Hempcrete – An Environmentally Friendly Material?

Abstract

In the last decade, society has been looking at sustainability of construction. The pressure for improved construction methods also leads to the search for new materials. One possible material with suitable technical properties based on renewable resources is hemp fibre concrete – hempcrete.Hempcrete is a construction material made from hemp fibres, lime and water. This composite breathes, as well as having good thermal and acoustic-insulation properties. The paper provides an overview of international literature and its relevance to New Zealand (where hempcrete has already been used) and the Czech Republic (where the first hempcrete house is under construction).A life cycle analysis of hempcrete will be used to examine its ecological footprint, especially in reducing carbon dioxide emissions. The construction in 2014 of a New Zealand house provides data which can be used to model performance in both countries. The preliminary results suggest that hempcrete offers both environmental and construction opportunities which can help to deliver sustainable housing solutions.

Full text

Hempcrete – an environmentally friendly material?
Hana Bedlivá
1,a*
, Nigel Isaacs
2,b
1
Faculty of Civil Engineering, BUT Brno, Veveří 331/95, Brno 60200, Czech Republic
2
School of Architecture, Victoria University of Wellington, PO Box 600, Wellington, New Zealand
a
bedliva.h@fce.vutbr.cz,
b
nigel.isaacs@vuw.ac.nz
Keywords: Hempcrete, Building Material, Lime, Hemp, New Zealand, Czech Republic.
Abstract. In the last decade, society has been looking at sustainability of construction. The pressure
for improved construction methods also leads to the search for new materials. One possible material
with suitable technical properties based on renewable resources is hemp fibre concrete – hempcrete.
Hempcrete is a construction material made from hemp fibres, lime and water. This composite
breathes, as well as having good thermal and acoustic-insulation properties. The paper provides an
overview of international literature and its relevance to New Zealand (where hempcrete has already
been used) and the Czech Republic (where the first hempcrete house is under construction).
A life cycle analysis of hempcrete will be used to examine its ecological footprint, especially in
reducing carbon dioxide emissions. The construction in 2014 of a New Zealand house provides data
which can be used to model performance in both countries. The preliminary results suggest that
hempcrete offers both environmental and construction opportunities which can help to deliver
sustainable housing solutions.
Introduction
Environmental issues, ranging from global warming responses to life cycle analysis, are becoming
more important in the design, construction and use of buildings. There has been an increase in the
demand for nature-friendly methods of construction, leading to design tools and construction
systems for improved energy-efficient houses. The next step is to focus on energy-efficient
construction processes, as well using local raw materials to minimize the building environmental
footprint.
The most common, man-made, building material is undoubtedly concrete. Every year world
production is about 1 m
3
of concrete per head of population [1] but each ton of cement produces
about 900 kg CO
2
. Is it possible to find an eco-friendly material that would also have suitable
construction properties?
Clay mixtures do not have suitable thermal properties, as shown in Table 2. Houses made of
straw bales are not sufficiently resistant to moisture. [2]
One possible option is to use other, lower
environmental impact, materials. Lime, manufactured from limestone but at a lower temperature
than cement, offers an option. Like cement concrete, lime concrete is good in compression but poor
in tension, so some form of reinforcing is required. Steel is widely use, but other materials may
offer greater environmental benefits. It was found that the addition of hemp fiber resulted in a high
performing construction material. [3] But is hemp really an environmentally friendly material? This
paper explores answers to this question, using both published literature and on site experience.
History
Hemp is not a new construction material. Archaeologists have confirmed the use of hemp fiber (also
called ‘shive’) in the construction of a bridge, dated to the 6th century AD, in southern France. The
first modern use of hemp fiber composite construction was in France in 1990 for the renovation of
historic timber-framed buildings, casting the hemp lime mixture around the timber frame. These
buildings are clear proof of the durability of materials based on lime. Cement plaster found to be
unsuitable as it did not breathe, stopping the escape of moisture and promoting rot; and was not
flexible, resulting in surface cracking. The hemp and lime product proved to be a natural alternative
to cement based concrete. Further experiments indicated that the hemp lime mixture could be used
Advanced Materials Research Vol. 1041 (2014) pp 83-86 Submitted: 10.06.2014
© (2014) Trans Tech Publications, Switzerland Revised: 20.07.2014
doi:10.4028/www.scientific.net/AMR.1041.83 Accepted: 06.08.2014
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
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for floor screeds and in roofs (see Fig. 3). In the following years several residential projects, mainly
in France and Great Britain, were built. This technology is new in the Czech Republic.[4].
Hempcrete
Hempcrete is bio-composite mixture of hemp shive, lime
binder and water. A lightweight material, it is about one eighth
the weight of concrete. Hempcrete can be used to construct
walls, floors and roofs; or moulded (monolithic), sprayed or
precast (e.g. hemp bricks or panels).
Literature review
There are three different types of literature on Hempcrete:
enthusiast; instruction books; and research papers. The
following review explores the full range of publications.
Raw materials: Industrial hemp can grow, under suitable
conditions to 4 m height in 12 weeks. One Irish seller
advertises that on 1 ha of land is possible to grow 8-10 tons of
hemp a year [5], enough to build a small house. 60% the hemp
plants is shive which is extracted by machine.
Reduction of CO
2
: As with other plants, during its growth
phase, it absorbs atmospheric CO
2
by photosynthesis, trapping it in the shive and hence in the
construction for the lifetime of the building. 1 ha of hemp absorbs during its growth 4 times more
CO
2
than the same tree forest area [4]. Table 1 briefly summarises hempcrete data:
Table 1. Hempcrete Summary Data. [6][4][5]
1 [ha] hemp field
Hemp from 1 [ha] = 8 [t] hemp Shiv from 1 [ha] = 4,8 [t] shiv
Hemp from 1 [ha] = 18 [t] CO
2
absorbed Shiv from 1 [ha] = 10 [t] CO
2
absorbed
1 [m
3
] hempcrete wall
110 [kg] hemp shiv 202 [kg] CO
2
absorbed
220 [kg] lime binder 94 [kg] CO
2
emitted
Summary for a small house 108 [kg] CO
2
absorbed
Benefit of substitution of traditional brick wall by hempcrete [1m
2
] wall
A traditional brick and block wall emits in its construction 100 [kg/m
2
] CO
2
A 300 [mm] Hempcrete wall absorbs in its construction -40 [kg/m
2
] CO
2
Nett benefit 140 [kg/m
2
] CO
2
Typical house
Typical house the wall area = 140 [m
2
] Equates to = 20 [t] CO
2
For a typical house the embodied carbon dioxide 50 [t] CO
2
Carbon dioxide saving 40%
Material properties
Table 2 compares hempcrete properties with those of a number of other building materials.
Breathability: Vapour permeable (or "breathable"), helping to facilitate healthier buildings.
[7]
Acoustic: Mean Acoustic Absorption Coefficient is 0,69 NRC
[8]
.
Flexible thickness: Hempcrete is non-structural, so its thickness depends on structural
components or other requirements. It can be shaped into window and door frames, minimising
thermal bridges.
Design Flexibility: Structural design is not limited by shape, as curves can be made.
Low density: Reduced weight of the building permits shallower foundations
Figure 1. Hempcrete wall.
[Hana Bedlivá; April 2014]
84 enviBUILD 2014

