The material data used in these simulations have not been included in WUFI's material database because straw bales, being a "natural" material, may differ greatly in their hygrothermal properties (depending on type of straw, alignment of straws, degree of compression, possible chemical treatments etc).
Due to popular demand, we list below some material data for straw bales and clay rendering, excerpted from the project report "Grundlagen zur bauaufsichtlichen Anerkennung der Strohballenbauweise- Weiterentwicklung der lasttragenden Konstruktionsart und Optimierung der bauphysikalischen Performance.
These may be useful as a rough guide or as a starting point for your investigations. Because of the strong variations to be expected for products coming from different regions or manufactured by different processes, we can not guarantee that these data are also applicable to the materials used in your project. The data given below reflect some of the possible variation.
Refer to the report for the individual sources of the data and details on the various measurement procedures.
You can download a WUFI project file which contains material data for straw bales and clay rendering that have been used for simulations of straw bale walls. However, given the strong variation of material data evident from the lists of data given below, this file may be applicable to your straw bale walls, or it may not.
Straw Bales:
Density measurements on rye straw bales used for investigations on mechanical stability:
p. 43, measured by Hochschule Magdeburg- Stendal
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mean bulk density of 36 samples: 130.12 +- 8.73 kg/m³
(Variation of these samples is
larger than that of typical
bales used for construction).
Measured by ZUB
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Dry density, computed (p. 87) from the measured moist mass and
the measured moisture content for six samples:
#1: 76.96 kg/m³
#2: 69.71 kg/m³
#3: 71.69 kg/m³
#4: 117.73 kg/m³
#5: 111.08 kg/m³
#6: 107.41 kg/m³
(p. 100)
Material data for straw
as used by Österreicherisches Ökologieinstitut für
angewandte Umweltforschung (and related by ZUB, p. 126):
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Straw: bulk density rho = 100 kg/m³
porosity psi = 0.9
heat capacity cp = 2000 J/(kg K)
thermal conductivity lambda = 0.045 W/(m K)
diffusion resistance factor mu = 1.3
The moisture storage function has been derived by re-scaling the
moisture storage function of wood for the bulk density of the straw.
See report p. 139, Fig. 5-1 for a plot
Liquid transport (both suction and redistribution) has been ignored
"because of the large distances between the straws".
Thermal conductivity
Design value determined for a straw bale sample with
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bulk density 90 kg/m³: lambda_design = 0.045 W/(m K)
Material data of construction straw bales
measured by FIW (related by ZUB, p. 129)
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bulk density : 81 kg/m³
lambda_10_dry: 0.067 W/(m K)
mu: 2
Material data for straw
p. 283; measured by IBP and used in the simulations
for the present report:
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bulk density of straw bale, air-dry: 90 kg/m³
sorption moisture content at 23°C, 65% RH: 1.21 vol.-%
sorption moisture content at 23°C, 80% RH: 1.80 vol.-%
sorption moisture content at 23°C, 93% RH: 2.51 vol.-%
is expected to be too small to be relevant (p. 227).
Missing material data, such as thermal conductivity or
diffusion resistance factor have been taken from measurements
performed by FIW.
See fig. 10-22 (p. 270) for a comparison of the measured (black) and
simulated (blue) temperatures (top) and relative humidities (bottom)
for a straw bale wall with exterior lime rendering.
Measurements of heat conductivity:
Measurements by FIW.
(p. 299 and pp. 429 ff.)
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Wheat straw, along the straws:
bulk density, dry: 105 kg/m³ ref.: p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
bulk density at 23°C, 80% RH: 120 kg/m³ p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
mass-specific moisture content u_23/80: 14.2% p. 433 & 442 (L1-06-134)
lambda_10_dry: 0.0670 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
lambda_10_(23/80): 0.0800 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
Wheat straw, along the straws:
bulk density, dry: 116 kg/m³ p. 299 (Ergebnisse 2008)
bulk density at 23°C, 80% RH: 133 kg/m³ p. 299 (Ergebnisse 2008)
lambda_10_dry: 0.065 W/mK p. 299 (Ergebnisse 2008)
lambda_10_(23/80): 0.0715 W/mK p. 299 (Ergebnisse 2008)
Wheat straw, across the straws:
