The Influence of Ventilated Rainscreens on the Interstitial Hygrothermal Environment ofStraw Bale Walls

Kyle Holzhueter*1 and Koji Itonaga2
1

Research 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. Researchers in Europe andNorth America have found the use of ventilated rainscreens to help control interstitial moisture in straw balewalls. The indoor, outdoor and interstitial hygrothermal environment of six straw bale structures in Japan
have been monitored. The six buildings are organized into two groups. The first group includes buildings
consisting of straw bales walls with an earthen or lime plastered exterior finish applied directly to the balewalls. The second group includes buildings consisting of straw bale walls utilizing ventilated rain screens.
The purpose of the present study is: (one) evaluate the potential for mold growth in the six buildings, (two)clarify moisture dynamics, and (three) determine the effectiveness of ventilated rainscreens to control
moisture in straw bale walls. As a result of the study, the potential for mold growth was found to vary by
structure. Buildings utilizing rain screens were found to have lower interstitial relative humidity and a lower
risk of mold growth.
Keywords: straw bale building; ventilated rainscreens; hygrothermal environment; mold growth; interstitial 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,
and many other countries with high humidity and*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 7, 2014 ; accepted November 12, 2014 )precipitation, 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).
Straw bale walls are generally finished with earthen
and/or lime plasters applied directly to the bale walls.
In recent years, ventilated rainscreens have been
employed to protect straw bale walls from moisturedamage.
The term "rainscreen" refers to the use of a drainage
plane/air gap between exterior siding and bale wall.
Permeable plasters tend to be porous and absorb
exterior moisture. In many cases, this moisture is thentransferred to interstitial bales through capillary action.
In order to prevent liquid water, i.e. rain, from directcontact with a plastered bale wall, rain-screens can beused. Previous research has found that rainscreens help
control interstitial moisture in straw bale walls (Carfrae
et al., 2009; Lawrence et al., 2009; Otto et al., 2008;
Thompson, 2006).
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

Journal of Asian Architecture and Building Engineering/January 2015/212

205

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
screens.
The purpose of the present study is to (one) evaluate
the potential for mold growth in the six buildings,(two) clarify moisture dynamics, and (three) determine
the effectiveness of ventilated rainscreens to control
moisture in straw bale walls.

2. Materials and Methods
2.1 Monitoring the Hygrothermal Environment
T and D Corporation's Thermo Recorder
sensors monitor the indoor, outdoor and interstitial
hygrothermal conditions of the six straw bale
structures (Fig.2.). These sensors measure temperatureand relative humidity, and data is recorded at one hourintervals by data loggers.
In Group A buildings, a "stack" of nine interstitial
sensors have been installed at various heights andGroup B

Group A

Shinrinbokujo Tango
Kyoto
LeyendaToyamaMadenaie
Fukushima

Yurikagonoie
Mie
Earthen 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
Group

A

Building Name

Prefecture

City

Architect

Principle Builder ofStraw Bale Walls

Construction

One Year
Observation Period

Start

FinishStart

FinishTakao Kobayashi

Oct-07

Jun-08

Sep-08Aug-09Minamiboso Goichi Oiwa

Takao Kobayashi

Mar-08Nov-08

Feb-09Jan-10Tsu

Masatoshi
Sakamoto

Hideto Oshima

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

Hokkaido

Asahikawa

Mikio andMasami Sakai

Stefan BellMay-09Jun-10Nov-11

Oct-12

Madenaie

Fukushima

Itate

Yoshiyuki
ToyotoKyle Holzhueter,
Koji Itonaga

Nov-09Mar-10Dec-11

Nov-12

LeyendaToyamaToyamaShoko
YoshimotoHiroaki Yoshimoto,Kyle Holzhueter

Aug-10

Sep-11

Apr-12Mar-13

Shinrinnobokujo
Tango

Kyoto

Kyotango

Goichi Oiwa

Furyu

Chiba

Yurikagonoie

Mie

Square One

B

206 JAABE vol.14 no.1 January 2015

Kyle Holzhueter

Group A

Shinrinbokujo Tango, Kyoto

Yurikagonoie, Mie

Furyu, Chiba

Square One, Hokkaido

Group B

HE

HI

ME

MI

LE

LI

Madenaie, Fukushima

Leyenda, Toyama

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

JAABE vol.14 no.1 January 2015

Kyle Holzhueter

207depths perpendicular to the plane of the wall in eachbuilding (Fig.3.). 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. Due
to funding limitations, interstitial sensors were notinstalled. Previous research by Holzhueter and Itonaga(2010, 2014) and the results of the present study
suggest that a thorough investigation of the interstitial
hygrothermal environment is possible with the sensor
arrangement of Group B.
Collecting data over several years from multiple
buildings has resulted in the loss of some data.
However, despite the absence of some data, an accurate
and thorough investigation is attainable.
No data is available from Furyu's indoor sensor
between 17:00 September, 15, 2009 and 23:00 January
31, 2010.