Table 2. Comparing hempcrete with other building materials. [11][10][8]
Comparison
Observation of the construction of a hempcrete house in both New Zealand and the Czech Republic
provides an opportunity to explore how the material can be used. Table 3 compares the climate and
legally required R-values in the Czech Republic and New Zealand.
Table 3. Comparison of climate and R-values. [11][12][13]
Location
Annual
Temperature
[°C]
Annual
Sunshine
Hours
Annual
Rainfall
[mm]
λ of
hempcrete
[W/mK]
Hempcrete
wall
thickness[m]
Hempcrete
Wall
R
N
-value
[m
2
K/W]
Required
R
N
-value
Wall
[m
2
K/W]
NZ (New Plymouth)
12,1-14 2000-2200 1500-2000 0.065 0,3 4,62 1,90
CZ (Karviná)
7-8 1446 600-700 0,4 6,15 4,83
The houses have different designs, reflecting different climates and cultural expectations,
although both houses were made by casting the hempcrete on site. The New Zealand house (Fig. 2)
is only single storey, reflecting both the design tradition as well as providing earthquake resilience.
The Czech building (Fig.3) is two storeys, with a ground floor and attic. In the Czech Republic
internal walls are commonly made of the same material as the external walls, but in New Zealand
hempcrete is used just for the external walls, with timber framing used for internal wall.
Figure 2. New Zealand hemp house.
[Nic Farbrother; March 2014]
Figure 3. Czech hemp house.
[Marek Šedivý; August 2013]
Technologically, the houses had comparatively simple structures, requiring 3-4 semi-skilled
workers who can if necessary learn on the job in 1-2 days. Over two weeks, a timber frame was
built on a prepared concrete base. Electric wiring and water piping were laid out to minimise later
cutting into the walls. Reusable formwork was then prepared for the walls, and was lifted as each
wall level hardened. An electric drum mixer mixes hemp, lime and water in the ratio of
approximately 4:1:1. The precise ratio of ingredients depends on where it is to be placed. The
mixture was placed in the formwork and pressed, in the tradition of pisé using clubs, or in New
Zealand big hammers. Roofs and floors are raked rather than pressed. As lime is corrosive, personal
protective equipment is used.
Lime is susceptible to frost damage, so it is necessary to avoid construction in temperatures
lower than 5°C. Drying under optimal conditions takes about 2-3 months. Hempcrete can also be
applied by spraying. This method is suitable for large buildings, but carries with it the high cost of
Hempcrete
Straw Bale
Brick
POROTHERM 44
Mud brick
Thermal comfort
λ=0.065 [W/mK] λ=0.052 [W/mK] λ=0.137 [W/mK] λ=0.510 [W/mK]
Construction cost Lowest Lowest Lowest More expensive
Workability
Easy and fast Easy and fast Experience required Difficult
Pest
Pest proof Risk pests get into walls Pest proof Termites
Fire resistance
Fireproof thanks to lime Depends on render Externally fireproof Externally fireproof
Density
275 [kg/m
3
] 90-120 [kg/m
3
] 750-790[kg/m
3
] 800 [kg/m
3
]
Embodied energy Low EE Low EE High EE Low EE
Carbon footprint
Absorbs CO
2
Absorbs during growing High (kiln fired) Neutral
Damp & rot
Rot resistant Can rot if wet Damp proof Relies on roof overhang
Recycling
Fully recycled Used as mulch Recycled but high costs Fully recycled
Advanced Materials Research Vol. 1041 85

