Proceedings of the 55th International Convention of Society of Wood Science and Technology
August 27-31, 2012 - Beijing, CHINA
The Hygrothermal Performance of Wood Frame Wall
System in Suzhou Lake Tai Climate Zone
Xiaohuan WANG
Assistant Research Fellow, Beijing Forestry Machinery Research Institute of
State Forestry Administration, China
Benhua FEI
Research Fellow, International Bamboo and Rattan Network Center, China
Jun NI
President, Suzhou Crownhomes co., Ltd., China
Abstract
Long-term on-site assessment of hygrothermal performance is necessary for developing
energy-efficient and durable wood frame wall system. In this paper the hygrothermal
performance of cavity insulation wall was examined from temperature, relative humidity, air
pressure in cladding ventilation and wood material surface temperature throughout the year in
Suzhou Lake Tai of hot summer & cold winter climate zone. The results clearly demonstrated
the effect of the cavity insulation, cladding cavity ventilation, air and vapor barrier. Thermal
performance was proved very well because of the wall cavity insulation. Cladding ventilation
including openings was effective at lowering relative humidity of insulated wall cavities.
Condensation and mold growth were not found inside the wall during the whole testing period.
Wood frame wall system exhibited very good hygrothermal performance to be widely used in
hot summer & cold winter climate zone in central China.
Key words: hygrothermal performance, wood frame wall, insulation, vapor retarder
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Introduction
Long-term on-site assessment of hygrothermal performance is necessary for developing
energy-efficient and durable wood frame wall system. Dynamic changes of temperature,
relative humidity and moisture content can reveal the function of each material in multilayer
wall. Heat and moisture transfer through wall was measured under the different materials and
climatic conditions[1-4]. These studies revealed the factors that affect the wall hygrothermal
performance, such as ventilation, airtightness, insulation, infiltration and vapor diffusion.
Experiments and simulation results have helped to improve hygrothermal performance of the
exterior wall system, and professional guidance or recommendations were provided [5-7].
China is divided into five climate zones for having a vast terrain territory. Suzhou is
identified as a hot summer & cold winter climate zone, which is a north subtropical monsoon
climate of hot summer, cold and dry winter, wet and humidity. Complicated environment
conditions increase the difficulty of building wall design to maintain energy saving and
durability. The increasingly wood frame houses were built in hot summer & cold winter
climate region. However, hygrothermal performance of wood frame wall system is still
unknown under this climatic condition.
This study is developing and implementing an on-site systems engineering approach to
monitor long-term changes in the hygrothemal responses of wall system. The medial and
lateral wall, ventilation cavity and insulated wall cavity conditions were monitored for air
temperature, relative humidity, air pressure in cladding ventilation and wood material surface
temperature in Suzhou Lake Tai of the hot summer & cold winter zone climate. The results
will be used to support the future design standards regarding exterior wall requirements
Tested building and Wall configuration
Suzhou Crownhomes co., Ltd established a low-carbon demonstration wood house at
park by the side of Suzhou Lake Tai (see figure 1). The building is a post and beam
construction consisted of glulam. The wall thickness was 125mm filled in the main frame
structure. Configuration and materials of multilayer wall illustrated in figure 2. Thermal
insulation was by inserting the premium loose-fill fiberglass insulation into the cavity
between studs in the exterior wall, which was then covered on interior panel with gypsum
board and the exterior panel with OSB sheathing. The continuous polyethylene films were
used as interior vapor retarders placed on the interior surface of cavity insulation.
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Proceedings of the 55th International Convention of Society of Wood Science and Technology
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Figure 1 Demonstration test house
Figure 2 Configuration of multilayer wall
Instrumentation and sensor locations
One frame in the south exterior wall was chosen to be tested according to the field
conditions of the house. Air temperature and relative humidity were monitored by
JWSK-6ACC05 sensor with accuracy of 0.1. Surface temperature was measured using type T
thermocouples, calibrated to 0.1K. Ventilated cavity pressure was measured by JQYB
atmospheric pressure transmitter with accuracy of 0.1kPa.
Sensors were connected to the data collection equipment, which continuously monitor
the hygrothermal performance of the test wall. Data were recorded and stored in computer
every minute, and could be extracted every 30 min or every hour flexibly to analyze by
operating software. The monitor process began in June 2010, and the testing has been done
continuously 2 years.
