Download Compressed Earth Block & Strawbale Home: Monitoring and

Compressed Earth Block & Strawbale Home: Monitoring and
Assessment of a Hybrid Wall Assembly
Greg Leskien1,a, Jordan Hoogendam1,b and Henry Wiersma2,c
1Zon
2970233
Engineering Inc., Kitchener, Ontario, Canada
Ontario o/a Earth Block Construction, Cobourg, Ontario, Canada
[email protected], [email protected], [email protected]
with contributions from Colin MacDougall, Queen’s University and John Straube, University
of Waterloo
This project was partially funded by Canada Mortgage and Housing Corporation (CMHC)
under the terms of the External Research Program, but the views expressed are the
personal views of the authors and do not represent the official views of the CMHC.
Keywords: Compressed, Earth, Block, Straw Bale
Abstract
The primary objective of this study is to measure, analyze and comment on the moisture and
thermal characteristics (and ultimately the durability) of the wall assembly in question: a
compressed earth block (CEB) and straw-bale hybrid assembly. In support of this, the following
tasks will be discussed:
•
•
•
Long term monitoring of moisture and heat conditions and flows within the wall enclosure
assembly
Air infiltration testing of the demonstration home
Measurement of the energy performance and identification of heat loss/gain characteristics of
the test home
In the end, the intent is that the observations and conclusions made in this report will be used in
conjunction with the findings presented in the structural study titled “Compression Testing of
Weathered Compression Earth Block (CEB) Prisms” as published by Queens University to help
answer the following question:
"Can compressed earth blocks (CEB) and straw bales be used in conjunction with one another to
produce a wall enclosure assembly that is environmentally friendly, energy efficient, and durable?"
Introduction
Project Description. The demonstration home is two storey structure constructed by and on the
client's property north of Cobourg, Ontario. The building has a usable floor area (measured to the
inside of the walls) of roughly 84.5 m2 (910 ft2) and an interior volume of approximately 217 m3
(7,660 ft3).
Fig. 1: Southwest Facade of the Demonstration Building
Of particular interest are the above-grade wall assemblies, which are a hybrid construction of
compressed earth blocks (CEBs) and straw-bales. The 4" high by 10" wide by 14" long CEBs were
manufactured onsite using ground-source clay excavated on the property. They play a dual role in the
wall assembly, creating the support structure for the second floor and roof framing above, as well as,
serving as the interior finish for both the ground and second floor levels. The interior face of the CEBs
have been left unfinished and exposed, while the exterior face was parged with a surface coat of clay
slurry (1/8" to 1/4" thick) to protect the blocks from the weather during construction. Straw-bales, 18"
in thickness, have been placed on the exterior side of the CEBs and serve as the insulation layer in
the wall assembly. The exterior face of the straw-bales were first dipped in a 2" deep clay slip mixture
(a "soupy" mix of clay and water) which served as the first coat of plaster, and eliminated the need
for wire or plastic mesh. After the bales were in place, they were finished with earthen plaster (3/4"
to 1" thick). A section of the wall assembly is provided in Fig. 2.
Fig. 2: Typical Above-Grade Wall Section
Windows are comprised of vinyl frames (with wood cladding inside) with 7/8" thick insulated
glazing units complete with 2 panes of 3mm clear glass, with a Cardinal CG LowE-272 low-
emissivity coating on surface #2 and an XL Edge spacers separating the glass. Doors are handcrafted
out of solid wood and are 2 1/4" thick.
The structure of the roof assembly is provided by 3x6 reclaimed hemlock joists spaced at 24"
on centre. 2x8 lumber at 4'-0" centres and 2x4 lumber at 2'-0" centres were placed on top of the
hemlock as strapping. Three 2" layers of reclaimed black rigid foam (waste from the manufacturing
process of foam board) was installed between the 2x8 roof joists. Samples sent for testing in a guarded
hot plate revealed an average R-value of 3.1 (h°F ft²)/Btu. A 6 mil polyethylene sheet was placed on
the interior side of the insulation and finished with 3/4" pine boards treated with hemp oil. On the
exterior, a layer of 1/2" spruce plywood was attached to the 2x4 lumber strapping and covered with
shingles.
Fig. 3: Second Floor - Southeast and Southwest Dormers
Foundation walls are constructed with two (2) rows of 8" expanded polystyrene ICF blocks,
16" in height. In addition, 3 1/2" of concrete was cast-in-place directly against the interior surface of
the innermost ICF blocks in order to make the foundations wide enough to bear the walls above. This
concrete was not reinforced or tied to the ICF wall in anyway. Supporting the foundation walls are 8"
deep and 39" wide concrete footings that run around the perimeter of the demonstration home. Four
(4) feet of 2" thick extruded polystyrene insulation was also installed, and extended out from the
foundation, horizontally away from the building.
