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Straw bale sound insulation: Blowing away the chaff

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Popular opinion states that straw bale walls are good at isolating sound. Cheap load bearing straw bale houses could contribute substantially to low carbon sustainable construction. However, literature on the subject was found to be highly anecdotal. The paper presents a summary of nine laboratory and field sound insulation test reports and two especially commissioned tests. Data were compared to European party wall sound insulation criteria, and it was found that straw walls could perform as well as, but sometimes worse than, conventional constructions, due to poor performance at low frequencies. Better performance could help to promote the use of straw bales in multi-unit housing. It was found that by adding a plasterboard layer on studs to just one side of a plastered straw bale wall would allow the construction to pass all of the criteria reviewed.
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INTRODUCTION
Materials such as straw, dung and mud have been used for centuries to construct buildings in many different
countries and societies. In modern times there has been a widespread move towards the use of brick, cement
and concrete as construction materials. Man made materials such as Portland cement and steel consume large
amounts of energy in their procurement and manufacture. The use of these to form reinforced concrete or
steel framed structures provide a construction technique that can adapt to countless situations, but they are
viewed as having negative properties in regards to sustainability and environmental impact.
One material enjoying a renaissance is straw and, from the authors previous interest in the subject it
appeared that the general consensus was that straw was not just a good sound insulator but an excellent one,
outperforming modern materials by a healthy margin. Until recently most straw bales houses have been
detached properties[1], often in rural locations and as a result sound insulation has, up until now, not been a
particular hurdle for straw bale builders. There appears to be limited research into the sound insulating
properties of straw bale walls and hence it should be quantified, Allowing straw bale constructions to be
widely adopted as a mainstream construction technique for cheap, natural and sustainable houses and
schools. For this, straw bale walls must be good enough to be used as party walls and must be able to pass
commissioning tests where required.
Research has already been carried out into the suitability of straw bale housing for the United Kingdom [2]
which concludes that it would meet building regulations. Whether straw can provide sustainable acoustics,
identified as important to a low carbon future by the United Kingdom's Institute of Acoustics[3], as well as
meeting other sustainability requirements, is the major question posed in this paper.
AIR BORNE SOUND INSULATION STANDARDS
There are a number of British, International and America standards for the measurement and calculation of
air borne sound insulation appropriate for laboratory [4] or in the field [5]. A method to rate airborne sound
insulation by a single number method is set out in [6] using reference frequency curves in 1/3 octave band
data whether in R, R', DnT format.
The spectrum adaptation terms C and Ctr offer a method of adapting the single number value to take into
the account the shape of noise spectra [7]. C spectrum is described as characterizing the difference between
the sound levels in the source room and receiving room for pink noise and the Ctr spectrum for road traffic
noise. The effect of adding these spectrum adaptation terms to the single number quantities is mainly in
penalizing poor low frequency performance of partitions tested especially in the case of Ctr. DnT,w(C :Ctr) the
values for C and Ctr are normally negative thus reducing the single number weighting value when combined
with it .
Sound Transmission class is very similar to Rw and is used in the USA. Laboratory tests similar to ISO 140-
3:1995 are performed to the ASTM E90 and then a single number weighting is applied to the results in a
similar manner to ISO 717-1:1997 using the ASTM E413. The main difference in the weighting method is
that the ASTM method frequency range is 125 Hz - 4 kHz as opposed to the 100 Hz - 3150 Hz range of the
ISO standards. The reference curves are identical where they overlap in frequency but are applied in a
slightly different way.
Rasmussen and Rindel discuss the wide variety of criteria and descriptors in Europe in Concepts for
evaluation of sound insulation of dwellings - from chaos to consensus? [8]. It has been updated with current
Northern Ireland and Scottish legislative requirements, as shown in Table 1.
Table 1. European sound insulation descriptors and criteria.
REVIEW OF STRAW BALE SOUND INSULATION TESTS
An extensive review of print and electronic media revealed an overwhelmingly a primitive view of the
notion of super sound insulation performance but with little empirical evidence. Much of the literature failed
to provide sources for claims of good sound insulation and when they were provided they often led back to
experiments that were of an amateur nature or intended as rough assessments of performance.
The search revealed four laboratory airborne sound insulation tests on straw bale walls (Eindhoven [9],
FASBA [10], GraT [11] and BRE-Modcell [12]) and five field sound insulation tests (DELTA [13], Genesis
Centre [14], Waddington [15], Martin [16]and MACH-Modcell [17]); all nine of these tests were room to
room except for the Modcell test which was for a facade wall. The tests are for various construction types and
thicknesses and the results are given in several different descriptors. All of the data from the nine tests above
will ,where possible, be converted to the Approved Document E standard of D nT,w. A full literature review is
available [18].
STRAW BALE SOUND INSULATION TESTS
Two field airborne sound insulation tests were carried out to ISO 140-4:1998 on the houses at Waddington
(see Figure 1) with one for the ground floor party wall and the other for the first floor. At Ralegh's Cross (see
Figure 2) just one set of tests were performed for the ground floor. The equipment used for the tests consisted
of a Bruel and Kjaer Type 2250 sound analyzer with 1/3rd octave and reverberation software and a Lookline
D301 omni-directional loudspeaker with internal power amplifier and signal generator (Figure 3).