Group AGroup B

Fig.3. Stack of Nine Interstitial Sensors in Group A Buildings and Stack of Six Interstitial Sensors in Group B Buildings
Table 2. Monitoring Results Summary of Six Buildings

Group

Building

One Year
Observation
PeriodInterstitial Hours Surpassing 80% Relative Humidity
and 10℃ Guideline

Total
Max Consecutive
Location Total Hours Location Hours
Date
LI
1705
HE
303
MI
169
Shinrinnobokujo
2008/09-2009/08
HN
50
LI1369 2009/7/5-8/31
Tango
HI
34
ME
33
LN
15

Furyu

2009/02-2010/01

LN
LE
LI
HN
HE
ME
MN
MI
HI

Yurikagonoie

2010/07-2011/06

LI
LE

105
6

2011/11-2012/10

ME
HE

4177
1158

A

Square One
B

Outdoor
Difference
Maximum
betweenInterstitial Maximum
Total
Total
Monthly
Total
Monthly Mean
OutdoorIndoorMean
Outdoor andRelative Humidity
Hours
Hours
Relative
Greatest
Surpassing Surpassing
Humidity
Total
Guideline
Guideline
Interstitial(％) Date Location (％) Date
Hours

275

2849

1144

86 Aug-09

LI

87 Nov-08

2849
2730
2682
1512
1387
1227
1165
1093
961

LN

1865

2009/6/2-8/19

661

3700

851

92

LI

75

2010/07/18-21

8

1711

1606

76 Aug-10

LI

73 Aug-103183

2012/05/2810/08

0

1990

-2187

95 Feb-12

HE

93

ME

Jul-09LN, LE92

Jul-09Jan-12

Madeinaie

2011/12-2012/11

na

0

na

0

na

0

2875

2875

76 Aug-12

LE

89 Sep-12

Leyenda2012/04-2013/03

LE

4

LE

4

13/3/7

29

2485

2481

75

LE

84 Dec-12

208 JAABE vol.14 no.1 January 2015

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Kyle Holzhueter

100
90
80
70
60
50
40
30
20
10
0

LI ﾟC
LI %RH

JAABE vol.14 no.1 January 2015
Guideline %RH

Guideline ﾟC

Leyenda, Toyama
Guideline %RH

Shinrinnobokujo Tango, Kyoto

100
90
80
70
60
50
40
30
20
10
0
-10

2008/09/01
2008/09/15
2008/09/29
2008/10/13
2008/10/27
2008/11/10
2008/11/25
2008/12/09
2008/12/23
2009/01/06
2009/01/20
2009/02/03
2009/02/18
2009/03/04
2009/03/18
2009/04/01
2009/04/15
2009/04/29
2009/05/14
2009/05/28
2009/06/11
2009/06/25
2009/07/09
2009/07/24
2009/08/07
2009/08/21

90
80
70
60
50
40
30
20
10
0
-10
HE ﾟC

LE ﾟC

Yurikagonoie, Mie
LE %RH

2010/12/06
2010/12/19
2011/01/01
2011/01/15
2011/01/28
2011/02/10
2011/02/23
2011/03/09
2011/03/22
2011/04/04
2011/04/17
2011/04/30
2011/05/14
2011/05/27
2011/06/09
2011/06/22

Guideline ﾟC2010/07/01
2010/07/14
2010/07/27
2010/08/09
2010/08/22
2010/09/05
2010/09/18
2010/10/01
2010/10/14
2010/10/27
2010/11/10
2010/11/23

2009/04/12
2009/04/26
2009/05/10
2009/05/24
2009/06/07
2009/06/21
2009/07/05
2009/07/19
2009/08/02
2009/08/16
2009/08/30

2008/09/01
2008/09/14
2008/09/29
2008/10/13
2008/10/26
2008/11/09
2008/11/23
2008/12/07
2008/12/21
2009/01/04
2009/01/18
2009/02/01
2009/02/15
2009/03/01
2009/03/15
2009/03/29
LI %RH