Fig. 4: Radiant Floor Heating Installed for Ground Floor
The ground floor consists of 4" thick CEBs placed on 1" of sand over 1 1/2" of extruded
polystryene board insulation. Inside the sand are radiant heating tubes attached to a layer of 6x6 wire
mesh.
Fig. 5: Interior - Ground Floor
Heating was provided during the study period by radiant floor heating (ground floor only)
supplied with hot water from a 40 gallon storage type electric water heater mounted inside the building
on the ground floor. No cooling or ventilation were provided to the building.
Long Term Monitoring
Methods. Long term monitoring of the of the demonstration building was carried out for a period of
one year (August 2011 through August 2012). During this time, temperature and relative humidity
readings were collected at fifteen (15) minute intervals at ten (10) locations around the building. Each
location consisted of five (5) temperature sensors (thermocouples) and three (3) relative humidity
sensors located at various points on and within the wall assembly. For the purposes of this report, 4
locations distributed around the building have been selected for analysis and are shown in Fig. 6 and
summarized in Table 1.
Fig. 6: Sensor Locations
Table 1: Wall Sensor Summary
Type1
A
Mounting Description
T and RH Near ground, exterior face of CEB
T and RH 42" up, exterior face of CEB
T and RH Bale embed, 42" up, 14" from exterior face of CEB
T only
78" up, exterior face of concrete at this location
T only
78" up, exterior face of CEB
B
T and RH Near ground, exterior face of CEB
T and RH 42" up, exterior face of CEB
T and RH Bale embed, 42" up, 14" from exterior face of CEB
T only
C
42" up, embedded in CEB
T and RH Near ground, exterior face of CEB
T and RH 42" up, exterior face of CEB
T and RH Bale embed, 42" up, 14" from exterior face of CEB
D
T only
42" up, embedded in CEB
T only
78" up, exterior face of CEB
T and RH Near ground, exterior face of CEB
T and RH 42" up, exterior face of CEB
T and RH Bale embed, 42" up, 14" from exterior face of CEB
T only
Under window, exterior face of CEB
T only
78" up, exterior face of CEB
Humidity readings at the wall locations were acquired using Honeywell HIH-4021-003
humidity sensors mounted within a vented rigid plastic tube as depicted in Fig. 7. Also mounted inside
the tube is a type T thermocouple. Type T thermocouples mounted in separate plastic tubes were are
also used for the remaining two (2) temperature only sensor points at each location.
Fig. 7: Integrated Humidity and Temperature Sensor
Temperature readings from the Type T thermocouples were measured by two (2) Campbell
Scientific AM25T solid-state multiplexers. Humidity sensors were measured by one (1) Campbell
Scientific AM16/32B relay multiplexer. All measured temperature and humidity readings were then
acquired and stored by a single Campbell Scientific CR1000 measurement and control system. Fig. 8
shows the multiplexers and measurement and control system mounted to the interior walls in the north
corner of the building.
Fig. 8: Data Acquisition System for Wall Temperature and Humidity Sensors
Interior temperature and relative humidity was also recorded at fifteen (15) minute intervals
over the monitoring period. These readings were collected by an Onset HOBO Pro v2 data logger
located on the ground floor, at the center of the building, and mounted at a height of 2.0 metres. Fig.
9 shows a photo of this data logger. Its location is shown on Fig. 6.
Fig. 9: Interior Temperature and Humidity Data Logger
Similarly, exterior temperature and relative humidity was collected at fifteen (15) minute
intervals over the monitoring period by an Onset HOBO Pro v2 data logger mounted to the fascia
board on the northeast facade of the building. The external temperature/humidity sensor of this data
logger sensor was mounted in a solar radiation shield to acquire ambient temperature readings. Fig.
10 shows a photo of this data logger. Its location is shown on Fig. 6.
Fig. 10: Exterior Temperature and Humidity Data Logger
During the one-year monitoring period there were instances where readings from the interior
and exterior monitoring stations were unavailable. For these occurrences, interior temperature
readings were replaced with those recorded using the onboard thermocouples present in the
acquisition equipment used for wall readings. In instances where data collection was interrupted,
exterior readings were obtained from the "Cobourg (Aut)" weather station via the Environment
Canada website. Since weather station readings are only available at one (1) hour intervals, the same
readings were used at the fifteen (15) minute intervals to augment missing data.
