An Evaluation of WUFI-Bio to Predict Mold Growth in Straw Bale Walls in Japan

Kyle Holzhueter*1 and Koji Itonaga2
1Research Associate, Graduate School of Bioresource Sciences, Nihon University, Japan
2
Professor, College of Bioresource Sciences, Nihon University, Japan

Abstract
The greatest challenges facing straw bale building in Japan, and many other countries with high humidity
and precipitation, are moisture and the susceptibility of straw to microbial decay. WUFI-Bio, developed by
the Fraunhofer Institute for Building Physics, is a computer program that assesses the risk of mold growthin buildings based on measured or computed climatic conditions. Researchers in Japan have monitored thehygrothermal conditions of six straw bale buildings in Japan and evaluated the risk of mold growth basedon an interstitial temperature and relative humidity guideline. The purpose of the present study is: (one)reevaluate the potential for mold growth in the six buildings using WUFI-Bio and (two) evaluate the accuracyof Holzhueter and Itonaga's interstitial temperature and relativity guideline vis-à-vis WUFI-Bio. As a result ofthe study, the potential for mold growth was found to vary by structure. Buildings utilizing rainscreens were
found to have a lower risk of mold growth. Holzhueter and Itonaga's interstitial temperature and relativity
guideline was found to over exaggerate the potential for mold growth. Also, although Holzhueter's guideline
does indicate a potential for mold growth, the number of hours above the guideline alone is not a sufficiently
accurate indication of severity.
Keywords: straw bale building; WUFI-Bio computer program; hygrothermal environment; mold growth; interstitial temperature and moisture

1. Introduction
Straw bales are blocks of compressed straw. In straw
bale construction, bales are stacked to create bearing or
infill walls.
Straw bale building has numerous ecologicaladvantages. Straw bales are low in embodied energy(Centre for Building Performance Research, 2010).
Since straw consists of approximately 36% carbon,
straw bale walls function as a carbon sink, sequestering
carbon during the life of the building (Wihan, 2007).
Straw bale walls are also highly insulative, reducing
energy use and CO2 emissions due to heating andcooling (Bigland-Pritchard, 2005). And lastly, upon
deconstruction, straw bales can safely decompose
without becoming landfill.
Straw bale building is relatively new to Japan.
According to the Japan Straw Bale House Association(2009), the first straw bale home in Japan wascompleted in 2001 in Tochigi Prefecture. The greatest
challenges facing straw bale buildings in Japan,*Contact Author: Kyle Holzhueter, Research Associate,
Graduate School of Bioresource Sciences, Nihon University,
1866 Kameino Fujisawa Kanagawa 252-8510 Japan
Tel: +81-466-84-3364 Fax: +81-466-84-4464
E-mail: nihondaigaku.kairu@gmail.com( Received April 4, 2016 ; accepted March 10, 2017 )DOI http://doi.org/10.3130/jaabe.16.357

and many other countries with high humidity andprecipitation, are moisture and the susceptibility ofstraw to microbial decay.
However, controlling moisture to reduce microbial
decay and improve building durability is an issuecommon to conventional construction and particularly
buildings utilizing organic insulations such as straw,
rice hulls, etc. (Lee et al., 2013).
Holzhueter and Itonaga of Nihon University (2015)
have monitored the indoor, outdoor and interstitial
hygrothermal environment of six straw bale buildings
in Japan. The potential for interstitial mold growth wasevaluated given a temperature and relative humidity
guideline. Holzhueter (2011) found hygrothermal
conditions of 80% relative humidity and 10ºC to be asafe guideline for straw bale walls. Above 80% relative
humidity and 10ºC, mold growth was predicted. Atand below 80% relative humidity and 10ºC, somebiological activity may be present, but was not believed
to impact the life of the building.
Given this guideline, Holzhueter and Itonaga (2015)
found that rainscreens were an effective means tocontrol interstitial moisture. That is, buildings utilizing
rain screens were found to have lower interstitial
relative humidity and a lower risk of mold growth.
Klaus Sedlbauer (2001) of the Fraunhofer Institutefor Building Physics developed a bio-hygrothermal
procedure for evaluating the potential of mold growth
based on three prerequisites: temperature, relative

Journal of Asian Architecture and Building Engineering/May 2017/362

357

humidity, and substrate. Based on these variables, theboundary conditions (Isopleths) of spore germinationand mold growth were identified. Sedlbauer reviewed
over 250 sources regarding mold development andsynthesized the data creating isopleths which represent
mold development on various building materials.
Sedlbauer's predictive model corresponded well with theliterature review of experimental results and case studies.
Led by Sedlbauer, the Fraunhofer Institute for
Building Physics developed WUFI-Bio, a transient
prognosis model to assess the risk of mold growth on
and in building components.

