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Passive cooling of outdoor urban spaces. The role of materials
L. Doulos, M. Santamouris *, I. Livada
Group Building Environmental Studies, Section Applied Physics, Physics Department, University of Athens, Panepistimioupolis,
Athens 157 84, Greece
Received 18 September 2003; received in revised form 22 January 2004; accepted 27 January 2004
Available online 4 May 2004
Communicated by: Associate Editor Volker Wittwer
Abstract
This paper presents the results of a comparative study aiming to investigate the suitability of materials used in
outdoor urban spaces in order to contribute to lower ambient temperatures and fight heat island effect. The study
involved in total 93 commonly used pavement materials outdoors and was performed during the whole summer period
of 2001. The thermal performance of the materials was measured in detail using mainly infrared thermography pro-
cedures.
The collected data have been extensively analysed using statistical techniques. Comparative studies have been
performed in order to identify the major advantages and disadvantages of the materials studied. Materials have been
classified according to their thermal performance and physical properties into ‘cool’ and ‘warm’ materials. The impact
of color, surface roughness and sizing has been analysed as well.
The study can contribute to selection of more appropriate materials for outdoor urban applications, and thus assist
to fight the heat island effect, decrease the electricity consumption of buildings and improve outdoor thermal comfortconditions.
2004 Elsevier Ltd. All rights reserved.
Keywords: Passive cooling; Pavement materials; Outdoor comfort
1. Introduction
The continuously growing size of the urban envi-
ronment and the careless development of buildings and
open spaces have a major impact on the urban micro-climate. The building’s energy behavior and perfor-
mance are heavily influenced by the density of the
building space. The observed ‘heat island’ effect is
mainly influenced by urban design, namely the canyon
radiative geometry, anthropogenic heat and the mate-
rial’s street physical properties (Santamouris, 2001; Oke
et al., 1991). The emitted infrared radiation from the
various buildings and street surfaces impinges on the
surroundings surfaces and is entrapped inside the can-
yon. Besides, the total amount of the absorbed solar
radiation is increased due to multiple reflections between
the buildings (Santamouris and Assimakopoulos, 1997).Also the anthropogenic heat increases the intensity of
the ‘heat island’ effect through the use of fuels from ei-
ther mobile or stationary sources. Finally the incident
solar radiation and every available heat form can in-
crease the storage of sensible heat in the city’s structure
during the daytime. The stored heat is released into the
urban atmosphere during the night period. Therefore
the total amount of the energy balance is increased and
air temperatures become greater (Santamouris et al.,
1998).
A more positive thermal balance can be achieved by
reducing the thermal gains in the urban environment
* Corresponding author. Tel.: +30-1-727-6934; fax: +30-1-
729-5282/81.
E-mail address: [email protected] (M. Santamouris).
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.solener.2004.04.005
Solar Energy 77 (2004) 231–249
www.elsevier.com/locate/solener
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and in particular by reduction of the absorbed solar
radiation. The role of building materials is decisive for
the reduction of the thermal gains and overheating. The
thermal performance of the building materials is mainly
determined by their optical and thermal characteristics;
the albedo to solar radiation and the emissivity to long
wave radiation are the most significant factors. The useof appropriate materials, the so-called ‘‘cold’’ materials,
can improve thermal comfort conditions during the
summer period. They are characterized by a high
reflectivity factor to the short wave radiation and high
emissivity factor to the long wave radiation. They reduce
the amount of solar radiation absorbed by the building
envelopes and urban structures and keep their surfaces
cooler. Respectively, they are good emitters of long wave
radiation and release the energy that has been absorbed
as short wave radiation. Using ‘‘cold’’ materials in urban
environmental planning contributes to lower surface
temperatures that affect the thermal exchanges with theair (Akbari et al., 1992, 1997; Bretz and Akbari, 1997).
In this paper, the surface temperature distribution of
the total number of 93 selected materials is presented. A
theoretical analysis is followed aiming at the investiga-
tion of the thermal performance of the selected building
materials. Also, a methodology for their classification in
‘cold’ materials is been developed. An experimental
campaign was set up at an open space at the National
and Kapodistrian University campus in Athens, during
August 2001. For the theoretical analysis the selected
materials were grouped according to their construction
material, their surface texture and surface color (Table
1). The measurements were obtained through infrared
(IR) thermograph imaging.
