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Technical & Practical |
TECHNICAL SECTION
Thermal
The thermal performance of buildings is very
complicated. Heat moves by three mechanisms,
conduction, convection and radiation. However,
building regulations have focussed on the heat
lost by conduction. Consequently most building
professionals have become obsessed by the
U-value of building elements. The U-value
gives a figure for the heat energy conducted
through a square metre of wall, floor or roof
for each degree difference in temperature
on each side (in watts per square metre per
degree centigrade). This simplification of the
true thermal dynamics of building materials
leads to some erroneous conceptions of thermal
performance of buildings.
Insulation
To calculate the thermal conductivity of a
material it is measured in a laboratory test
involving a hot plate/hot box. The nature
of this test dries the material. Their actual
performance in buildings where a degree of
moisture is present may be very different from
the dry laboratory test. Consequently very few
buildings perform thermally as predicted at the
design stage. Some buildings perform better,
but most fail to perform as well as anticipated
by the designer, despite their following current
"best" practice.
Thermal Mass
There is no doubt that walls with the same
U-values transmit heat at the same rate when
they are in a steady state. However, walls in
buildings are very rarely, if ever, in a steady
state. In fact only a portion of the heat energy
on one side of a wall is conducted through
to the other side. The rest is simply stored
and then released later. The higher the heat
capacity of a material, the greater its ability to
store heat. Tradical® Hemcrete® exhibits the
property of good thermal mass.
Air-tightness
Significant amounts of heat are lost from
buildings through air leakage. This simply
means the hot air leaks through gaps
and takes the heat with it. Hemcrete® is a
monolithic material that is inherently air-tight
and it is easy to use. There is no need for lots
of complicated layers. Just a simple solid
cast or spayed wall. This high level of air-tightness
minimises the heat lost through air
leakage and draughts. Air-tightness values of
better than 2 air changes per hour are readily
achievable.
Comfort
The human body perceives comfort by subconsciously
averaging the air temperature and
the surface temperature of surrounding walls.
It is possible to feel cold in a warm room if the
walls are cold. However it is also possible to feel
warm in a cooler room if the walls are warm.
The thermal effusivity of Hemcrete® is low
which is why it feels warm to the touch. This
warm feeling greatly improves the thermal
comfort of a building. Experience of Hemcrete®
buildings in France shows that sub-conscious
feelings of thermal comfort are achieved at an
air temperature of 1 to 2 degrees lower than in
conventional masonry structures. This means
that you feel warm even though the heating
is turned down, potentially saving energy and
money each year in reduced heating costs, as
well as saving on further carbon emissions.
Health
It is now widely recognised that Healthy
buildings are ones that deal well with moisture.
The word "breathability" is often used to
describe materials in these buildings. There
are various ways in which materials deal
with moisture (water vapour permeability,
capillarity and hygroscopicity). Tradical®
Hemcrete® performs well in all these and
therefore naturally provides a healthy internal
environment to all buildings, commercial as
well as domestic.
Research and Monitoring
It is important to design and construct
buildings that perform in reality rather than
just on paper. In order to do this we need to
understand all the properties of a material
and how these properties relate to the real
performance. There is no doubt that walls
should prevent as much heat being lost as
possible, and they should contribute to the
feeling of comfort within the building.
The hemp lime houses built at Haverhill in
2000 and monitored by the BRE yielded lots of
interesting data that confirms the information
above - see www.projects.bre.co.uk/
hemphomes/. On paper the hemp houses
had a very poor U-value (around 0.58W/m2.K)
however the monitoring showed that they out
performed the conventional brick and block,
cavity-walled, neighbouring houses with a Uvalue
of 0.3W/m2.K.
Research in Belgium, France and Germany has
shown that Hemcrete® subjected to (simulated)
sudden cooling of 20°C takes over 100 hours
to reach a steady state of heat transfer
compared to 30 hours in cellular concrete
and 12 hours in mineral wool of the same
thickness. The amount of energy lost from the
internal environment in the first 24 hours is
less in Hemcrete® than mineral wool (despite
the mineral wool having better thermal
conductivity) and less than half that of cellular
concrete despite having a similar thermal
conductivity.
Thermal diffusivity is very low for Tradical®
Hemcrete® compared to other materials, which
means it will take longer to heat up. When
subjected to sudden heating of 20°C, in the
above test, it took 850 hours for the Hemcrete®
to achieve a steady state and the effect of latent
heat transfer within the Hemcrete® was shown
to reduce the need for cooling by nearly 10%.
This is very important as we see air conditioning
costs already rising and set to rise still further
in the future.
This research highlights the importance of
considering the response of materials under
dynamic conditions to assess their thermal
performances. It also confirms that thermal
performance of Hemcrete® in transient
conditions is very good. Just 250mm of
Hemcrete® was shown to completely dampen
a sinusoidal change in external temperature of
20°C to 0°C over a 24hr cycle.
Some conclusions of the research were that:
"A specific combination of hygrothermal
parameters gives strong thermal and hygric
inertia to Hemcrete®. A high thermal capacity,
with a medium density and a quite low thermal
conductivity correspond to a low thermal
diffusivity and a relatively low effusivity.
These elements help to create a comfortable
environment in winter as well as summer
conditions".
"Bioclimatic architecture takes into account
the dynamic reality of climate, and it appears
that transient performances of such a wall
element are definitely higher than what
permanent transfer calculations would assess.
This conclusion is often observed in wood and
earth constructions. Combined parameters can
be defined on the basis of material's transfer
and storage parameters to help architects and
designers to choose materials when they wish
to optimize comfort feelings and low energy
demand of their buildings".
Hemp plasters can also be used in the
refurbishment of solid wall masonry structures
to improve their thermal performance. In a
recent French study 65mm of hemp plaster
used as part of a range of energy saving
measures reduced the heating bills of a
traditional stone building by as much as 75%.
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