Using WUFI-Bio, the present study evaluates thepotential for mold growth in the same six straw balebuildings investigated by Holzhueter and Itonaga in 2015.
The purpose of the present study is to (one)reevaluate the potential for mold growth in the sixbuildings using WUFI-Bio and (two) evaluate theaccuracy of Holzhueter's interstitial temperature andrelativity guideline vis-à-vis WUFI-Bio. By doing so,
the authors hope to show the potential to use WUFIBio in future studies examining the potential for moldgrowth in straw bale walls.

Group BGroup A

Shinrinbokujo Tango
Kyoto
LeyendaToyamaMadenaie
Fukushima

Yurikagonoie
MieEarthen or
Lime Plaster
Finish

Furyu
Chiba

VentilatedRainscreen

OutsideInsideOutsideInsideSquare One
Hokkaido

Fig.1. Six Straw Bale Buildings Divided into Two Groups
Table 1. Details of Each Building
Start

FinishOne Year Observation
PeriodStart
Finish

Oct-07

Jun-08

Sep-08Aug-09Minamiboso Goichi Oiwa Takao Kobayashi Mar-08
Masatoshi
MieTsuHideto Oshima
Feb-09
Sakamoto
Mikio andHokkaido Asahikawa
Stefan BellMay-09Masami Sakai
Yoshiyuki
Kyle Holzhueter,
Fukushima
Itate
Nov-09ToyotoKoji Itonaga
Shoko
Hiroaki Yoshimoto,
ToyamaToyamaAug-10
YoshimotoKyle Holzhueter

Nov-08

Feb-09Jan-10Jul-09Jul-10Jun-11

Jun-10Nov-11

Oct-12Mar-10Dec-11

Nov-12

Sep-11

Apr-12Mar-13

Group Building Name PrefectureA

Shinrinnobokujo
Tango
Furyu
Yurikagonoie
Square One

B

Madenaie
Leyenda

Kyoto
Chiba

358 JAABE vol.16 no.2 May 2017

CityArchitect

Principle Builder ofStraw Bale Walls

Kyotango

Goichi Oiwa

Takao Kobayashi

Construction

Kyle Holzhueter

2. Materials and Methods
2.1 Monitoring the Hygrothermal Environment
The indoor, outdoor and interstitial hygrothermal
environment of six straw bale structures in Japan havebeen monitored. These six straw bale buildings areorganized into two groups according to construction
details (Fig.1., Table 1.). The first Group A includes
buildings consisting of straw bale walls with an earthen
or lime plastered exterior finish applied directly to thebale walls. The second Group B includes buildings
consisting of straw bale walls utilizing ventilated rain

Group A

Shinrinbokujo Tango, Kyoto

Yurikagonoie, MieFuryu, Chiba

Square One, Hokkaido

Group BHEHIMEMILELIMadenaie, Fukushima

Leyenda, Toyama

Fig.2. Wall Section and Sensor Locations for Each Building

JAABE vol.16 no.2 May 2017

screens. Further details regarding the buildings can befound in Holzhueter and Itonaga, 2015.
T and D Corporation's Thermo Recorder sensors
monitor the indoor, outdoor and interstitial hygrothermal
conditions of the six straw bale structures. These sensors
measure temperature and relative humidity, and data isrecorded at one-hour intervals by data loggers.
In Group A buildings, a "stack" of nine interstitial
sensors have been installed at various heights anddepths perpendicular to the plane of the wall in eachbuilding (Fig.2.). Each interstitial sensor is given a
name consisting of multiple letters. The first letter
designates the height: "L" stands for low, "M" formiddle, and "H" for high. The second letter describes
the sensors depth: "I" stands for interior, "N" forinterstitial and "E" for exterior.
In Group B buildings, a "stack" of six interstitial
sensors have been installed at various heights anddepths perpendicular to the plane of the wall. The
nomenclature of the sensors follows Group A. Dueto funding limitations, interstitial sensors were notinstalled. Previous research by Holzhueter and Itonaga(2010, 2014, and 2015) and the results of the present
study suggest that a thorough investigation of theinterstitial hygrothermal environment is possible withthe sensor arrangement of Group B.
2.2 Predicting Mold Growth
H o l z h u e t e r (2011) f o u n d t h a t h y g r o t h e r m a lconditions of 80% relative humidity and 10ºC are
understood to be a safe guideline for straw bale walls.
Above 80% relative humidity and 10ºC, mold growthis predicted. At and below 80% relative humidity and10ºC, some biological activity may be present, but is
not believed to impact the life of the building.
WUFI-Bio uses three parameters, temperature,
relative humidity and substrate, to evaluate thepotential for mold growth. In addition to relying on
temperature and relative humidity, WUFI-Bio takes
into consideration the influence of the substrate (andthe probability of contamination) on the formation ofmold. Based on research by Sedlbauer (2001), WUFIBio divides substrates into four categories:(0) Optimal culture medium.
(I) Biodegradable building materials such as
wallpaper, wood, etc.
(II) Building materials with a porous structure such as
plaster, mineral building materials, insulation, andsome species of wood not covered by I or possible
contamination by mold fungus.
(III) Building materials that are neither degradable nor
contain any nutrients.
Assuming a worst-case scenario, for purposes of thisstudy, straw bales are assumed to be in the substratecategory (I).
Based on a review of the literature, Sedlbauer
developed an isopleth model for each substrate category.
For a given substrate, isopleths are three-dimensional
diagrams using temperature, relative humidity, andKyle Holzhueter359

