Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 534

insulated (but low thermal mass) buildings. In short, hempcrete is a combination of
modern building elements (insulation through trapped air) with traditional ones (high
thermal mass), and in many ways makes use of the best features of each technology
to achieve its unique thermal performance.
Indeed, the potential of the heat store within the building created by the thermal mass
of the hempcrete can be maximized by specifying hempcrete for the internal walls as
well as the external ones. Using hempcrete internally also makes use of its excellent
acoustic insulation properties to dampen the transfer of sound between rooms. For
more information on the thermal and acoustic properties of hempcrete, see Chapter 7 .
As discussed in Chapter 21 , the render can usually be sealed to other materials using a
render stop bead and a suitable sealant such as burnt sand mastic or linseed oil mastic seal-
ant, but the hempcrete will require an expandable material to take up any shrinkage (the
material will also need to be able to withstand the wet environment if fitted against wet
hempcrete). If a membrane is used in conjunction with cladding, it can be sealed with pro-
prietary sealing tape or by compression when screwed between a batten and other building
element (see page 331 ) .
It is worth noting that the actual airtightness levels achieved by hempcrete in tests are very
good, as illustrated by the following examples. During research carried out on the 'Hem-
pod', an experimental hemp building with 200mm-thick walls built at Bath University:
The air permeability of the Hempod was tested prior to the co-heating test in
accordance with the procedures detailed in ATTMA TS1 [13] and BS EN 13829
[14] test Method A; thus the building was tested in its finished state with no tem-
porary seals. The number of air changes per hour at 50Pa (n 50 ) was 0.55, which
is within the Passivhaus limiting value of 60.6 air changes per hour. 1
An airtightness test on a timber-frame construction with hempcrete infill as part of the
Serve Project in 2010, at Cloughjordan Ecovillage in Co. Tipperary, Ireland, showed “an
excellent airtightness level of 1.12m 3 (m 2 .h)@50Pa”. 2