equipment. On the other hand, the mixture gets more air so has improved thermal and sound
insulation properties.
Hempcrete can also be used as prefabricated blocks and panels with higher density and lower
thermal insulation. Particular attention must be paid to filling the gaps to avoid thermal bridges.
In both countries it is permitted to grow industrial hemp. In New Zealand, hemp is grown near
the town of New Plymouth, which was where the house was built. However, no one is able to
process hemp, possibly due to the cost of setting up plant, requiring the hemp shive for the house to
be shipped from Holland. In the Czech Republic the hemp was obtained from Poland.
Hemp has low bulk density, resulting in high storage and transport costs. Local production
minimizes the environmental footprint and associated CO
2
emissions.
Summary
Hempcrete meets condition of eco-friendly material. It is made of renewable resources in sufficient
quantity. Production is less energy-intensive. It has negative greenhouse gas emissions. It provides
resistance and durability construction and healthy living condition. This material is recyclable. It is
proposed to undertake a LCA (Life Cycle Assessment) of hempcrete in the future.
Construction technology is simple and building in the Czech Republic and on New Zealand was
without unexpected problems.
This brief overview of the use and performance of hempcrete suggests that hempcrete can be
considered an environmentally friendly material. Further research is now being undertaken into the
details of the production, use and performance of hempcrete. In particular the context of use is also
being considered, as this can significantly affect the overall environmental impact. Although
hempcrete achieves very good properties, the performance of alternative materials, including other
natural fibers such as straw or flax, will also be examined.
References
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