Adobe and Latent Heat: A Critical Connection
John J. Morony
Department of Biology
Southwest Texas Junior College
Del Rio, TX 78840
Mailing Address:
P.O. Box 421627
Del Rio, TX 78842
jmorony@delrio.com
Note: The author’s use of the term “cinder block”
refers to the common cement block found in virtually all modern residential
and commerical construction. — Fernco Metal Products Web Master
Abstract
A series of ongoing experiments provide evidence supporting the oft-told adage
that adobe houses are “warmer in the winter and cooler in the summer”
than houses made of other materials. Two modular structures of equal dimension,
one of adobe and the other of cinder block, were constructed with 8-inch thick walls,
and roofs and floors of identical material. Each structure has an identically
constructed and fitted small door for entry of data-gathering instruments. Simple
experiments illustrate the thermal properties of adobe (i.e., soil). Adobe still
remains soil after its incorporation into a building and thus adobe has the thermal
dynamics of soil. Phase change from liquid water to vapor or the reverse will result
in a high rate of latent heat to lower or raise the temperature of adobe.
On a dry day, with an out door ambient temperature of 98ºF, interior temperatures
were 90ºF in the adobe structure and 103ºF in the cinder block structure. It is proposed
that the 13º variation in temperature in the two structures is a direct result of
the adobe having lost 8º by way of latent heat of vaporization (in accord with known
properties of soil), whereas the cinder block structure gained 5º due to simple heat
conduction. The reverse occurs when relative humidity is high and temperatures are
low. Adobe then takes in moisture from the air, thus releasing latent heat. During
cold weather, data loggers for temperature and moisture were placed in each of the
modules for ten days. During each diurnal cycle the lowest and highest temperate were
restricted to the cinder block.
Clay, the binder in adobe, is hygroscopic and its water content varies with
available moisture. Such variation precludes adobe being assigned a specific heat
capacity comparable to conventional building material. More importantly, any
evaluation of adobe needs to take into consideration dynamic properties of soils
(especially the role of latent heat) and not be restricted to the parameters of
sensible heat (a static property) by the building industry. Experimental data
gathered by the author provides strong evidence that as a construction material
adobe blocks keeps a building warmer in the winder and cooler in the summer than
cinder block. The explanation for this phenomenon appears to lie in the role of
latent heat, not sensible heat -– a critical distinction.
Introduction
Use of cinder blocks for construction of small buildings, especially housing,
has almost completely replaced adobe along the Texas-Mexican border. In the Mexican
city of Ciudad Acuña, across the river from Del Rio, Texas, perhaps as much as 95%
of new home construction, and essentially all government built houses, are of
cinder block.
This trend from earthen structure to a cinder block one appears throughout the
non-industrial world. Even still, in land where adobe construction had once dominated,
the belief of the older populace persists: “Adobe is cooler in the summer and warmer
in the winter.”
The means for temperature moderation in adobe houses may come from the ease
at which moisture enters and leaves permeable and hygroscopic soil in response
to changing atmospheric conditions. The movement of moisture in and out of the
adobe is more than a simple transfer of water. It is the transfer of latent heat
that must take place when there is a phase change in water that raises or lowers
the temperature of the building fabric. While adobe and compressed earth blocks
have been assigned an R-value of .25/inch, it is the latent heat exchanges that
appear to be the dynamic factor to consider most when comparing it to other
building materials.
Adobe differs profoundly from all other type building material in that adobe
comes from soil and remains soil after its incorporation into a building. Latent
heat flux is of elementary concern to soil science. Attempts to evaluate adobe
exclusively in terms of sensible heat, as with the use of the R-value, or thermal
mass, have resulted in confusion in evaluating abode in terms of thermal properties.
Adobe and its suitability for exceptionally hot climates (as exists along the
Texas-Mexico border) are of special interest to this study. Traditional concerns
in the United States have been for development of building materials for use in
cold climates. Adobe vs. cinder block construction is being studied with a series
of simple experiments including the use of two modular structures, one of cinder
block and one of adobe. Studies were conducted in Del Rio, Texas in 2003 and
early 2004.
Two Modules
Experiment 1: Two modular structures with 8" walls were constructed:
one of adobe blocks (8" x 16" x 4") and one of cinder blocks
(8" x 16" x 8"). The cinder block was stuccoed with cement and the
adobe with lime. Both were left with their natural color. Outside dimensions of both
modules are approximately 62" x 48" x 26" with interior volumes about
22 cubic feet each. The roofs and floors of both are constructed of the same material.
Both face west and were free of shadows throughout the day (Figure 1). Recording of
data was made 27 August 2003 at 4:30 p.m. Modules are located at the Casa de la
Cultura in Del Rio, Texas.