bulk density, dry: 93 kg/m³ p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
bulk density at 23°C, 80% RH: 105 kg/m³ p. 433 & 442 (L1-06-134)
mass-specific moisture content u_23/80: 12.6% p. 433 & 442 (L1-06-134)
lambda_10_dry: 0.0439 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
lambda_10_(23/80): 0.0472 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
Wheat straw, across the straws:
bulk density, dry: 97.2 kg/m³ p. 299 (Ergebnisse 2008)
bulk density at 23°C, 80% RH: 109 kg/m³ p. 299 (Ergebnisse 2008)
lambda_10_dry: 0.0397 W/mK p. 299 (Ergebnisse 2008)
lambda_10_(23/80): 0.0469 W/mK p. 299 (Ergebnisse 2008)
Rye straw, along the straws:
bulk density, dry: 115 kg/m³ p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
lambda_10_dry: 0.0820 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
Rye straw, across the straws:
bulk density, dry: 93 kg/m³ p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
bulk density at 23°C, 80% RH: 104 kg/m³ p. 433 & 442 (L1-06-134)
mass-specific moisture content u_23/80: 11.8% p. 433 & 442 (L1-06-134)
lambda_10_dry: 0.0422 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
lambda_10_(23/80): 0.0465 W/mK p. 299 (Prüfbericht 2006), p. 433 & 442 (L1-06-134)
Heat conductivity of straw bales
(including some clay seaped into the bales),
determined from the measured U-value of a wall:
lambda_10_dry: 0.065 W/mK p. 299, p. 453 (I1-08/06)
Wheat straw dried at 70°C; across the straws:
bulk density dry: 115 kg/m³ p. 434 & 443 (F.2-589/08)
lambda_10_dry: 0.0553 W/mK p. 434 & 443 (F.2-589/08)
Wheat straw, along the straws
bulk density dry: 116 kg/m³ p. 299 (F.2-909/08, F.2-605/08), p. 437 & 438 & 446 (F.2-605/08,
same sample as in F.2-909/08)
bulk density at 23°C, 80%: 133 kg/m³ p. 299 (F.2-909/08, F.2-605/08), p. 435 & 444 (F.2-909/08)
mass-specific moisture content u_23/80: 14.0% p. 435 & 444 (F.2-909/08)
lambda_10_dry: 0.0650 W/mK p. 299 (F.2-909/08, F.2-605/08), p. 437 & 438 & 446 (F.2-605/08)
lambda_10_(23/80): 0.0715 W/mK p. 299 (F.2-909/08, F.2-605/08), p. 435 & 444 (F.2-909/08)
Wheat straw, across the straws
bulk density dry: 97.2 kg/m³ p. 299 (F.2-714/08, F.2-916/08), p. 447 (F.2-714-08,
same sample as in F.2-916/08)
bulk density at 23°C, 80% RH: 109 kg/m³ p. 299 (F.2-714/08, F.2-916/08), p. 436 & 445 (F.2-916/08)
mass-specific moisture content u_23/80: 12.5 % p. 436 (F.2-916/08)
lambda_10_dry: 0.0397 W/mK p. 299 (F.2-714/08, F.2-916/08), p. 447 (F.2-714-08)
lambda_10_(23/80): 0.0469 W/mK p. 299 (F.2-714/08, F.2-916/08), p. 436 & 445 (F.2-916/08)
Interior Clay Rendering
Material data for clay rendering
as used by Österreicherisches Ökologieinstitut für
angewandte Umweltforschung (and related by ZUB, p. 126):
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Clay rendering, (three coats): bulk density rho = 1514 kg/m³
porosity psi = 0.4
heat capacity cp = 850 J/kgK
thermal conductivity lambda = 0.7 W/mK
diffusion resistance factor mu = 11.3
Clay rendering, mu=30: bulk density rho = 1514 kg/m³
porosity psi = 0.4
heat capacity cp = 850 J/kgK
thermal conductivity lambda = 0.7 W/mK
diffusion resistance factor mu = 30.0
The moisture storage function has been derived from the sorption measurements
of IBP. See fig. 5-2, p. 139 for a plot of the moisture storage function.
Material data for clay rendering
Measured by IBP, related by ZUB (p. 129):
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bulk density: 1514 kg/m³
true density: 2630 kg/m³
diffusion resistance factor (23°C, 0/50% RH): 11.3
diffusion resistance factor (23°C, 50/93% RH): 9.6
water absorption coefficient (A-value): 2.8 kg m^-2 h^-0.5
sorption moisture content at 23°C, 65% RH: 1.3 vol.-%
sorption moisture content at 23°C, 80% RH: 1.88 vol.-%
sorption moisture content at 23%C, 93% RH: 2.54 vol.-%
free saturation : 29.4 vol.-%
liquid transport coefficient, suction: Dw0: 2.0e-10 m²/s
Dwf: 7.7e-8 m²/s
liquid transport coefficient, redist.: Dw0: 7.0e-10 m²/s
Dwf: 8.0e-5 m²/s
Material data for interior clay rendering
Measured by IBP (p. 281).
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bulk density, dry: 1798 kg/m³
true density : 2707 kg/m³
porosity : 34 vol.-%
diffusion resistance factor (23°C, 0/50% RH): 14.3
diffusion resistance factor (23°C, 50/93% RH): 8.7
water absorption coefficient (A-value) w1 : 4.6 kg m^-2 h^-0.5
water absorption coefficient (A-value) w24: 2.1 kg m^-2 h^-0.5
sorption moisture content at 23°C, 65% RH: 1.53 vol.-%
sorption moisture content at 23°C, 80% RH: 1.88 vol.-%
sorption moisture content at 23°C, 93% RH: 2.91 vol.-%
free saturation : 35.1 vol.-%
liquid transport coefficient, suction: Dw0: 2.0e-10 m²/s
Dwf: 2.4e-8 m²/s
liquid transport coefficient, redist.: Dw0: 2.5e-9 m²/s
Dwf: 5.0e-7 m²/s
