Passive Thermal Performance of Chez Soleil,
Self Sufficient in Central Texas
Paul N. Breaux
Texas Solar Energy Society
11301 Lime Creek Rd.
Volente, Texas 78641
e-mail: pbreaux@hotmail.com
A unique opportunity for studying the passive solar and
passive cooling performance of a house in the hot/humid
Sunbelt is presented in this paper. “Chez Soleil” is an
entirely self-sufficient house built in 1996 in central Texas
near Austin. Results of over 5 years worth of thermal data
show that the ground-coupled high thermal mass design
works quite well even in a challenging Texas climate.
The thermal performance of this house is presented during 4
extreme cases as well as during the entire heating and
cooling seasons. Temperature fluctuations are studied from
completely sunny and from completely overcast days within
six weeks of the winter and summer solstices. These
extreme conditions are picked from 4 years worth of
temperature monitoring data from 1998-2001.
Hourly temperature measurements were made at strategic
locations within the house. Dallas SC 1820 temperature
sensors were placed in strategic locations throughout Chez
Soleil and its environs. These semiconductor temperature
sensors were hooked up to a HotTherm apparatus provided
by Spiderplant. For this paper, only data from one sensor
from a central point on the “First Floor” is used. This
sensor is located at a height of 4.5 feet above the floor.
Examining this data can accurately describe the thermal
performance of this passive house design. This data
includes a variety of climatic conditions occurring over a 4
year period 1998-2001.
A nearby weather station run by the Texas Natural
Resource Conservation Commission (now TCEQ) provides
hourly data for outside temperature, wind speed and solar
radiation among other parameters. Outside temperature data
is taken from the CAMS 38 air monitoring station at 12200
Lime Creek Rd., which is about 1 mile, southeast. This also
is hourly data, which has been monitored since 1997.
Figure 1 shows results of temperature measurements during
completely sunny Summer days. This chart describes the
temperature profile during 13 different days within six
weeks of the Summer Solstice. During these Summer days,
the house orientation and shading devices prevent any
significant amount of solar radiation from entering the
house. As a result, any heating would have to occur by
conduction or convection except for a small amount of bluesky radiation.
The lighter (color) broken line in figure 1 shows the actual
outside temperature as measured at the nearby TCEQ station.
This outside temperature fluctuates from about 70 to 105
degrees, which is typical during the summer in central
Texas. The dark solid line describes the hourly temperature
taken in a central part of the house on the first floor.
Due to the ground-coupled high thermal mass design of this
house, this inside temperature fluctuates very little. The
temperature inside the house on the first floor is shown to
vary by only 1 or 2 degrees even with no added active
heating/cooling systems. The temperature appears to
actually peak in the evening at about 10 pm during these
sunny Summer days. This is a good measure of the entirely
passive cooling performance of Chez Soleil.
Figure 2 describes results of 13 selected cloudy Summer
days. Actually, only the first 3 days were completely
cloudy while the rest were mostly cloudy. It is hard to find
many completely cloudy days in the middle of Texas during
the summertime! The temperature curves in this case are
quite flat, especially for the first 3 completely cloudy days.
The first floor temperature even stays very stable during the
first day (5/4/01) even though the outside temperature
fluctuates quite a bit. This shows the stabilizing effects of
the high thermal mass design.
Figure 3 presents results from 13 selected sunny Winter
days within one month of the Winter solstice. These
examples show a variety of outside temperatures ranging
from 25 to 75 degrees. In most cases, the first floor house
temperature is seen to rise up to about 5 degrees warmer as
a result of the incoming solar gain during the daytime. A
similar temperature rise is seen inside the house regardless
whether the outside temperature is much colder or warmer.
Figure 4 shows the results of 13 completely cloudy Winter
days. The first floor temperature is shown to maintain a
fairly level temperature even when the outside temperature
is up to 30 degrees colder. This temperature can decline up
to about 5 degrees per day during extreme conditions with
no solar gain. A variety of cloudy wintry conditions are
presented in the chosen days described in this figure.
The next two figures describe the more long-term
performance of the passive heating/cooling at Chez Soleil
during the Winter and Summer. The monitored temperature
on the first floor of the house was averaged for each hour of
each day during the 4-year period 1998-2001. This was
done in an attempt to average out some of the short-term
fluctuations, which normally occur.
The outside temperature was also averaged for each hour of
each day during this same time period. In addition, a
running average of the following 24 hours of average
outside temperatures was done to smooth out this data.
Well over 95% of the outdoor temperature data is available
during these monitoring periods. However, for a variety of
reasons, about 90% of the first floor data is available during
June-Sep and only about 65% is valid from November-Feb.
The results are presented in figures 5 and 6.