Results and Observations. Fig. 11 through 14 present the relative humidity readings collected at
Locations A, B, C and D over the course of one year. Each location includes readings taken at the
following positions:
•
Exterior face of CEB - approximately 6" above the top of the foundation wall
•
Exterior face of CEB - approximately 42" above the top of the foundation wall
•
Exterior face of the straw bales at a height of 42" above the top of the foundation wall approximately 14" from the exterior face of the CEB
The values collected at these locations were plotted along with interior relative humidity values,
as well as, interior and exterior temperatures. [Note that interior relative humidity values for the period
of mid November 2011 through April 2012 are unavailable, and have been discontinued during this
time on the graphs below].
Fig. 11: Relative Humidity at Location A from August 2011 through 2012
Fig. 12: Relative Humidity at Location B from August 2011 through 2012
Fig. 13: Relative Humidity at Location C from August 2011 through 2012
Fig. 14: Relative Humidity at Location D from August 2011 through 2012
It is important to consider three pertinent details when reviewing the data presented above.
First, the building remained incomplete for much of the test period. From the period of August 2011
through March 22, 2012, the building was protected by a series of plastic tarps to allow the final coat
of earthen plaster to be applied to the exterior face of the straw bales. Fig. 15 shows the demonstration
home in this state.
Fig. 15: Demonstration Home Protected by Tarps During Finishing
With the tarps in place, the lower half of the building was never fully exposed to the elements.
This had two side effects. Firstly, it protected the wall assembly from rain and snow, minimizing the
moisture that would have potentially been added to the wall had it been exposed to these sources.
Second, it sheltered the building from the drying potential it might have experienced had it been
exposed to wind and an environment with lower relative humidity values. Although not fully air tight,
it is thought that that tarps functioned as a vapour retarder to some degree, slowing evaporation and
vapour transport by diffusion to the exterior environment.
Second, the permanent in-floor heating system was not operated until March 28, 2012.
Without a constant source of heat, the interior temperature of the building drifted with exterior
temperature and solar gain. As a result, the drying potential of the building was drastically reduced,
limiting the ability of materials to dry to the outdoors by vapour diffusion. From time to time an open
flame portable propane heater (i.e. salamander) was used inside the building to keep workers warm
as they finished interior details. Although this would have increased the drying potential during these
times, it is important to note that it would also add moisture to the inside air as propane fuel was
burned.
Third, the types of materials and construction used to build the wall assemblies of this home
inherently result in high amounts of "built-in" moisture. The CEBs, CEB grout, CEB exterior parging,
straw-bales, and exterior earthen plaster finish all contain high moisture contents when installed. It is
important to deal with these high amounts of moisture soon after the building is complete in order to
avoid potential durability issues (e.g. mould and rot). This is done by maximizing the drying potential;
mainly diffusive drying to the outdoors (during heating periods) and evaporation from the exterior
surface of the straw-bales.
When considering Fig. 11 through 14, it is clear that the relative humidity readings at all three
positions in the wall at locations A, B, and C are high; greater than 60% for much of the year, and
more typically around 80%. This is also true of location D, except for the measurements taken within
the straw bale. [Note: The measurements taken within the straw bale at location D fell consistently
between 40 and 50% throughout the year. The reason(s) for the comparatively "low" readings within
the bale are not known at this time. Plans have been made to acquire and insert a portable straw bale
moisture probe next to the sensor to check that the embedded sensor is operating properly.]
The high, and relatively constant, relative humidity readings observed within the walls are
thought to be a combined result of the high built-in moisture and a low drying potential. This is in
some part reinforced when studying the data acquired after the tarps were removed (March 22, 2012)
and the in-floor heating system was operational (March 28, 2012). During the entire month of April
and the early part of May, the building was being actively heated (except during thermal mass
observations). When looking at this time period for all four locations (A, B, C and D), the relative
humidity readings at all three sensors in the walls drop considerably (20% in most cases). This would
make sense as the outward vapour pressure has been increased (increasing drying by vapour diffusion
to the outdoors), as well as, the evaporation from the straw bale surface (which is now exposed directly
to wind and sun). Fig. 16 and 17 present the relative humidity and corresponding temperature values
recorded during this time period at location B.
Also evident in Fig. 11 through 14 are the large increases in relative humidity readings
throughout the walls at all four locations during the month of March 2012. This is thought to be a
direct result of the procedures that were used to install a plaster drip edge to the bottom of the straw
bales around the perimeter of the building. During this time an open flame portable propane heater
was used (between the temporary tarps that enclosed the building and the exterior walls) to heat the
air and surface of the walls to allow for the application of the plaster. The heated air was continuously
circulated around the building by two large fans, ensuring that heat was evenly distributed within the
tarps. The builder also noted that the ground inside the tarps had started to thaw as a result of the
added heat, creating a wet and muddy walking surface. Given these details, it is evident that a
significant amount of moisture was introduced into the space between the tarps and the exterior wall;
moisture from burning fuel, moisture from the thawing ground and moisture for the plaster drip edge.