2. Materials for pavements and their role in the thermal
balance of the urban environment
The use of appropriate materials to reduce heat is-
land and improve the thermal characteristics of the ur-
ban environment has gained increasing interest during
recent years. Many research works have been carried out
to evaluate the possible energy and environmental ben-
efits when light colored surfaces are used. Research triesto investigate the impact of the materials optical and
thermal characteristics on the urban temperature as well
as the possible energy conservation during the summer
period. A detailed guide on light colored surfaces has
been published by US EPA (Akbari et al., 1992). Re-
search shows that important energy gains are possible
when light color surfaces are used in combination with
the plant of new trees.
The use of materials define the global albedo of the
cities. Typical albedo of European and American cities
are close to 0.15–0.30. Much higher albedo have been
measured in some North African cities (0.45–0.6). Taha
(1997) has compiled data given by (Taha, 1994; Kung
et al., 1964; Dabberdt and Davis, 1978; Vukovich, 1983;
Brest, 1987; Coppin et al., 1978; Rouse and Bello, 1979;
Mayer and Noack, 1980; Steyn and Oke, 1980; Aida,
1982; Oguntoyinbo, 1970, 1986) for snow free urban
albedos for several cities and has published the difference
between the urban and rural albedo. Cantat (1989) hasestimated the albedo of various types of surfaces as well
as their temperature in the major Paris area, It is found
that urban areas have a much lower albedo while the
albedo in Paris is to about 16% lower than in the sur-
rounding rural areas.
Various studies have been performed to understand
better the thermal and optical performance of materials
used for pavements and their impact to the city climate.
Lower surface temperatures contribute to decrease the
temperature of the ambient air as heat convection
intensity from a cooler surface is lower. Such tempera-
ture reductions can have significant impacts on coolingenergy consumption in urban areas, a fact of particular
importance in hot climate cities.
Yap (1975) has reported that systematic urban–rural
differences of surface emissivity hold the potential to
cause a portion of the heat island. Robinette (Santa-
mouris, 2001) reports relative temperatures of 38 C
over grass, 61 C, over asphalt, and 73 C over artificial
turf. Santamouris (2001) reports asphalt temperatures
close to 63 C and white pavements close to 45 C.
Oke et al. (1991) have simulated the effect of the
optical and thermal characteristics of ‘urban’ materials
to the heat island intensity during the night period. They
report that the role of emissivity is minor. As the emis-
sivity increased from 0.85 to 1.0 there was a slight in-
crease of 0.4 C of DT between the urban and rural
environment for very tight canyons, where there was
almost no change for higher view factors. On the con-
trary, the effect of the thermal properties of the materials
was found to be much more important. For a flat land, it
is found that if the urban admittance was 2200 J/m 2/K,
and the rural one was 800 units lower a heat island
of about 2 C was developed during the night period,
while when the urban admittance was decreased to 600
J/m2/K, a cool island of over 4 C was formed during
night.In an other study, Asaeda et al. (1996) have reported
the experimental results of a study where the impact of
various pavement materials used commonly in urban
environments were tested during the summer period.
They found that the surface temperature, heat storage
and its subsequent emission to the atmosphere were
significantly higher for asphalt than for concrete and
bare soil. At the maximum, asphalt pavement emitted an
additional 150 W per square meter in infrared radiation
and 200 W per square meter in sensible transport com-
pared to a bare soil surface. They also found that the
rate of infrared absorption by the lower atmosphere
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over asphalt pavement was greater by 60 W/m2 than that
over the soil surface or concrete pavement. Gustavsson
and Bogren (1991) measured the influence of road con-
struction on road surface temperature. On a test road,
they had found a nocturnal maximum difference of 1.5
C between beds consisting of blast furnace slag and
those consisting of gravel.