With an ambient temperature of 98ºF, temperatures inside the modules were 103ºF in
the cinder block and 90ºF in the adobe (13º difference.) The cinder block was 5º
above ambient and the adobe 8º below ambient.
Reference to R-values, or thermal mass, cannot fully explain the 13 degree
difference in interior temperature. An 8-inch adobe wall has an R-value of 2
(0.25/inch for adobe) and the cinder block used has an R-value of 1.08. With the
lower R-value, the cinder block would be expected to exhibit a higher interior
temperature; however the significant difference is that the cinder block was
above ambient temperature whereas the adobe was below ambient.
This indicates that there is another important contributing factor beyond the
insulating properties of these materials.
Experiment 2: Data loggers were placed in the two previously described
modules during acute cold weather from the 25th to the 30th of January, 2004.
Data was recorded for temperature, relative humidity and dew point. Only
temperature data is illustrated in Figure 2a and 2b.
Figure 2. Temperature data loggings during a cold period (25th to
30th of January 2004). The solid bold line represents adobe; the dashed line
represents cinder block and the solid light line represents ambient temperature.
Note that for every temperature extremes the cinder block had temperatures higher
and lower than the adobe. Also fluctuation of temperature was greater for the
cinder block than for the adobe. On January 27, 2004, the range of temperature
was 12ºF in the adobe and 24ºF in the cinder block.
Experiments on Latent Heat of Vaporization/Condensation
Effect of latent heat, especially of vaporization, is first demonstrated with
simple experiments prior to more discussion. The initial experiment relates the
nature of clay and the permeability of clay-rich material to observed results of
evaporative cooling or latent heat of vaporization under full sun.
Experiment 3. Four small plastic flower pots are used to demonstrate that heat
of vaporization moderates temperature. Three red clay-colored plastic pots and one
slightly larger red clay pot were used. One plastic pot was painted black, another
painted white and the third was left its original color. The clay pot is left with
its natural clay color. The pots had their bottom holes sealed. Each was filled with
500 ml of water and covered with a corresponding colored plastic lid and placed in
full sun. Ambient temperature at the time was 94ºF in the shade. After being left
in full sun for three hours (2:00-5:00 p.m. CST), data were recorded (Figure 3.)
| No. |
Pot Color/Type |
Temp |
Diff |
Comments |
| 1 |
Black |
113ºF |
+19º |
No measurable loss of water |
| 2 |
White |
102ºF |
+8º |
No measurable loss of water |
| 3 |
Natural Clay Color |
105ºF |
+11º |
No measurable loss of water |
| 4 |
Clay Pot |
86ºF |
-8º |
56% loss of water |
Figure 3. Test flower pots and vaporization of water.
Ambient temperature was 94ºF.
The most dramatic difference is in the temperature of the clay pot;
a full 8º below ambient, whereas all the plastic pots were well above ambient.
The clay pot was 19º cooler than the plastic pot of similar color. Also of note
is the large amount of water lost from the clay pot. An explanation is that the
clay pot, while being waterproof to liquid water, it is permeable to water
vapor that readily diffuses through the sides of the pot. Such movement of water
molecules involves a phase change from liquid to water vapor, resulting in the
latent heat of vaporization. For each gram of water going from liquid to a vapor
state about 580 calories per gram of heat (540 calories per gram for vaporization
with the boiling of water) are removed from the clay pot. As the clay pot lost
280 ml of water (one ml of water is equal to one gram) by diffusion there was
a total of some 160,000 calories of heat removed from the water! As the heat
lost is incorporated into the vaporized water molecules, it is not subject to
measurement by a thermometer nor can it be felt — it is thus 'hidden' heat or
latent heat of vaporization as opposed to 'sensible heat' (heat that can be felt
and measured).
The plastic pots, being impermeable to water vapor, evaporative cooling
was not possible. The difference in temperature of the plastic pots is
associated with differing capacity of colors to absorb solar radiation.
Black mostly absorbs radiant energy while white mostly reflects it. The
rather dark natural clay color is in-between. The contrasting colors of
black and white pots translate into difference in temperature in the two
pots of 11 degrees.
Experiment 4. The important role of clay and aggregates (sand and silt)
in adobe are demonstrated with a simple experiment. Besides serving as the
binder in adobe, clay also contributes important thermal dynamics properties.