time as variables to depict iso-lines that indicate spore
germination time or growth per time unit. Isoplethsvisually depict the hygrothermal prerequisites for fungal
growth and germination on a specific substrate. The
isopleth for spore germination and mycelium growth on
substrate category I are seen in Fig.3.
In Fig.3., LIM stands for Lowest Isopleth for Mold,and represents the limit of fungal activity below whichspore germination and mycelium growth is excluded.
For spore germination (left), the other lines represent
the hygrothermal conditions and time needed for sporegermination. And for mycelium growth (right), theother lines represent the hygrothermal conditions andrate of mycelium growth in mm/day.
Given a specific substrate category and temperatureand relative humidity data over a one-year period,
WUFI-Bio calculates the mold growth rate (Fig.4.).

Fig.4. is the results of Shinrinnobokujo Kyotango's
LI sensor. According to WUFI-Bio, the straw in thelocation of the LI sensor is calculated to have a moldgrowth rate of 110 mm/year.
WUFI-Bio classifies mold growth rates into three
categories:
Green Light - Mold growth is below 50 mm/year,
which is usually acceptable.
Yellow Light - Mold growth is between 50 mm/yearand 200 mm/year. Additional criteria or investigations
are needed for assessing acceptability.
Red Light - Mold growth exceeds 200 mm/year,
which is usually not acceptable.
The signal light presents a general assessment of themold growth risk and the severity of the infestation.
Wi t h a m o l d g r o w t h r a t e o f 110 m m / y e a r,
Shinrinnobokujo Tango was given a yellow signal

Fig.3. Generalized Isopleth System for Spore Germination (Left) and Mycelium Growth (Right) Valid for All Fungi of Substrate
Category I (Sedlbauer, 2001)

Fig.4. WUFI-Bio Results for Shinrinnobokujo Tango's LI Sensor Location

360 JAABE vol.16 no.2 May 2017

Kyle Holzhueter

light. That is, additional criteria or investigations areneeded for assessing acceptability.
2.3 Comparing Holzhueter's Guideline and WUFI-BioHolzhueter and Itonaga (2015) never confirmed thepresence of mold in the six buildings they monitored.
Practical limitations prohibit the opening up of wallsand extraction of straw from the walls of private

homes. On the other hand, Sedlbauer's predictivemodel corresponded well with the literature review ofexperimental results and case studies.
The results of Holzhueter and Itonaga's 2015 study
are compared to the results of using WUFI-Bio toevaluate the same data.

Table 2. Results of the Study Using Holzhueter's Guideline and WUFI-BioGroup

BuildingOne YearObservation
PeriodShinrinnobokujo
2008/09-2009/08
TangoFuryu

2009/02-2010/01

A

Yurikagonoie

Square One

Madeinaie

2010/07-2011/06

2011/11-2012/10

2011/12-2012/11

B

LeyendaJAABE vol.16 no.2 May 2017
2JAABE vol.1 No.1 Feb.2002

2012/04-2013/03

SensorLocationInterstitial Hours
Surpassing Guideline

Mold Growth
WUFI-Bio
Rate
Signal Light(mm/year)

