From a nursery rhyme to award winning construction: a modern tale of straw bale construction

Pete WalkerBRE Centre for Innovative Construction MaterialsDept. Architecture & Civil EngineeringUniversity of Bath

Straw bale construction• Developed in Nebraska, USA in the late 1800s• Only material readily available• Technique declined with availability of ‘modern’

industrial materials

Straw bale construction

• Resurgence in the 1970/80s as a result of the oil crisis

• 1990s saw the first straw bale buildings in the UK – may be?

• Three main types of construction:– Load-bearing– Post and beam with straw infill– Prefabricated panels

Straw bale construction

Properties:– Low thermal conductivity– Low embodied carbon– Renewable resource– Co-product of cereal production– Locally and widely available– Low strength and stiffness (but still suitable for

low-rise load-bearing applications)

Opportunities for natural materials in modern construction

• Reduced GHG emissionsLower embodied carbon (stored carbon)Improved building performance

• Resource efficiencyRenewable supplyReduced waste

• Healthier buildings• New agricultural markets

Straw bale self build

Straw bale loadbearing wall tests

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Standard bale constructionHalf balesNo hazel spikesCreep test wallLime Render

Static load tests

If straw bale is such a good building method and material….

……why aren't there more straw-bale buildings being constructed?

Perceptions…

• Poor fire resistance• Full of bugs and vermin• Only suited to self-build/hippy market• Poor durability• Little strength – wall hangings• Unsuited to mainstream construction

Market development: barriers for natural materials

• Certification (lack of)• Cost• Financing• Perceptions of poor performance• Supply chain• Warranty (lack of)

Products not Materials

Spot the difference…

Prefabricated Straw Bale Insulated Panels: ModCell• Main components:

• Timber framed panels • Straw bale infill• Lime:sand render

• Manufactured off-site in temporary flying factories

• Panels’ designed to be dismantled, reused and recycled

UWE, 2001

Eco-Depot, York City Council, 2006

Manufacture

Construction

Racking Shear testing

• Full size panel tests

• Horizontal in plane load applied

• Displacements and load measured

• Four different panels tested

Structural Panel Tests

Fire resistance• Fire test in accordance with BS EN 1364-1:1999

– >1000°C– 2 ¼ hours

Full scale structural testing

Environmental Performance TestingAir permeability: 0.86 m3/hr m2 @ 50Pa

Environmental Performance Testing

Co-heating test:

• 36 kWh/m2 per annum

• Represents around 70% savings on current UK stock average

Environmental Performance Testing

Wall 1: comprises 300 mm Mineral Wool insulation

U = 0.15 W/m2K

Wall 2: comprises 300 mm Hemp-Lime

U = 0.30 W/m2K

Bio-based insulation performance

Time to reach steady-state, ts-s

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Figure 1 -Temperature change in 300 mm HL wall after sudden temperature drop

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Mineral Wool 24 hours

Figure 1 -Temperature change in 300 mm HL wall after sudden temperature drop Figure 1 -Temperature change in 300 mm HL wall after sudden temperature drop

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Mineral Wool 24 hours

Figure 1 -Temperature change in 300 mm HL wall after sudden temperature drop

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Mineral Wool 24 hours

Figure 1 -Temperature change in 300 mm HL wall after sudden temperature drop

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Mineral Wool 24 hours

Figure 1 -Temperature change in 300 mm HL wall after sudden temperature drop

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Mineral Wool 24 hours

Condition monitoring

Case study

• Gravimetric MC measured at 28.4% behind render > 25% limit

• Plotting monthly average hygrothermal data showed high mould risk

However, serious degradation was not apparent.

• What does this mean in terms of assessing degradation risk?

• Are we being too conservative?• Is there something else happening?

28 days exposure 7 days drying (20°C/70%)

28 days exposure

Hygrothermal Testing (ETAG 004)

Heat-rain cycles (80)• Raise temp to 70oC and

10-30% RH (over 3 hours)• Spray rain (15oC):

1 /m2/min for 1 hour• Leave 2 hours before

repeating

Heat-cold cycles (5):• 50oC for 8 hours• -20oC for 16 hours

Inspire, Bradford, 2012

Hayesfield School, 2012

LILAC housing, 2013

Product certification

• Performance requirements:– Structural safety– Environmental performance– Durability

• Quality assurance– Materials and components– Manufacturing process– Installation

ModCell core

ECO-SEE project

Eco-innovative, Safe and Energy Efficient wall panels and materials for a healthier indoor environment

The ECO-SEE project aims to develop new eco-materials and components for the purpose of creating both healthier and more energy efficient buildings.

18 partners.

Various studies have confirmed that airtight buildings with low air exchange rates lead to deterioration in indoor environmental quality for occupants.*

*Yu, Chuck W. F.; Kim, Jeong Tai: Low-Carbon Housings and Indoor Air Quality. In: Indoor and Built Environment, 21(1), 2012, pp. 5 - 15

Several factors affect a healthy indoor environment, including:

• Volatile Organic Compounds (VOCs) • Radon• Fibres• Particulate matters• Moisture and humidity• Rotting and microbiological/mould growth

Moisture buffering

From Rode et al., 2005

VOC capture• Reaction between

formaldehyde and proteins.

• Reduce the VOCs and formaldehyde levels in indoor air by the sequestration and chemisorption of VOCs.

Biocomposites Centre, Bangor University

Concluding comments• Development of prefabricated panels undertaken to

address practical concerns for straw bale construction.

• Successful development of panels has provided opportunity for addressing much wider barriers to market acceptance caused by lack of certification and warranty.

• Wall panels using bio-based materials, including wood based panels and insulation products, aim to contribute to improved indoor environmental quality, and occupant well-being, through Moisture buffering and VOC capture.

Acknowledgements– Dr Katharine Wall– Dr Andy Shea– Neil Price– Dan Maskell– Dr Mike Lawrence– Dr Chris Gross– Sophie Hayward– Will Beazley– Dr Andrew Thomson

– White Design Associates– ModCell Ltd– Integral Engineering Design– Lime Technology

– TSB UK– Carbon Connections UK– EACI Eco-Innovation– FP7