaster and/or render finishes are
applied.
Due to its good thermal mass, hempcrete allows the use of natural ventilation of the interior
space (see below right). This avoids the risks associated with high levels of airtightness
in conventional buildings built with non-porous synthetic materials, and brings benefits in
terms of air quality.
For more information on airtightness in a hempcrete building, see Chapter 22 .
Indoor air quality
As we saw earlier in this chapter, synthetic insulation materials can cause health problems
for a building's occupants, as a result of chemicals they contain 'off-gassing' into the in-
door atmosphere. This is not an issue with hempcrete, which contains no toxic materials.
We have also seen that hempcrete is valuable in helping to manage humidity levels within
the building because of its hygroscopic nature. This too is important for indoor air quality,
because maintaining internal levels of humidity has important health consequences. If the
humidity level drops too low, there is an increased risk of allergies and asthma, and if it
is too high, the risk of the growth of moulds, fungi and mites rises. There is an increased
risk of bacterial or viral infections, especially of the respiratory tract, if the humidity rises
above or drops below the ideal range of 40-60 per cent relative humidity. 'Relative humid-
ity' describes the amount of water held in a body of air, at a given temperature, expressed
as a percentage of the maximum amount of water that body of air would hold if completely
saturated.