Figure 1. Waddington Semi-detached House. Figure 2. Ralegh's Cross Semi-Detached House
The party wall bales were approximately 450mm wide with a 30mm coating of lime plaster on each side.
Both walls failed the commercial tests and those carried out for this project. A further set of commercial tests
were undertaken after a stud wall had been built on one side of the party walls in one of the houses. This
consisted of two layers of high density 12.5mm plasterboard mounted on timber studs with 755mm of
Isowool insulation behind and a minimum 10mm air-gap. In this instance the results greatly exceeded the
criteria in Approved Document E at the expense of floor area in one of the houses, see Figure 5.
For Ralegh's Cross it was discovered that the plaster work on the party wall between the two kitchen areas
had only received a rough coat, so was not complete. In addition, 4 apertures were apparent and only two
18mm plywood boards were available to block them downstairs. These factors meant that full compliance
with ISO 140-4:1998 was not possible but it was felt that a test was still worth doing as it would still provide
data which could possibly be compared with a full test once the housing is completed at a later date, see
Figure 5.
Figure 3. Equipment in source room - Ralegh's Cross. Figure 4. Viewing Panel in Ralegh's Cross
At Ralegh's Cross viewing panels were placed in the party wall directly opposite each other (Figure 4).
Although the viewing panels were located in the hallway away from day to day areas of the house and not yet
finished with their glass, when pink noise was played in the source room at maximum volume it was apparent
that the noise increased by the viewing panel in the receiver room. This is a design feature that would warrant
re-measurement on the completion of the house and should be avoided in the future, as it compromises
performance.
Measurement Problems
Both homes were not ready for testing on the specified day. Ralegh's Cross also contained building
materials which could also have influenced the measurements. Additionally, the house had all of its windows
open and the authors were told by a member of staff at the Inn that this was to let it dry out; whether this was
because the plastering had been completed in the recent months or there had been a damp problem was not
clear. When attempting to close the windows it was found that many of them were stiff and one or two could
not be closed properly and had small air gaps. The front doors had similar problems. It was not clear whether
these problems were caused by damp, by the building shifting or by poor installation but if may be a
contributory factor in the tests.
LABORATORY AND FIELD TEST COMPARISONS
All of the 450mm straw bale field sound insulation tests are shown in Figure 5. For clarity, the 1/3rd octave
DnT results for Waddington were arithmetically averaged as were those for the Genesis tests (ADE method).
Apart from the MACH (Modcell) tests there seems to be a shared coincident dip at around 125 Hz to 200 Hz.
The MACH (Modcell) results seem to follow a very different pattern from the rest of the tests with the best
low frequency performance at 100 Hz to 125 Hz but with worse performance even than Ralegh's Cross
further up the frequency range. The graph is not entirely correct as the Y-axis is labelled DnT but of course
the MACH (Modcell) tests were to a different standard (ISO 140-5:1998) and use the R'45º descriptor so
cannot be properly compared; the data has been included however as the shape of the curve still shows the
same inter-frequency relations. Poor sealing of the window may have compromised the MACH (Modcell)
test.
Table 2 European criteria compliance
Both the laboratory and field sound insulation tests exhibit a wide range of results. Nearly all partitions
exhibit fairly poor sound insulation at low frequencies usually caused by a resonance or coincident dip
between 125 Hz and 315 Hz. The only partitions with good low frequency sound insulation were the GrAT,
Martin and final Waddington partitions. These partitions all feature a composite design of straw bale with an
added panel element. Table 2 reanalyses the data for the purposes of European compliance.
CONCLUSIONS
It was found that most types of plastered straw bale wall would either not meet the criteria or only exceeded
it by small amounts. It has been established that a straw bale wall with plaster one side and a double layer
plasterboard partition the other side can satisfy all European sound insulation criteria for dividing walls
between dwellings. However, the use of plasterboard and extra labour needed for this reduces the
sustainability of the walls as well as increasing the width of a wall which some might already find
unacceptable, especially in parts of the United Kingdom where land is at a very high premium and space
therefore limited by cost.
To summarize the project: the idea that straw is a super sound insulator is not apparent from the evidence.
Reasons for this belief may be down to the lack of multi-unit housing using straw bales to date.
Improvements to the performance will need additional layers of material and careful flanking design. A
serious programme of research is needed to improve sound insulation and if directed at Nebraska style
construction, the cheapest technique, could help push straw bale construction as a mainstream technique for
house building in the UK and Europe.