Since the air surrounding the exterior walls was warmer than the inside of the building, and because
the walls of the building are more vapour permeable than the tarps enclosing it, any moisture present
between the tarps and exterior walls would likely have been driven into the building at this time. This
is supported by the graphs, which show very similar increases humidity at all location during this time
period.
Also worth noting are the occurrences of 100% relative humidity readings measured at
locations C and D for sensors positioned on the exterior face of CEBs, approximately 42" above the
top of the foundation wall. At location C, these high readings are observed as "spikes", and with the
exception of the occurrence in mid-October, appear to coincide with drastic reductions in exterior
temperatures (i.e. 10°C), and the because the building was not permanently heated, drastic reductions
in interior temperatures as well (refer to Fig. 18). Reduction in temperature will increase the relative
humidity, but not to the extent observed. Although the building was enclosed at this time, this location
was on the most exposed side of the building and nearest to the temporary door in the tarped structure.
This location may have been exposed to "free" water at these times, but may have had the ability to
dry out relatively quickly. This location will continue to be monitored to see if these occurrences
continue as the building is heated and occupied.
As can be observed on Fig. 14, the 100% relative humidity readings at location D are much
more consistent, occurring from January through March 2012. This suggests that moisture in liquid
form may have entered the wall at the sensor location and was unable to sufficiently dry out until
building tarps were removed in late March and the building heat turned on at the beginning of April.
It is unclear why the sensor located in the same plane, but mounted 6" above the foundation wall had
much lower readings during this period. This location will continue to be monitored for periods of
high relative humidity, and if required, a test hole will be drilled, and a portable straw bale moisture
probe will be inserted into the wall to verify proper sensor operation.
Fig. 16: Relative Humidity at Location #B for April 2012
Fig. 17: Temperatures at Location #B for April 2012
Fig. 18: Relative Humidity at Location #C for from August to November 2011
Concluding Remarks. It is important to consider the relative humidity and moisture content
thresholds related to the durability of straw bales and CEBs. For straw bales, mould growth is typically
thought to occur at relative humidity values 80% (or 20% moisture content) and higher, over sustained
periods of time [1]. Further to this, rot is known to occur in wood when exposed to extended periods
of warm temperatures and humidity values higher than 95%. Straw is thought to have the potential to
rot in slightly lower relative humidity conditions because of its higher surface area per unit volume.
When reviewing the straw bale readings shown in Fig. 11 through 14, it is evident that relative
humidity values at locations A, B and C were in the 80% range for up to five consecutive months
(August 2011 through January 2012). This suggests that the potential for mould growth may have
existed for these locations. After January 2012, the relative humidity readings for these locations
steadily declined and remained below 80% until June 2012, when values started to increase toward
this threshold again. The readings at location D never approached 80%, consistently remaining
between 40% and 50% relative humidity. The highest relative humidity reading recorded was 90%
and occurred at location A (March 2012). This value is below the 95% threshold, and was not
sustained over a significant period of time, suggesting that risk of rot is remote.
For CEBs, the primary durability concern relating to moisture content is spalling resulting from
freezing of moisture present within the blocks. For this to occur the blocks would need to be near
saturated (100% relative humidity) and temperatures below freezing. During the one year monitoring
period presented in this report, the only sub-zero temperature recorded on the exterior face of the
CEBs was -0.4°C at a relative humidity of 81.7%. This suggests that the risk of freeze/thaw damage
did not occur at any of the locations during the monitoring period.
One aspect of the wall assembly that will continue to be monitored into the future (especially when
the home becomes habited by occupants) is its ability to control vapour diffusion. In a more typically
straw bale home, a vapour control layer is provided on the interior of the wall system by applying a
low vapour permeable paint (≤ 300 metric perms) to the interior facing of stucco [1]. This is done to
throttle the amount of vapour that moves from the building interior into the wall assembly, to a point
where it can condense within the straw. In the demonstration home, this control layer don't appear to
exist, as only the CEBs (and an exterior parge coat) are present on the interior side of the straw bales.
Durability may be compromised if the CEBs and exterior parging coat are unable to provide sufficient
vapour diffusion resistance. This is more prevalent in air tight buildings such as this one (refer to “Air
Tightness Testing” section) when vapour diffusion contributes more significantly to the amount
moisture that is present within the enclosure assembly. If it is determined that the CEB does not offer
suitable vapour resistance, one solution may be to seal the interior face with a product that serves as
a suitable vapour retarder.