The sensor locations are illustrated in figure 3. Sensors of air temperature and relative
humidity (RHT1~ RHT10) were distributed in each layer from the medial to lateral wall.
These were installed 300mm from the top and bottom of the test wall respectively. Sensors of
air pressure (P1 and P2) were placed in cladding ventilation cavity, which locations were
same as RHT4 and RHT9.
Sensors of surface temperature (T1~T6) of stud and OSB were located in cavity
insulation. T1 and T5 were located in the center of top and bottom wood plate, T2 and T4
were located in exterior sheathing board, 300mm from the top and bottom wood plate
respectively. T3 and T6 were located in stud and the exterior sheathing board respectively,
centered vertically between the top and bottom plates.
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Figure 3
Schematic drawing of sensor locations in the test wall
Results and discussion
The trend was basically identical between RHT1~RHT5 and RHT6~RHT10 described in
[7]. Figure 4 and figure 5 only present air temperature and relative humidity changes of
RHT1~ RHT5 in the last one year in addition to the August data failed in this paper. Air
temperature of the medial wall varies were less during the period of April to October than the
period of November to May. Air temperatures of the medial wall were closer to air
temperatures of the interface between the gypsum board and insulation layer. Air temperature
changes trend of the interface between the insulation layer and OSB sheathing board was
closer to air temperature of the lateral wall, which confirmed that effective function of the
insulation layer. January was the indoor coldest month from the changes of the whole year.
Although air temperature of the lateral wall was near to -3.0℃, air temperature of the medial
wall could maintain 11℃. The peak of each interface air temperature clearly showed the
hysteresis behavior from the medial to lateral of the test wall.
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40
35
Temperature / ℃
30
25
20
15
10
5
0
-5
6/24/11
7/9/11
9/16/11
10/18/11
11/12/11
12/12/11
1/12/12
2/12/12
3/12/12
4/12/12
5/12/12
Date(Month/Day/Year)
the medial wall
Figure 4
gypsum/insulation
insulation/OSB
air space ventilation cavity
the lateral wall
Changes of interface air temperatures for the whole year
Relative humidity changes over time clearly suggested the opposite state compared with
temperature. Relative humidity range of the lateral wall was from 30% to 99%, but relative
humidity range of the medial wall was lower, more steady and proper to live. Relative
humidity of the medial of wall was always little lower and nearer to relative humidity of
interface between the gypsum board and insulation layer. Relative humidity changes trend of
the interface between the insulation layer and OSB sheathing have been shown the same as
those of the lateral wall, which were different with relative humidity of the gypsum board
cavity-side obviously. It was shown that the vapor retarder of polyethylene film really
prevented the vapor into the indoor through wall cavity.
Relative humidity of the interface between the insulation layer and OSB sheathing was
stable comparatively, and lower than relative humidity of inside the cladding cavity.
Apparently, the waterproof and moisture permeable building paper was very indeed effective
in condition of the high relative humidity of outdoor. But relative humidity of the OSB
cavity-side was lowest when relative humidity of each the medial and lateral wall was high in
summer, especially in July. The moisture was restrained from indoor into wall cavity by vapor
retarder of polyethylene film. The temperature of ventilated cavity up to 36.7℃ in July, when
the highest relative humidity was 83.9%. However, the time was not so long under this high
temperature and relative humidity state. There was not mold growth according to reference
[8].
Both the stucco and air space ventilation cavity resistance to moisture was obviously
effective since data showed relative humidity inside the cladding cavity before reached the
maximum was always lower than relative humidity of the lateral wall, and the peak of relative
humidity inside the cladding cavity clearly showed the hysteresis behavior.