Figure 5 presents the averaged temperatures during the
Winter months in central Texas (November through
February). The first floor temperature is described by the
thicker solid upper line, while the outside temperature is
shown by the thinner dashed line. It is apparent that the
house maintains a significantly higher temperature than the
average outside temperature during the entire wintertime
even without any additional active heating. The average of
all the first floor temperature measurements during this
winter period is about 63 degrees.
A spreadsheet calculation was done in EXCEL to determine
the temperature difference between outside and inside (first
floor) temperatures for each hour. During the month of
November, the average hourly difference in temperature
was calculated to be nearly 8 degrees F. For December,
January, and February; the average temperature difference
was determined to be 11, 9.4 and 5.9 degrees respectively.
The average total temperature difference for the entire 4month period was calculated to be 8.6 degrees.
Figure 6 illustrates similar temperature characterization for
the summer months June – September. As before, the
outside and first floor temperatures were averaged during
the 4 year period of monitoring 1998-2001. The first floor
temperature remains relatively stable with small daily
fluctuations throughout the Summer. The temperature
peaks during August and quickly declines by midSeptember because of outside cooling and ventilation
through windows. The average of all the first floor
temperature measurements during this Summer period is 79
degrees.
Spreadsheet calculations determine that the averaged hourly
temperatures in the first floor remain at about 3.7 degrees
cooler that the corresponding average smoothed outside
hourly temperatures for June. Similarly, this average
temperature difference is 3.4, 2.3, and –0.8 degrees for July,
August, and September. The total average temperature
difference for June through August is slightly over 3 degrees
Fahrenheit. These numbers were very similar when using
the unsmoothed non-running averaged data for the outside
temperature.
Figure 7 presents the results of ground temperature
measurements taken at the foundation level and beneath.
The ground temperatures are obtained from direct
measurements every 2 weeks and averaged over the 5-year
period 1998-2002. The annual average for measurements at
all depths is very close to 70 degrees. The deepest
temperature is taken from a well under the house at a depth
of 6 feet below the foundation in the “Dungeon”. This
temperature varies from 66 to 75 degrees and peaks during
September and is shown as an even dotted line in figure 7.
The temperature is monitored just below the foundation
level (near the solar hot tub). The thicker unbroken line in
figure 1 describes this “Foundation” temperature. This
temperature fluctuates from 63 to 77 degrees and peaks
during August-September.
The temperature was also monitored in an enclosed water
tank in the garage. This temperature ranges from 58 to 83
degrees and peaks during August. Rainwater is collected in
this tank and the water volume varies, so this measurement
can only be considered a rough estimate of the true average
temperature in the garage throughout the year.
The effects of wind were also investigated but showed no
significant effect on the inside house temperature. The
results of the passive performance of “Chez Soleil” show
that this design works well for the Texas climate.
Sunny Summer Day Temperatures
100
95
90
85
80
75
First Floor Temp
TNRCC Outside Temp
70
65
5/22/00
5/23/00
5/24/00
5/28/98
6/1/98
6/2/98
6/3/98
6/28/00
8/11/99
8/12/99
8/13/99
8/14/99
8/15/99
Fig. 1: Sunny Summer day temperatures.
Cloudy Summer Day Temperatures
95
90
Degrees F
85
80
75
70
First Floor
Outside
65
60
5/4/01
6/20/99
8/22/98
5/5/00
Fig. 2: Cloudy Summer day temperatures.
5/9/99
5/13/98
5/19/01
5/27/00
6/5/98
6/7/01
6/14/01
7/4/98
Sunny Winter Day Temperatures
75
First Floor
Outside Temperature
65
55
45
35
25
12/13/98
12/14/98
12/16/98
12/13/99
12/14/99
12/16/99
12/17/00
1/5/00
1/4/99
1/10/99
1/9/00
1/15/99
1/16/99
Fig. 3: Sunny Winter day temperatures.
Cloudy Winter Day Temperatures
75
65
55
45
35
First Floor
25
Outside Temperature
12/1/00
12/12/00
12/9/98
12/11/99
15
Fig. 4: Cloudy Winter day temperatures.
12/12/99
12/18/98
12/20/99
1/7/00
1/12/00
1/26/00
1/27/00
1/28/00
1/29/99
Average Temperatures
Winter 1998-2001
75
70
65
60
55
50
45
40
35
30
First Floor
Outside Average
November
December
January
February
Fig. 5: Average temperatures during Winter (1998-2001).
Average Temperatures
Summer 1998-2001
100
95
90
85
80
75
70
First Floor
65
Outside
Average
60
June
July
55
Fig. 6: Average temperatures during Summer (1998-2001).
August
September
Average Ground Temperatures 1997-2002
90
Foundation
Dungeon
85
Garage Tank
80
75
70
65
60
55
Jan
Feb
Mar
Apr
May
50
Fig. 7: Average ground temperatures 1997-2002.
June
July
Aug
Sep
Oct
Nov
Dec