Looking to the future, it would be beneficial to continue to monitor the demonstration home for
changes in relative humidity values for at least the next year. In particular, the following points should
be considered when reviewing the data:
•
Reduction in "Built-in" Moisture: Based on the high relative humidity readings observed over
the monitoring period, it is thought that "built-in" moisture from the materials is still present
in the building. Over the next year, values are expected to decrease and stabilize (to some
extent) with exposure to exterior environment (i.e. wind and sun) and a constant source of
interior heating during the winter months.
•
Changes in relative humidity with building use: While it is expected that relative humidity
values will drop over the next year as "built-in" moisture is reduced, it is also anticipated that
the building will be occupied at some point during this time. When this happens, occupants
and their activities (e.g. cooking) will contribute to moisture within the building.
•
Sustained relative humidity values of 80% or higher: The first three months of data collected
during the monitoring period showed sustained RH values near 80%. It will be important to
ensure that this trend does not continue and lead to problems such as mould growth.
•
Relative humidity values nearing 100%: Values of 100% mean the air has reached saturation,
suggesting that moisture in liquid form is present inside the wall. In addition to posing the risk
of rot for straw, this condition is risky for the CEBs if the temperature at the same location is
below freezing.
Air Tightness Testing
Methods. Air tightness testing was carried out on the demonstration building for two (2) reasons. The
first was to demonstrate the low-infiltration potential for this type of construction; the second reason
was to obtain data to educate the heat/loss gain testing that is explained in “Whole Home Heat
Loss/Gain Testing” section.
Testing was carried out on July 30, 2012 starting at 1:00pm. The weather during the test was
clear, the wind relatively calm (11 km/h from the SSW from Environment Canada), the exterior
temperature approximately 27°C and the interior temperature 25°C. Tests were carried out using a
Minneapolis Model 3 blower door fitted with a "D" ring (suitable for a flow range of 115 cfm down
to 30 cfm) and equipped with the DG700 pressure and flow gauge to record measurements.
Before any measurements were made, the building was depressurized to -50 Pa and a general
inspection of the building was carried out to identify locations of air leakage. This was done by
observing movements in the smoke produced by sticks of burning incense held closely to suspected
leakage points. Locations with noticeable air flow were noted. After leakage locations were identified,
the blower door fan was turned off, allowing the building to reach pressure equilibrium with the
outdoors.
Immediately following the general inspection, two multi-point tests were carried out (1:30 pm
and 1:45 pm) to quantify the air tightness characteristics of the building. For both tests a pre-test
baseline pressure measurement was made and "zeroed out" using the DG700 gauge, allowing the
actual change in building pressure made by the blower door fan to be displayed and recorded. Testing
started by depressurizing the building to ±60 Pa and slowly reducing the pressure across the enclosure
to ±15 Pa. A total of eight (8) building pressure (Pa) and eight (8) corresponding blower door fan flow
(cfm) measurements were made during each test across the depressurization range. This data was then
used to calculate air leakage characteristics for the building.
Fig. 19: Blower Door Used During Air Tightness Testing
Results. The air leakage characteristics measured with the blower door are presented in the Table 2.
Based on the test results, the demonstration home is extremely air tight, and is a testament to the care
and attention to detail with which this house was constructed. With an air change per hour (ACH) of
approximately 0.7 at 50 Pa, this house far exceeds the R-2000 requirement (1.5 ACH50) and falls just
short of the stringent Passivhaus target (0.6 ACH50).
Table 2 - Demonstration Home Air Leakage Characteristics
Test #1
Test #2
40 L/s
(85 cfm)
42 L/s
(88 cfm)
Air Changes per Hour (ACH) at 50 Pa
0.66
0.69
Air Flow at 50 Pa per unit of floor area
0.47 L/s/m2
(0.0931 cfm/ft2)
0.47 L/s/m2
(0.0970 cfm/ft2)
Airflow Characteristics
Air Flow at 50 Pa
0.092 L/s/m2
(0.0181 cfm/ft2)
0.096 L/s/m2
(0.0188 cfm/ft2)
Equivalent Leakage Area (EqLA) at 10
Pa
40.0 cm2
(6.2 in2)
47.1 cm2
(7.3 in2)
Effective Leakage Area (ELA) at 4 Pa
17.4 cm2
(2.7 in2)
21.9 cm2
(3.4 in2)
Flow Coefficient (C)
2.9
4.1
Exponent (n)
0.82
0.79
0.99437
0.99432
Air Flow at 50 Pa per unit of surface area
Leakage Areas
Building Leakage Curve
Correlation Coefficient
The air leakage locations, although minor, are identified on Fig. 20 and in Table 3. The flow
exponent (n) values measured in the two tests were both above 0.7, which indicates that smaller holes
tend to dominate the leakage area. This is consistent with the observations that were made during the
general inspection prior to the testing.