2010/12/06
2010/12/19
2011/01/01
2011/01/15
2011/01/28
2011/02/10
2011/02/23
2011/03/09
2011/03/22
2011/04/04
2011/04/17
2011/04/30
2011/05/14
2011/05/27
2011/06/09
2011/06/22

2010/07/01
2010/07/14
2010/07/27
2010/08/09
2010/08/22
2010/09/05
2010/09/18
2010/10/01
2010/10/14
2010/10/27
2010/11/10
2010/11/23

90
80
70
60
50
40
30
20
10
0
LI ﾟC

GROUP A
HE %RH
Guideline ﾟC

Guideline ﾟC

Guideline %RH

Furyu, Chiba

Guideline %RH

Square One, Hokkaido

GROUP B

Madeinoie, Fukushima

Fig.4. Evaluation of Mold Growth Given an 80% Relative Humidity and 10ºC Temperature Guideline

Kyle Holzhueter209

No data is available from Square One's HI, HN and
HE sensors between 11:00, October 8, 2012 and 23:00,
October 31, 2012. Data is also not available from
Square One's Outdoor sensor between 0:00, November
1, 2011 and 17:00, November 9, 2011.
No data is available from Leyenda's Outdoor sensor
between 19:00 November 13, 2012 and 0:00 December
6, 2012.
No data is available from Madeinaie's MI sensor
between 15:00 February 27, 2012 and 23:00
November, 30, 2012.
2.2 Predicting Mold GrowthFirst, in order to evaluate the potential for moldgrowth in straw bale walls, the interstitial hygrothermal
environment is evaluated given a relative humidity andtemperature guideline. Holzhueter (2011) found that
hygrothermal conditions of 80% relative humidity and

10ºC are understood to be a safe guideline for straw balewalls. Above 80% relative humidity and 10ºC, moldgrowth is predicted. At and below 80% relative humidity
and 10ºC, some biological activity may be present, but
is not believed to impact the life of the building.
The straw bale walls of the six buildings areevaluated given this guideline.
2.3 Clarifying Moisture Dynamics
The sensor locations which surpass the guideline arefurther investigated to clarify moisture dynamics.
Simultaneously depicting the relative humidity
readings of multiple sensors recorded at one-hourintervals over an entire year on the same graph results
in incomprehensible figures. In order to visually depict
the moisture dynamics over an entire year in an easily
understandable manner, monthly averages are graphed.
Trends in relative humidity will be identified.

100
90
80
70
60
50
40
30
20
10
0

HE

IndoorOutdoorRelative Humidity (%)

Relative Humidity (%)

GROUP A
LI

100
90
80
70
60
50
40
30
20
10
0

Sep- Oct- Nov- Dec- Jan- Feb- Mar- Apr- May- Jun- Jul- Aug08
08
08 08
09
09
09
09
09 09
09 09

LI

LE

IndoorOutdoorFuryu, Chiba

OutdoorRelative Humidity (%)

Relative Humidity (%)

LI

Indoor

Feb- Mar- Apr- May- Jun- Jul- Aug- Sep- Oct- Nov- Dec- Jan09
09
09
09
09
09
09
09
09
09
09
10

Shinrinnobokujo Tango, Kyoto
90
80
70

LN

60
50
40
30
20
10
0

100
90
80
70
60
50
40
30
20
10
0

ME

HE

IndoorOutdoorNov- Dec- Jan- Feb- Mar- Apr- May- Jun- Jul- Aug- Sep- Oct11 111212
12
12 12 12
12 12
12
12

Jul-10 Aug- Sep- Oct- Nov- Dec- Jan- Feb- Mar- Apr- May- Jun10
10
10
10
10
11
11
11
11
11
11

Yurikagonoie, Mie

Square One, Hokkaido

100
90
80
70
60
50
40
30
20
10
0

LE

IndoorOutdoorRelative Humidity (%)

Relative Humidity (%)

GROUP B

Apr- May- Jun- Jul- Aug- Sep- Oct- Nov- Dec- Jan- Feb- Mar12
12 12
12 12
12
12
12 12
13
13
13

Leyenda, Toyama

100
90
80
70
60
50
40
30
20
10
0

LE

IndoorOutdoor

Dec- Jan- Feb- Mar- Apr- May- Jun- Jul- Aug- Sep- Oct- Nov11
12
12
12
12
12
12
12
12
12
12
12