Figure 5 Comparison of all 450mmm straw bale wall sound insulation tests
REFERENCES
1. Simons, M. W. and Waters, J. R. Sound Control in Buildings: A Guide to Part E of the Building
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the United Kingdom? Sustainability: The Journal of Record, 2(6):368{374, 2009
3. Rogers, P. Acoustics and the UK's approach: Towards sustainable acoustics. In ICA Madrid 2007
proceedings, Madrid, September 2007
4. BS EN ISO 140-4:1998 Acoustics - Measurement of sound insulation in buildings and of building
elements - Part 4: Field measurements of airborne sound insulation between rooms
5. BS EN ISO 140-3:1995 Acoustics - Measurement of sound insulation in buildings and of building
elements - Part 3: Laboratory measurement of airborne sound insulation of building elements
6. BS EN ISO 717:1997 Acoustics - Rating of sound insulation in buildings and of building elements - Part
1: Airborne sound insulation.
7. BS EN ISO 717-1:1997 acoustics - rating of sound insulation in buildings and of building elements - part
1: Airborne sound insulation, 1997
8. Rasmussen, B. and Rindel, J. H. Concepts for evaluation of sound insulation of dwellings - from chaos to
consensus? Budapest, Hungary, 2005
9. Dalmeijer, R. Straw bale sound isolation and acoustics. The Last Straw - The International Journal of
Straw Bale and Natural Building, (53):8{9, 2006
10. Teuber, W. FASBA Airborne sound insulation test. Number A 59829/3950. June 2008. A report. Institut
Fur Akustik Und Bauphysik
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12. Straw bale house project. Available at: http://www.bath.ac.uk/features/balehaus/ [Accessed 03.01.2011
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14.The genesis project. Website. Available at: http://www.genesisproject.com/ [Accessed 13.04.2010].
15.BBC NEWS England Lincolnshire council's straw homes to be built. Website. Available at:
http://news.bbc.co.uk/1/hi/england/lincolnshire/7948843.stm[Accessed 30.11.2009].
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17 Mach acoustics. acoustic solutions for buildings. Website. Available
at: http://www.machacoustics.com/?gclid=CKSz5M6M8KsCFYMKfAodahSAKQ [Accessed 19.03.2011].
18. Herwin, P. Straw Bale Sound Insulation- Blowing away the chaff, Masters dissertation, London South
Bank University 2011.
... 16. Sound insulation field studies from Deverell [89], Dance [90], Cascone [18], D'Alessandro [19,62], Wall [66]. ...
... In general, the airborne sound insulation performance of a straw bale wall depends on the finishing such as plaster, acoustic board, etc. as reported in Teslik et al. [88] and Marques et al. [14], and less influenced by the straw bale itself which are typically protected within the finishing. The field tests on the straw bale buildings showed a mixed results, but in general poor performance was observed by Dance and Herwin [90], Cascone et al. [18], and D'Alessandro et al. [19], even if studies from Deverell et al. [89] and Wall et al. [66] showed a satisfactory result. However, the poor airborne sound performance on the tested walls may not be due to the straw bale itself, but by the workmanship, flanking factor or finishing methods. ...
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Sound Control in Buildings: A Guide to Part E of the Building Regulations
  • M W Simons
  • J R Waters
Simons, M. W. and Waters, J. R. Sound Control in Buildings: A Guide to Part E of the Building Regulations. Wiley-Blackwell, first edition first printing edition, January 2004.
Acoustics and the UK's approach: Towards sustainable acoustics
  • P Rogers
Rogers, P. Acoustics and the UK's approach: Towards sustainable acoustics. In ICA Madrid 2007 proceedings, Madrid, September 2007
Acoustics -Measurement of sound insulation in buildings and of building elements -Part 4: Field measurements of airborne sound insulation between rooms
  • Bs En
BS EN ISO 140-4:1998 Acoustics -Measurement of sound insulation in buildings and of building elements -Part 4: Field measurements of airborne sound insulation between rooms
Acoustics -Measurement of sound insulation in buildings and of building elements -Part 3: Laboratory measurement of airborne sound insulation of building elements
  • Bs En
BS EN ISO 140-3:1995 Acoustics -Measurement of sound insulation in buildings and of building elements -Part 3: Laboratory measurement of airborne sound insulation of building elements
Acoustics -Rating of sound insulation in buildings and of building elements -Part 1: Airborne sound insulation
  • Bs En
BS EN ISO 717:1997 Acoustics -Rating of sound insulation in buildings and of building elements -Part 1: Airborne sound insulation.
Straw bale sound isolation and acoustics. The Last Straw -The International Journal of Straw Bale and Natural Building
  • R Dalmeijer
Dalmeijer, R. Straw bale sound isolation and acoustics. The Last Straw -The International Journal of Straw Bale and Natural Building, (53):8{9, 2006
FASBA Airborne sound insulation test. Number A 59829/3950
  • W Teuber
Teuber, W. FASBA Airborne sound insulation test. Number A 59829/3950. June 2008. A report. Institut Fur Akustik Und Bauphysik
Straw Bale Construction -Autonopedia. Website, Available at
  • Autonopedia
Autonopedia.com. Straw Bale Construction -Autonopedia. Website, Available at: http://autonopedia.org/buildings_and_shelter/Straw_Bale_Construction.html#Acoustics_of_straw_bale_struc tures[Accessed 13.05.2011].