There are two factors to consider in relationship to this: clay particles
carry a negative charge and thus water, a polar compound, is readily attracted
and attached to clay particles; and simple diffusion of water vapor from high
to low concentration varies throughout the day in response to changes in
atmosphere moisture. The presence of aggregates in the adobe provides
pathways for capillary action, allowing water molecules to move in and out.
| Weight |
Totals |
Weight |
Total |
| High RH, a.m. |
Low RH, p.m. |
loss of moisture |
loss of heat |
| 261.0 g |
257.9 g |
3.1 g |
1,674 cal |
| 261.0 g |
257.4 g |
3.6 g |
1,944 cal |
| 260.2 g |
258.4 g |
1.8 g |
972 cal |
| 261.2 g |
258.7 g |
2.5 g |
1,350 cal |
Figure 4. Moisture absorbed by clay in response to changes in relative
humidity. The result in exposing a cube of a compressed earth block to conditions
of a hot dry climate (Del Rio, Texas from August to 20 to 24, 2003.)
Weights were recorded in early morning and late afternoon.
Percent of weigh gain may be small, but the latent heat of vaporization that
it represents is extremely great. The specific heat of water is much higher than
any conventional building material.
Experiment 5. Three clay pots were used to determine the effects of color on
evaporative cooling. One pot was painted with white enamel, one with white lime
wash and the third was left its natural clay color. The bottoms of the pots were
sealed, the pots filled with water, covered with a cap of similar color and
placed in full sun. Any differences in evaporation between the while colored
posts, related to the nature of the coating material, will be revealed.
| Limewash |
Enamel Paint |
Unpainted |
| 78ºF |
94ºF |
88ºF |
| 16º below ambient |
no change |
6º below ambient |
Figure 5. Small clay flower pots filled with water: #1 lime wash;
#2 enamel paint; #3 unpainted clay color. Pots exposed to full sun with for
three hours in late afternoon. Ambient temperature of 94ºF.
The limewashed clay pot is now 16º degrees below ambient temperature!
The high reflectance of the white limewash significantly limits the amount of radiant
energy absorbed to convert into thermal energy as sensible heat. At the same
time, lime remains vapor permeable and thus permits evaporative cooling.
The white enamel on the pot succeeds in greatly reducing the conversion of
radiant to thermal energy, but because it is impermeable to water vapor it
prevents evaporative cooling.
Experiment 6. Three clay flower pots were used to determine the effects of
color on temperature when no evaporative cooling was allowed to occur (Figure 6.)
One pot was painted with white enamel, one with white limewash and the third was
left its natural red clay color. The pots were placed upside down in full sun.
Inside temperature was measured with a thermometer inserted in the hole in the
bottom of the pot.
Figure 6. Large clay pots turned upside down exposed to ambient
condition in full sun; #1 enamel white; #2 white limewash; #3, natural clay color.
Inside temperatures recorded after three hours exposure and subsequent gain
in temperature is recorded. Ambient temperature of 94ºF.
| 1 |
2 |
3 |
| 104ºF |
98ºF |
104ºF |
| +10º |
+4º |
+10º |
Note that the limewash is highly effective in reflecting solar radiation.
Limewash is a mixture of slaked lime (calcium hydroxide) and water. When applied
as a near water-thin paint it sets slowly by absorbing CO2 from the air, producing
crystals of calcite (CaCO3, calcium carbonate). Unlike paints that are organic
polymers, limewash is a mineral of dual reflective index and thus more
effective in reflecting solar radiation. The limewash is 6º lower than the enamel.
Latent Heat and Building Materials
Phase change material (PCM) is any substance capable of latent heat flux and
it has been of interest to the building industry since at least the 1940s.
Stored energy in latent form within a building fabric would lead to greater heat
storage capacity per unit volume than would be otherwise possible with conventional
building materials. The concern has focused almost entirely on providing warmer
indoor temperature in the winter. Interest in the matter appears to have been
restricted to heat of fusion and an inventory of PCM did not include soil.
It was initially restricted to a list of inorganic chemicals (largely hydrated
salts) that would have to be incorporated into a building fabric and none
constituting the building fabric itself. Nothing really workable emerged from
these efforts. Interest then turned to organic PCM but with like consequences.
Soil, suitable for earthen block making, is inherently phase change material
par excellence. Most significantly, it constitutes not only the entire
building fabric as to heat of fusion but to vaporization and condensation as well
— and it does so to a degree far in excess of almost all other materials man-made
or otherwise.
The Nature of Adobe vs. Cinder Block
Clay is the binding material of adobe with silt and sand serving as the aggregate,
often with the addition of fibrous organic matter by way of straw or horse manure.
In construction of an adobe block, clay remains chemically unaltered. Adding water
serves to facilitate rearrangements and compaction of the particles in making
adobe blocks. The clay in the adobe block retains its capacity to attract water
after the block is made. This water can move in and out via capillary action in
response to available moisture along the pathways created by the contained
aggregate.