Of the locations presented in Table 3, air leakage through the underground conduits supplying
data and power to the building were by far the worst, but could be addressed by injecting spray foam
into the top few inches of the opening. The remaining locations could easily be remedied with
caulking and by replacing weather-stripping at the base of the entry doors. Note that no detectable air
leakage locations were observed on the second floor within 8 feet of the floor.
Fig. 20: Air Leakage Locations
Table 3 - Air Leakage Locations
Description
Leakage
Appraisal1
Potential
Solution
Southeast Door
Leakage occurred between
the door shims adjacent to
the upper hinge.
Low
Caulking
or spray
foam
Light Switch
Electrical Box (West
of Southeast Entrance
Door)
Leakage was observed
where wires pierce the
body of the electrical box
Low
Caulking
Underground Data
Conduit
Leakage was noted at the
end conduit
High
Fill end of
conduit
with spray
foam
Underground Power
Conduit
Leakage was noted at the
end conduit
High
Fill end of
conduit
with spray
foam
Northwest Door
A section of the door
sweep was deformed allow
air to pass underneath.
Low
New door
sweep
Location
Note 1: Leakage appraisal is qualitative, with leakage locations being compared against one another.
Whole Home Heat Loss/Gain Testing
Methods. When this project was initially conceived, the approach to assessing the heat loss/gain
characteristics of the demonstration home was to carry out using a Short-Term Energy Monitoring
(STEM) test. This test protocol was developed in the 1990s by the National Renewable Energy
Laboratory (NREL) based in Golden, Colorado, to provide a fast and cost-effective means of verifying
energy performance and to afford a quantitative assessment of heat losses and gains, including
conduction losses, air infiltration, and solar heat gain. However, after contacting NREL, acquiring
procedures and past test reports, and reviewing the software package, it was evident that the
procedures and equipment required to carry out this type of analysis were far beyond the scope of this
project. The main limiting factors were the need for a complex data acquisition system and the
computer programming required to process and convert the data collected by it into a format that can
be used by the STEMWIN Version 2.02 software. It should also be noted that NREL no longer uses
the STEM test in part due to criticism the procedures have received over the years. In the end, a
decision was made to take a different approach to commenting on the heat loss/gain characteristics of
the demonstration home.
Borrowing from the STEM test procedures, a multi-day analysis was carried out and involved
"co-heating" and "cool-down" periods. During the "co-heating" period, the building was heated for a
two-day period by a storage-type electric water heater (located in the building) in conjunction with
the in-floor heating system (pump and tubing). A Dwyer Series TS Digital Temperature Switch was
used to turn on and off the electric water heater (and pump circuit) as required to maintain a constant
interior temperature of 22ºC (as measured at a height of 6'-6" on the northwest wall). During this
period all electrical energy being consumed in the building (i.e. water heater, pump, and temperature
controller) was measured by an EKM Metering EKM-25IDS-N v.2 single-phase energy meter.
Interior and exterior temperatures were recorded at 15 minute intervals by the same Onset HOBO Pro
v2 data loggers as were described in the “Long Term Monitoring” section.
Data collected during the "co-heating" period was used to comment on the building's effective
thermal resistance and its response to external factors such as temperature change and solar gain.
During the night time, conduction and air infiltration were expected to be the primary sources of heat
loss. During the day, the interior temperature was allowed to drift upwards (above the 22ºC set point)
as the building responded to solar energy absorbed through transparent and opaque surfaces. The
measurements made during these times were graphed to provide a visual illustration of the
demonstration home's response patterns.
Fig. 21: Temperature Controller and Energy Meter
Immediately following the "co-heating" period, the set-point of the temperature controller
was set down to 15°C, causing the water heater (and pump) to turn off, and allowing the building to
drift with the exterior temperature and effects of solar insolation. This "cool-down" period lasted a
total of six days. The purpose was to observe the heat capacity (thermal mass) characteristics of the
building and monitor how the building responded to changes in the exterior air temperature. It was
also used to study the effects of solar heat gain on the unheated building.
Results. The interior and exterior temperatures collected at 15 minute intervals over the 2 day "coheating" period and 6 day "cool-down" period are shown in Fig. 22. Also shown on the graph are the
period for which energy consumption was recorded as the building was being heated, and the heating
temperature set point during both the "co-heat" and "cool-down" periods.
Fig. 22: Heat Loss/Gain Characteristics
During the co-heating period, a total of 42.2 kWh of energy was consumed over the time that
the heating system operated. This value is the total energy delivered to the interior of the building to
overcome heat losses to the exterior by modes of conduction, convection and radiation. One of the
limitations of the energy meter that was used in this study is that it did not have the capability to
record energy consumption at intervals. However, if we make a general assumption that the energy
consumption varies linearly with the difference between interior and exterior temperature (and ignore
the effects of solar heat gain; a reasonable assumption giving the trends in Fig. 23) it is possible to
roughly approximate the total amount of energy consumed over each 15 minute period. If steady state
heat flow is assumed over each 15 minute interval, the constant heat flow (W) that is required to
produce the approximated energy consumption for that interval can be calculated. By using the
constant heat flow (W) value and temperature change (ºC) assumed for each interval, the in-situ
overall building heat loss coefficient (W/ºC) can be estimated. For the demonstration home, this value
is approximately as 64 W/ºC.