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100
Relative humidity /%
90
80
70
60
50
40
30
6/24/11
7/9/11
9/16/11
10/18/11
11/12/11
12/12/11
1/12/12
2/12/12
3/12/12
4/12/12
5/12/12
Date(Month/Day/Year)
the medial wall
Figure 5
gypsum/insulation
insulation/OSB
air space ventilation cavity
the lateral wall
Changes of interface relative humidity for the whole year
Temperature and relative humidity data measured at the coldest day of outdoor monthly
from October to March of next year were chosen to analyze for illustrating the state of heat
and moisture transfer through the multilayer wall. Temperature and relative humidity
gradients through the test wall profile were obvious in Figure 6. Temperatures were reduced
from medial to lateral wall, and the trends of relative humidity were opposite. Temperature
difference was the largest between gypsum board and OSB sheathing in the process of heat
transfer from medial to lateral wall. The largest temperature difference has reached 8.2℃
happened in December. The insulation kept the indoor warm. Temperature difference between
ventilated cavity and lateral wall was about 4℃. There was a certain heat barrier because of
air space. Figure 7 showed the surface temperature of SPF frame dimension lumber and OSB
sheathing. The surface temperatures of OSB sheathing (T2, T4 and T6) developed
continuously along with the changes of the lateral wall temperatures. The surface
temperatures of wood frame (T1, T3 and T5) developed smaller than those of OSB sheathing.
All surface temperatures were above 0℃. So the risk of water condensation in the wall could
be eliminated in winter.
Relative humidity gradients were largest between ventilated cavity and lateral wall. The
maximum difference was 34% happened in December. In winter relative humidity maximum
gradient was 14.6% from the interface between the gypsum board and insulation layer to the
interface between the insulation layer and OSB sheathing. So the vapor retarder of
polyethylene film was perfect. But at the same time relative humidity of medial wall didn’t
attain to 40%. Therefore, the indoor environment was felt little dry.
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Proceedings of the 55th International Convention of Society of Wood Science and Technology
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25
80
70
Relative humidity /%
Temperature / ℃
20
15
10
5
0
60
50
40
30
20
10
-5
0
0
15
124.5
85 94.5
0
15
10/25/11
1/23/12
Figure 6
124.5
85 94.5
Distance/mm
Distance/mm
12/17/11
3/12/12
11/21/11
2/3/12
10/25/11
1/23/12
11/21/11
2/3/12
12/17/11
3/12/12
Temperature and relative humidity gradients through the test wall profile
45
40
Temperature / ℃
35
30
25
20
15
10
5
0
-5
10/18/11
11/12/11
12/12/11
1/12/12
2/12/12
3/12/12
4/12/12
5/12/12
Date(Month/Day/Year)
T1
T2
Figure 7
T3
T4
T5
T6
the lateral wall
Surface temperature of wood materials of SPF and OSB
Figure 8 showed the air pressure of the top and bottom in cladding ventilation cavity.
The top air pressure P1 was always less than the bottom air pressure P2. The average
difference of air pressure was 0.12kPa. This indicated airflow entering low on the wall and
exiting at the top through ventilation holes in metal strips. Winter air pressure was higher than
other seasons’ from the whole year data.
Air Pressure /kPa
104
103
102
P1
P2
101
100
10/18/11
11/12/11
12/12/11
1/12/12
2/12/12
3/12/12
4/12/12
Date(Month/Day/Year)
Figure 8 Air pressure in cladding ventilation cavity
The result analysis of two years data seems that the application of wood frame wall
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Proceedings of the 55th International Convention of Society of Wood Science and Technology
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system in demonstration test house is very successful in Suzhou Lake Tai climate zone.
Thermal performance of this system was good and no mold growth and water condensation
occurred inside the wall during the testing period. Most time of temperature and relative
humidity indoor were perfect or acceptable in addition to January night. The increasing
humidity should be needed to protect the dry glulam against split in winter.
Conclusions
Long-term on-site assessment of hygrothermal performance was presented in this paper.
The results indicated that water vapor control strategy performed well at reducing
summertime mold growth and wintertime condensation in testing wall assemblies in Suzhou
Lake Tai of hot summer & cold winter climate zone. Vapor retarder, waterproof & moisture
permeable building paper and cladding ventilation cavity could increase the moisture
tolerance and reduce risks related moisture for multilayer wall. Thermal performance was on
perfect level because of cavity wall insulation from the whole testing period. Therefore, the
same configuration design as testing exterior wall can be widely used in China hot summer &
cold winter climate zone.
Acknowledgements
This research has been supported by the forestry industry research special funds for
public welfare projects, State Forestry Administration of China, contract No. 201204701 and
leading talent project in Suzhou Wuzhong District, contract No. WC201107.
References
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