Heat Insulation Performance of Straw Bales andStraw Bale Walls

R. Wimmer, H. Hohensinner, L. Janisch, M. Drack

GrAT - Center for Appropriate Technology / Vienna University of Technology

GrAT - Center for Appropriate TechnologyVienna University of TechnologyWiedner Hauptstrasse 8-10A-1040 Vienna, AustriaWeb: www.grat.tuwien.ac.atE-mail: contact@grat.tuwien.ac.atPhone: ++43 1 58801 49523Fax: ++43 1 786 42 05

Key references

McCabe, J. (1993): Thermal Resistivity of Straw Balesfor Construction (Master thesis), University ofArizona, TucsonStraube, J. (2000): Moisture Properties of Plaster andStucco for Strawbale Buildings, Canada, Mortage andHousingWimmer, R. et al. (2001): Wandsysteme aus Nach-wachsenden Rohstoffen (Wall Systems made ofRenewable Resources), BMVIT, Vienna

Test certificate, page 1

Conclusions

· The heat insulation value of straw bales dependsnot so much on the density as expected initially. Onthe basis of experiences and tests, which were con-firmed by the tests made within the project, the heatinsulation value rises with increased density.Surprising was the fact, that small bales with a lowdensity of 80-110 kg/m³ show rather high insulationvalues. There is of course a significant difference bet-ween one insulation and another regarding the wholeconstruction, because powerfully pressed bales (90-110 kg/m³) have a more exact shape and thereforegenerate less heat leakage. Nevertheless the testresults show, that not only Jumbo Bales with highdensities of more than 130 kg/m³ but also small baleswith densities between 80 and 110 kg/m³ meet thetechnical requirements for highly heat insulated strawbale constructions.

· In the tests wheat straw was used. The tests in theUS were cared out with rice straw. Both tests showsimilar results. Significant differences compared toother sorts of cereals can be excluded due to the simi-larity of the plants. It can be suggested that the heatinsulation values of the much heavier straw fromhemp and flax does not show significant differences.But to prove this assumption systematic tests stillhave to be made.

· To guarantee a high heat insulation value the opti-mal moisture content of straw is between 8 and 14 %.A long-term-test has shown an average moisture con-tent of 13 % in a plastered straw bale wall. This valueis independent from out- and inside temperature, cli-mate and season and remains to be almost constant.If the wall construction is diffusion-open (no vapourbarriers except between the foundation and the wall)the moisture content of the wall levels off at 13 % wit-hin a few weeks. (Straube, 2000) To prove the longterm behaviour, tests in a demonstration building (seewww.S-House.at) are recommended.

· The lower price of the straw bales can compensatethe costs of the more complex housing technique and3-layer glazing in passive solar houses.

Aim

Straw bale houses will play an important role in sus-tainable building all over the world. The fundamentaltechnical basis and the necessary building certificateslike thermal conductance tests, provided by this proj-ect, will prepare and facilitate the introduction to themarket of this highly functional, economical and envi-ronmental friendly building material. The insulationparameters thus have to meet the criteria of passivesolar houses.

Conductance tests

The specific thermal conductance is described by thematerial dependent numeric ll -value. More commonis the thermal transmittance (U-value) for a buildingcomponent (wall, roof, etc.).

Recently made thermal insulation tests of uprightstanding straw bales (as they are used for woodenframe constructions) have shown ll -values of 0,045W/mK (McCabe,1993).

The above described testing results were done accor-ding to US norms and do not correspond withEuropean norms and therefore can not be used inEurope. Hence within the project "Wall Systems madeof Renewable Resources" wheat straw bales with adifferent density were tested. The thermal insulationtests were accomplished by the MunicipalDepartment 39 (Vienna) according to ISO 8301 andÖNORM B6015 part 1.

The measurement according ISO 8301 showed thefollowing result: ll 10 = 0,0369 respectively 0,0337W/mK (at 10 °C and dry material)

The measurement according ÖNORM B6015 resultedin the following value: ll 10 = 0,0380 W/mK

According to EU-norm the reference value has to bea calculated ll -value, which includes 20% of moistu-re-supplement. Hence the thermal insulation referen-ce value for wheat straw bales with a density of 100kg/m³ is ll = 0,0456 W/mK.

Therewith the thermal conductance is in the range ofother insulation materials which are made of renewa-ble resources (flax, hemp, wool, cork, cellulose).

Acknowledgement

We would like to thank the Federal Ministry ofTransport, Innovation and Technology (BMVIT) forsupporting the project. (www.hausderzukunft.at)

Section of a straw bale wall

Thermal transmittance

Due to the low conductance value of straw, straw balewalls with a total thickness of 42 cm (35 cm strawlayer) show a U-value of 0,12 W/m²K. Including thewooden frame the U-value is around 0,14 W/m²K.This means that a straw bale construction with woo-den frames or post and beam structures has a U-value below 0,15 W/m²K and hence meets the criteriafor passive solar houses.

Cost reduction potential

A rough estimation of the costs of different insulationmaterials shows the huge cost reduction potential ofstraw bale insulation.

In the table below ll refers to a calculated value. Thethickness that is required to reach U = 0,15 W/m²K isgiven in the third column and the specific price of thedifferent materials stands in the fourth column. Thelast column shows the insulation material costs for anaverage 150 m² house. (prices incl. 20% VAT)

Material ll Thickness Spec. pr. Price W/mK cm Euro/m² Euro

Straw Bale 0,045 30 3,63 1453Cellulose 0,045 30 18,31 7325EPS 0,038 24 20,35 8139Rockwool 0,038 24 23,55 9418

Test certificate, page 2

Compared costs of different insulation materials

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