It is important to note that the total surface area of the demonstration home is dominated by a
few assemblies. As can be seen in Table 4, the roof and walls make up 65.6% and 19.3% respectively
(85% in total), of the total surface area of the building. This fact is highlighted to suggest that the
building heat loss coefficient, and the home's thermal performance in general, is heavily impacted by
these surface areas. This should come as no surprise, however, the effective thermal resistance value
of the roof is thought to be much less than that of the CEB-straw-bale walls, meaning that the energy
measured during this test is more of a comment on overall building performance (which is dominated
by roof performance) than wall performance.
Table 4: Breakdown of Building Surface Areas
Area (m2)
% of Total Area
Ground Floor
42.3
9.7%
Above-Grade Walls
84.0
19.3%
Roof
286.3
65.6%
Dormers
13.7
3.1%
Windows
6.7
1.5%
Doors
3.4
0.8%
Total
436.3
100%
Surface
A portion of the in-situ overall building heat loss coefficient is also attributed to the air leakage
heat loss coefficient (Uair) where,
Uair = 0.3 x n x V [2]
where,
n = number of natural air changes per hour (ACH) = 0.021
V = total air volume of the building (m3) = 217 m3
and,
Uair = 0.3 x 0.02 x 217 = 1.3 W/°C
[Note 1: The ACH value is the average annual natural infiltration rate estimated by the TECTITE
blower door software using the calculation procedure contained in ASHRAE Standard 136. It is based
on the average values calculated from the data collected during blower tests #1 and #2, and presented
in Table 2.]
The air leakage heat loss coefficient makes up 2% of the overall building heat loss coefficient.
The remaining 98% is attributed to heat loss through the enclosure surfaces; walls, floor, roof,
windows and doors.
As was mentioned previously, the heating system set-point was set to 22°C. This meant that
during the day, it was possible for the interior temperature of the demonstration home to rise above
that value in response to solar gains. Fig. 23 shows a detailed view of the temperatures measured
during the "co-heating" period. The interior temperatures during the daylight hours on April 5th and
6th vary little, and with no distinguishable pattern the would be attributed to solar gain through a
window. This suggests that solar heat gain is negligible in the demonstration home. This is supported
by the fact that both days were clear and had good solar insolation values being recorded consistently
at the nearby Burnley weather station (refer to Fig. 24 and 25).
Fig. 23: "Co-heat" Period
Fig. 24: Measured Solar Insolation on April 5, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
Fig. 25: Measured Solar Insolation on April 6, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
However, when studying the graph of the "cool-down" period (refer to Fig. 26) a slightly
different trend is apparent as the interior temperature begins to decrease. As can be seen in Fig. 27
and 28, 4.8, April 7th and April 8th were clear days that resulted in good solar insolation values
(very similar to April 5th and 6th). A small increase in interior temperature (approximately 0.5°C)
was recorded on both days just before noon. Conversely, if we look at the insolation plots for April
9th, 10th and 11th (Fig. 29, 30, 31), it is evident that these days were overcast, resulting in poor and
varied insulation values. On these days, no increase in interior temperature was observed. These
observations suggest that the demonstration home, does have a response to solar heat, albeit small.
This is thought to be a result of the relatively few number of window openings and their small
apertures.
Fig. 26: "Cool-Down" Period
Fig. 27: Measured Solar Insolation on April 7, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
Fig. 28: Measured Solar Insolation on April 8, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
Fig. 29: Measured Solar Insolation on April 9, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
Fig. 30: Measured Solar Insolation on April 10, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
Fig. 31: Measured Solar Insolation on April 11, 2012
(acquired from www. wunderground.com for Burnley, Ontario weather station)
Also evident from the "cool-down" period illustrated in Fig. 26 is the difference in the rate
of temperature decrease inside the building when compared to that of exterior temperature as it tries
to reach equilibrium with the outdoor environment. The slope of the interior temperature changes
are much more shallow than that of the exterior temperature changes. The difference results in very
different changes of temperature recorded over the same periods of time. For example, the exterior
temperature dropped from 15°C on April 8th to 2°C on the morning of April 9th; a temperature
change of 13°C over a period of 15 hours. If we look at the interior temperature trend over the same
time period, a change of only 1°C was recorded. These observations are in part attributable to the
volume of air in the building that needs to be cooled down. However, the high thermal mass of the
building is also suspected to be at play, effectively dampening the magnitude of interior temperature
swings from the changes taking place in the outdoor environment. This dampening effect, was also
evident during the "co-heating" period, allowing the heating system to easily react to changes in
exterior temperature, and never letting the building drop by more than 0.5°C below the 22°C setpoint during the coldest evening (-2°C).