LI

1705

110.00

Yellow

LN
LE

150

6.29
0.00

Green
GreenMI
MN

169
0

13.80
0.00

Green
GreenMEHI

33
340.00
2.46

GreenGreenHN
HE

50
303

1.47
6.07

GreenGreenLI
LN

2682
2849

264.00296.00

RedRedLE
MI

2730
1093

284.0065.40

RedYellow

MN
ME

1165
1227

69.10
75.10

Yellow
Yellow

HI
HN

961
1512

54.60
104.00

Yellow
Yellow

HE
LI

1387
105

91.40
12.70

Yellow
Green

LN
LE

060.00
0.00

Green
Green

MI
MN

000.00
0.00

Green
Green

ME
HI

000.00
0.00

Green
Green

HN

00.00

Green

HE
LI

000.00
0.00

Green
Green

LN
LE

000.00
0.00

Green
Green

MI
MN

000.00
0.00

Green
Green

ME
HI

4177
0428.00
0.00

Red
Green

HN
HE

011580.00
339.00

Green
Red

LI
LE

000.000.00

Green
Green

MI
ME

000.00
0.00

GreenGreenHI
HE

000.00
0.00

GreenGreenLILE040.00
0.00

GreenGreenMIME

000.00
0.00

GreenGreenHIHE000.00
0.00

GreenGreenKyle Holzhueter
Tomonari Yashiro

361

3. Results and Discussion
Table 2. summarizes the results of the monitoring.
Of the six buildings monitored, the interstitial
hygrothermal environment of five buildings surpassed
the 80% relative humidity and 10ºC guideline.
20 sensors surpassed Holzhueter's guideline, ranging
from 4 hours to 4,177 hours. According to WUFIBio, sensor locations surpassing the guideline for303 hours were found to be acceptable. 961 to 1,705
hours above the guideline was found to be uncertain
with additional criteria or investigations needed forassessing acceptability. More than 1,158 hours above
the guideline was found to be unacceptable.
Shinrinnobokujo's LI sensor surpassed the guideline
for 1,705 hours, and was given a yellow signal light
by WUFI-Bio. Square One's HE sensor surpassed theguideline for 1,158 hours and was given a red signal
light by WUFI-Bio. The results show that although
Holzhueter's guideline does indicate the potential for
mold growth, the number of hours above the guideline
alone is not a sufficiently accurate indication of moldgrowth.
According to WUFI-Bio, even sensor locations
surpassing the guideline for 303 hours were found tobe acceptable. This suggests that Holzhueter's guideline
over exaggerates the potential for mold growth.
A discussion of building details and hygrothermal
performance can be found in Holzhueter and Itonaga,
2015, 2014, and 2010.

4) Holzhueter K. and Itonaga K. (2010) The hygrothermal
environment and potential for mold growth within a straw balewall. Journal of Asian Architecture and Building Engineering, Vol.
9, No. 2, pp.495-499.
5) Holzhueter, K., Itonaga, K. (2014) The influence of passive
ventilation on the interstitial hygrothermal environment of a straw
bale wall, Journal of Asian Architecture and Building Engineering,Vol. 13, No. 1, pp.223-229.
6) Holzhueter, K., Itonaga, K. (2015) The Influence of VentilatedRainscreens on the Interstitial Hygrothermal Environment ofStraw Bale Walls, Journal of Asian Architecture and Building
Engineering, Vol. 14, No. 1, pp.205-212.
7) Japan Straw Bale House Association (2009) First straw bale housein Japan. Available Online: http://www.japanstraw.com/index/main/03hatuno/03hatuno.html
8) Lee, K., Yeom, D., Kim, E. (2013) Experimental research on thecorrelation of temperature, humidity, and CO2 level of a rice hull
insulated indoor environment, Journal of Asian Architecture andBuilding Engineering, Vol. 12, No. 2, pp.221-228.
9) Sedlbauer, K. (2001) Prediction of mould fungus formation on thesurface of and inside building components. Fraunhofer Institutefor Building Physics. Available Online: http://www.hoki.ibp.fhg.
de/ibp/publikationen/dissertationen/ks_dissertation_e.pdf
10) Wihan, J. (2007) Humidity in straw bale walls and its effect on
the decomposition of straw. Master's Thesis. University of EastLondon School of Computing and Technology.

4. Conclusion
The hygrothermal conditions of six straw bale
structures in Japan were monitored. The purpose of thepresent study was to (one) reevaluate the potential for
mold growth in the six buildings using WUFI-Bio and(two) evaluate the accuracy of Holzhueter's interstitial
temperature and relativity guideline vis-à-vis WUFIBio. (1) The potential for mold growth was found tovary by structure. Buildings utilizing rainscreens were
found to have a lower risk of mold growth. (2) Theresults show that although Holzhueter's guideline does
indicate the potential for mold growth, the number ofhours above the guideline alone is not a sufficiently
accurate indication of mold growth. Also, the results
suggest that Holzhueter's guideline over exaggerates
the potential for mold growth.
References

1) Bigland-Pritchard, M. (2005) An assessment of the viability ofstrawbale wall construction in buildings in maritime temperateclimates. PhD Thesis, University of Sheffield.
2) Centre for Building Performance Research, Victoria Universityof Wellington (2010) Table of embodied energy coefficients.
Available Online: http://www.victoria.ac.nz/cbpr/documents/pdfs/ee-coefficients.pd3) Holzhueter, K. (2011) The hygrothermal environment of straw
bale walls in Japan and building practices to control interstitialmoisture. PhD Thesis, Nihon University.

362 JAABE vol.16 no.2 May 2017

Kyle Holzhueter