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Helstråkompositter - Demonstration og teknisk vurdering
The Light Natural Sandwich (LNS) material is a new board-product that benefits from the
high strength and low weight of straw – nature’s own mast-material. Gathering
straw firmly and filling the gaps between the straw with stabilising foam produces LNS.
From the foamed bundles cores for LNS are sawn transversely to the straw direction, and a
top and bottom layer is glued on. In this project LNS was mounted in the ‘Prøvehus
II‘ in the eco-settlement ‘Andelssamfundet i Hjortshøj’, respectively as a
floor and as a wall surrounding a lavatory.
For the floor-elements straw of miscanthus, which gives the highest strength, was used.
The injected foam was from glutin, and the core was glued to 9mm birch plywood surfaces
with a casein-glue. Each element measured 875 x 1200 x 68 mm and was fitted to the next
element with a spring trim.
The not-bearing inner walls did not demand for high compression strength, and therefore
straw from rye foamed with glutin was used and glued to 9mm birch-plywood surfaces with
casein-glue. The wall elements measured 2200 x 1200 x 118 mm, weighed about 58 kg and thus
had a volumetric weight of about 186 kg/m3. Humidity sensors were built into
the elements before mounting. Two men mounted the 5m long wall including a 90o
angle and a door opening during about 4 hours.
For evaluation of humidity transport through the elements an evaporator was set up in
the lavatory and a temperature gradient was established to the surrounding room. During
the test run the humidity content of the LNS elements did not raise above what is normally
seen in outside wooden wall constructions. Inspection of the straw core 15 months after
mounting the walls showed no mould or any other sign of change. Accordingly, it is
considered that with respect to humidity the wall elements mounted in Hjortshøj are
usable for both inner and outer wall constructions. For outside walls a vapour seal must
be mounted on the warm side and the cool side must be ventilated between the wall and the
outer weather screen, analogous to a traditional light wall construction.
The rather thick LNS-elements produced for building did not have the same strength as
thinner LNS boards constructed earlier. The bending strength of the floor element was
measured to be 2.3 N/mm2, which has to be compared to LNS bending strengths of
up to 33 N/mm2 measured earlier. A prerequisite for achieving a high strength
is that the relation between straw length and diameter in the core is kept below 8.
The details of mounting LNS walls are not yet fully developed. Mounting LNS-elements to
outside walls must be optimised to ensure total air tightness. Based on the Danish
regulation for building construction (BR 95) it is considered that the low value of
airborne sound insulation (31 dB) impairs the use of the current type of LNS-elements as
walls separating apartments. On the other hand, the lightness and easy mounting of the
material opens for use as flexible inner walls.
As LNS materials are not yet produced on an industrial scale it is not possible to
perform an exact environmental assessment of the product. However, the environmental
impact is considered to be low as rye and miscanthus can be grown with low inputs and low
emissions, whole straw (no defibration) is used and the product can be combusted or
possibly composted after use. A preliminary assessment of energy use indicates that 25% of
the energy used for particleboard production is necessary for LNS production calculated
per volume of board.
LNS has been developed with the aim of creating a strong and lightweight material from
environmentally benign raw materials. The primary target area for the product was
industrial products for substitution of materials made from plastic, light-metals and
speciality wooden products such as plywood. Thus, LNS in its original form does not
necessarily fulfil the material demands for building construction e.g. with respect to
sound insulation and fire protection.
For building construction LNS may be improved by using gypsum boards as surface
materials instead of plywood. In this project a sample board using 12.5mm gypsum boards
was made. This element could, based on BR 95, fulfil the requirements for sound insulation
within one apartment, and the requirements for insulation between apartments if the wall
is build up as two separate elements with additional insulation in between.
Another idea for surface material is to use glass. With glass surfaces the attractive,
organic core material is clearly visible and gives the composite a very new and different
appearance compared to the traditional plywood and gypsum surfaces. Such a sample was
produced during the project.
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