Madeinoie, Fukushima

Fig.5. Mean Monthly Relative Humidity of Indoor, Outdoor and Interstitial Sensors with the Highest Monthly Mean Relative
Humidity

210 JAABE vol.14 no.1 January 2015

Kyle Holzhueter

2.4 Determining the Effectiveness of VentilatedRainscreens to Reduce Interstitial MoistureThrough an evaluation of the potential for moldgrowth and a clarification of moisture dynamics, theinfluence of rainscreens on interstitial moisture can be
examined.
Furthermore, it must be remembered that the sixbuildings monitored are located in unique locations
and climates. In order to compare buildings in different
locations and climates, the interstitial hygrothermal
environment is evaluated given its specific climate.
The sensor which surpasses the guideline by thegreatest number of hours is compared to the ambientenvironment. For example, the number of hours theinterstitial sensor surpasses the guideline is subtracted
from the number of hours the outdoor sensor surpasses
the guideline. A building with a positive value has
performed well given its specific climate. That is,although the ambient environment has surpassed theguideline for a given number of hours, the interstitial
sensors only surpassed the guideline for a fractionof those hours. A building with a negative value has
not performed well given its environment. That is,although the ambient environment only surpassed theguideline for a specific number of hours, the interstitial
sensor surpassed the guideline for more hours.
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.
The number of consecutive hours the guideline
was exceeded varies by building. The number ofconsecutive hours the interstitial environment of Group
B buildings surpassed the guideline was only fourhours. Both of these buildings utilize rainscreens. On
the other hand, the interstitial environment of three ofthe four Group A buildings surpass the guideline forover 1000 hours. These buildings have exterior plaster
finishes directly applied to bale walls.
The temperature and relative humidity of the two
sensors from each building that surpassed the guideline
the greatest number of hours are graphed in Fig.4.
From Group B, only one sensor from Leyenda
surpassed the guideline and no sensors surpassed theguideline from Madeinaie. Although it never surpassed
the guideline, Madeinaie's interstitial sensor with thegreatest monthly mean relative humidity, LE, waschosen to depict Madeinaie's interstitial hygrothermal
environment.
The monthly mean relative humidity of indoor,
outdoor and interstitial sensors with the highestmonthly mean relative humidity are graphed in Fig.5.
At some point during the observation period, all of theGroup A buildings' interstitial monthly mean relative
humidity surpass the outdoor monthly mean relative
humidity. However, during the observation period,

JAABE vol.14 no.1 January 2015

none of the Group B buildings' interstitial monthly
mean relative humidity surpass the outdoor monthly
mean relative humidity.
All of Furyu's sensors surpassed the guideline by
hundreds of hours. However, Furyu also had the most
severe climate.
When comparing the difference between interstitialand outdoor hours surpassing the guideline, Square
One performed the worst. However, despite having thesensor with the highest number of hours surpass theguideline, only two of Square One's nine interstitial
sensors surpassed the guideline. This suggests a
localized problem near the exterior finish.
Many of Shinrinnobokujo Tango's sensors surpassed
the guideline. Shinrinnobokujo Tango also had a severe
climate, but the climate was similar to Group B's
Madeinaie's climate in terms of monthly mean relative
humidity and hours surpassing the temperature andrelative humidity guideline.
Despite being located in rather severe climates
where the ambient environment surpassed theguideline for well over 2000 hours, the number ofhours the interstitial environment of Group B buildings
surpassed the guideline was only 4 hours.
The research results suggest that rainscreens are aneffective means to reduce moisture and prevent moldgrowth in straw bale walls.
4. Conclusion
The hygrothermal conditions of six straw bale
structures in Japan were monitored. The purpose of thepresent study was to (one) evaluate the potential for
mold growth in the six buildings, (two) clarify moisturedynamics, and (three) determine the effectiveness ofventilated rainscreens to control moisture in straw balewalls. (1) The potential for mold growth was foundto vary by structure. Buildings utilizing rainscreens
were found to have a lower risk of mold growth. (2)Buildings utilizing rainscreens were found to have lessinterstitial moisture. Presumably, the use of rainscreens
deflected rain from the plastered straw bale walls,
which in turn reduced interstitial moisture. And (3)the research results suggest that rainscreens are aneffective means to reduce moisture and prevent moldgrowth in straw bale walls.
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