Another trend visible from Fig. 26, is the time delay between the lowest exterior and interior
temperatures recorded for each day. On April 7th the lowest exterior temperature recorded was -1°C
which occurred at 6 am. On the same day, the lowest interior temperature was 20°C and occurred at
9 am, 3 hours after the exterior temperature. This "thermal delay" is also suspected to be a result of
both thermal mass and high volume of air needing to be cooled.
Concluding Remarks and Future Research Opportunities
The demonstration home is a prototype building that showcases construction techniques not
typical in the Canadian building industry, but that will likely be considered more frequently as
concerns about the condition of our environment escalate. It not only emphasizes the importance
and viability of using only local materials to construct our buildings, but also demonstrates that
concrete earth block structures can be combined with reclaimed materials and other natural
materials to produce an attractive, well insulated and extremely air-tight home. As energy security
and environmental issues continue to threaten our planet, it will be important to continue to develop
non-traditional forms of construction such as CEB and straw-bale in the years to come. Studying
and documenting the thermal performance, effects of moisture on, and the durability of these
buildings will be essential. Not only will this aid in the development and constant improvement of
the techniques used, but it will also inform those individuals resistant to the need for a change in our
approach to the built environment, making them aware of the potential of these construction styles.
Based on the observations presented in study, there are a number of recommendations that
should be considered. For the demonstration home in particular, it would be extremely beneficial to
continue to monitor it for changes in relative humidity values over the next year (at the very least).
In particular, the following points should be considered when reviewing the data:
•
Reduction in "Built-in" Moisture: Based on the high relative humidity readings observed over the
monitoring period, it is thought that "built-in" moisture from the materials is still present in the
building. Over the next year, values are expected to decrease and stabilize (to some extent) with
exposure to exterior environment (i.e. wind and sun) and a constant source of interior heating
during the winter months.
•
Changes in relative humidity with building use: While it is expected that relative humidity values
will drop over the next year as "built-in" moisture is reduced, it is also anticipated that the building
will be occupied at some point during this time. When this happens, occupants and their activities
(e.g. cooking) will contribute to moisture within the building.
•
Sustained relative humidity values of 80% or higher: The first three months of data collected
during the monitoring period showed sustained RH values near 80%. It will be important to ensure
that this trend does not continue and lead to problems such as mould growth.
•
Relative humidity values nearing 100%: Values of 100% mean the air has reached saturation,
suggesting that moisture in liquid form is present inside the wall. In addition to posing the risk of
rot for straw, this condition is risky for the CEBs if the temperature at the same location is below
freezing.
For future CEB-straw-bale buildings that will be studied, the following recommendations are made:
•
Finishing the Building: As is evident from the “Long Term Monitoring” section, high humidity
readings measured consistently for a number of months is a concern. Due to the nature of the
materials used in this type of construction, we know these buildings have the potential to start
out with high amounts of "built-in" moisture. To avoid potential problems (i.e. mould growth
and rot) it is be important to allow the structure to release the moisture to the outdoors as
quickly and as easily as possible. This means removing tarps (if used) and heating the building
sooner; steps that should not be a problem taking if the building is occupied soon after
construction.
•
Add People: In order to truly comment on the durability of this type of wall construction the
building must be used for its intended purposed. Monitoring moisture and temperature in an
occupied home will provide crucial insight into how the walls respond to varying moisture loads
associated with indoor activities such as cooking and bathing. The data collected would help
determine the type of vapour control that is required to avoid/minimize the presence of vapour
in the wall assemblies.
•
Energy Consumption: Energy consumption should be monitored over the course of at least one
year to provide a better indication of how the building performs during peak heating and cooling
conditions. If the effective thermal resistance value of the CEB-straw-bale wall assembly is
desired, it should be done in a guarded hot-box or similar controlled environment to eliminate
the effects of other building components such as complex and area-dominating roofs. Finally, a
energy monitoring device with interval logging capabilities is highly recommended for
commenting on trends observed in tests such as the "co-heating" and "cool-down" periods.
References
[1] J. Straube, Building Science for Strawbale Buildings (Building Science Digest 112), Building
Science Press. 2009-01-30.
[2] E.Burnett, J. Straube, Building Science for Building Enclosures, first ed., Building Science Press,
2005, pp 205.