In contrast, Portland cement (a highly complex and altered very fine powder
predominantly limestone) undergoes a chemical transformation into concrete when
mixed with water and an aggregate. While some capacity for capillary action may
remain, it is much reduced compared with adobe or other earthen building materials.
Importantly, the clay content of Portland cement has been chemically altered and
is no longer hygroscopic. This distinction between earthen material and products
incorporating Portland cement (or stone and brick for that matter) as building
material is critical to appreciating their thermal character.
A Scaled-up Model to Consider
To scale up from the small modules, previously discussed, an appreciation of
the thermal properties of an existing adobe dwelling is provided by a study
published in Earthbuilder (10th Anniversary Issue 42, 1984, p. 56, Adobe News, Inc.)
The house, described as an “old style adobe,” was located in Los Lunas,
Rio Grande Valley, New Mexico at an elevation of 4,750 feet. The building had
17-inch thick walls and an 8- to 12-inch thick earthen roof. Temperature was
recorded in two intervals: before and after expansion to the house. The initial
floor plan, of less than 1,000 square feet is illustrated below (Figure 7).
Figure 7. Original floor plan as of June 14, 1976. No insulation was used
and no cooling mechanisms or overhangs existed. There was one window on the
north side. Late in the day a large tree partially shaded the northwest
corner of the house. The building was kept closed during the period the
data was gathered.
Temperature data for the adobe house on June 14, 1976:
| Time |
Inside Temp. |
Outside Temp. |
| 12:30 pm |
79.0 |
99.0 |
| 1:30 pm |
79.5 |
101.5 |
| 2:30 pm |
80.0 |
102.0 |
| 3:30 pm |
80.0 |
99.5 |
| 4:30 pm |
79.0 |
88.0 |
| 6:30 pm |
79.5 |
89.0 |
Inside temperature of the adobe did not exceeded 80º F when outside temperatures
average in the mid- to upper 90s. Note that when outside temperature was 102ºF,
inside temperature was 80ºF (a 22º difference!) The authors state that there was
an inside temperature variation of only 5º in the house from May 27 to July 11 of
that year, and further note that this was with no roof insulation or cooling unit
of any kind. Significantly, the authors comment that it was noted that the inside
high temperature occurred during the morning hours, at roughly
12 hours after the outside high of the preceding day. Likewise, the inside low
temperature appeared in mid- to late afternoon, roughly 12 hour after the
morning outside low temperature. That inside temperatures of an adobe house would
be cooler when outdoor ambient temperature is highest and warmer inside when outdoor
temperatures are coolest is clearly counter-intuitive! However, the adobe is
responding not to sensible heat of the environment, but rather to a differential
of moisture content on either side of an adobe enclosure.
Latent heat of condensation would be expected to occur in the morning hours
when relative humidity is highest and outside temperature is coolest. The
absorption of moisture by the clay in the adobe would result in raising the
temperature of the adobe. In the late afternoon, when relative humidity is the
lowest, latent heat of vaporization (evaporative cooling) would exhibit a reverse
effect, i.e., adobe would actually cool. However, the explanation provided by the
author centered on what is said to be the 'flywheel effect'. This is an untested
assumption that a delay in the conduction of heat in and out of the adobe house
would be due to sheer mass of the wall. A question arises: what is the annual
energy cost required to maintain a comparable inside temperatures of a building
not susceptible to latent heat flux?
Summary
The preliminary results of a series of ongoing experiments may be summarized as follows:
-
Adobe is indeed cooler in the summer and warmer in the winter, and significantly so,
in comparison to cinder block and other non-earthen building materials. The reason
for this is not directly related to sensible heat of conduction, but rather to
latent heat and especially latent heat of vaporization and condensation.
Latent heat flux appears to stabilize internal temperatures within an adobe
enclosure.
-
Thermal qualities of adobe and other earthen materials cannot be accurately
expressed or understood using only the R-values of conventional building material.
The “guarded hot box,” used to determine the R-values, measures steady-state
heat flow of differential heat on either side of the material being tested. For adobe,
it is the latent heat flux promoted by a moisture differential on either side of a
wall of an enclosed adobe building that lowers and raises the temperature of the
adobe. The concept of insulation, as it is applied to conventional building
materials, is of doubtful use or significance.
-
Caution is suggested in the use of any material, modifications or structural
design that might impede the thermal dynamics of latent heat flux of earthen
structures.
-
Latent heat phenomena would appear to strongly favor what has come to be
known as a “green roof” for adobe structures.
-
Adobe and similar materials must be recognized for what they are — a very
superior building material both from the standpoint of their functional
value and cost. Economically, the price of soil is not tied to the price
of oil, and the costs for heating or cooling would be significantly reduced
in a rightly constructed earthen structure.