Berg and Quinn (1978) reported that in mid-summer
white painted roads with an albedo close to 0.55 have
almost the same temperature with the ambient envi-
Table 1
Description of the studied materials (Bl stands for black, Bli for black inlays, Br for brown, Gn for green, Gr for gray, R for red, Wh
for white, Whi for white inlays)
Material
number
Construction
material
Surface color Surface
texture
Material
number
Construction
material
Surface color Surface
texture
1 Mosaic Green Smooth 48 Concrete Bl–Whi Smooth
2 Mosaic Wh–Bl Smooth 49 Concrete Wh–Bli Smooth
3 Mosaic Light–Br Smooth 50 Concrete Black Rough
4 Mosaic Gray Smooth 51 Concrete Black Smooth
5 Mosaic White Smooth 52 Concrete Red Rough
6 Mosaic Brown Smooth 53 Concrete Red Smooth
7 Mosaic Red Smooth 54 Concrete Red–Whi Smooth
8 Mosaic Black Smooth 55 Concrete Red–Bli Smooth
9 Concretea Red Smooth 56 Pebble Green Rough
10 Concretea Black Smooth 57 Pebble Dark–Gr Rough
11 Granite Red Smooth 58 Pebble Gray Rough
12 Granite Black Smooth 59 Pebble Light–Gr Rough
13 Concretea Orange Smooth 60 Asphalt Black Rough
14 Concretea Brown Smooth 61 Concrete White Rough
15 Concretea
Gray Smooth 62 Concrete White Rough16 Concretea Gray Rough 63 Concrete White Smooth
17 Concretea White Smooth 64 Concrete Wh–Whi Smooth
18 Concretea White Rough 65 Concrete Or–Whi Smooth
19 Granite White Smooth 66 Concrete Orange Rough
20 Granite Wh–Bl Smooth 67 Concrete Orange Rough
21 Granite Green Smooth 68 Concrete Or–Whi Smooth
22 Granite Wh–Gn Smooth 69 Concrete Green Rough
23 Marble White Smooth 70 Concrete Orange Rough
24 Marble Wh–Bl–R Rough 71 Concrete Dark–Gn Rough
25 Marble Wh–Bl Smooth 72 Concrete Green Smooth
26 Marble Wh–Bl Rough 73 Pebble Gr–Wh Rough
27 Marble White Smooth 74 Concrete Wh–Blue Smooth
28 Marble White Rough 75 Pebble Wh–Gn–R Rough
29 Marble Wh–Bl Smooth 76 Pebble White Rough30 Marble White Smooth 77 Pave stone Red Rough
31 Marble White Smooth 78 Pave stone Gray Rough
32 Marble Pink Smooth 79 Pave stone Brown Rough
33 Marble Light–Br Smooth 80 Stone Black Rough
34 Marble Red Smooth 81 Stone Brown Rough
35 Marble Wh–Bl Smooth 82 Stone Gray Rough
36 Marble Dark–Gr Smooth 83 Stone Green Rough
37 Marble Gray Smooth 84 Stone Black Rough
38 Pebble Brown Rough 85 Stone Brown Rough
39 Pebble Light–Br Rough 86 Stone Brown Rough
40 Pebble Bl–Br Rough 87 Stone Green Rough
41 Pebble Light–Br Rough 88 Stone Brown Rough
42 Pebble Red Rough 89 Stone Brown Rough
43 Pebble Wh–Br Rough 90 Stone Brown Rough
44 Concrete Gray Smooth 91 Stone Brown Rough
45 Concrete Gray Rough 92 Stone Red Rough
46 Concrete Gray Rough 93 Stone Red Rough
47 Concrete Gray–Whi Smooth
a The size of the material tile is 30 cm · 30 cm.
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ronment, while unpainted roads with albedo close to
0.15 were approximately 11 C warmer than the air.
Taha et al. (1992) have measured the albedo and
surface temperatures of a variety of materials used in
urban structures. They found that white elastomeric
coatings having an albedo of 0.72 were 45 C, than black
coatings with an albedo of 0.08. They also report that awhite surface with an albedo of 0.61 was only 5 C
warmer than ambient air whereas conventional gravel
with an albedo of 0.09 was 30 C warmer than the air.
3. Implementation of the experimental measurements
3.1. Instrumentation and description of the experimental
site
The basic experimental equipment used for the
implementation of the measurements consists of aninfrared camera to measure surface temperatures.
Measurements were also performed by using a precise
contact thermometer in order to take into account
minor errors associated with reflected infrared radiation
and the non-complete knowledge of the material’s
emissivity.
The surface temperature measurements were taken
on an hourly basis from 9:00 to 18:00 (local time). The
ambient meteorological conditions, recorded from ameteorological station at the university campus, were
characterized by high air temperatures, low relative
humidity (Figs. 1 and 2) and clear sky. Wind speed and
direction were also measured. Wind speed was always
low during the experimental period (
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pebble and mosaic) of different surface color materials
(white, gray, black, red, brown and green) and of dif-
ferent surface texture materials (with smooth surfaces,
rough surfaces and anaglyph surfaces with marks and
designs). The tiles had a size of 40 cm · 40 cm. In order
to compare the thermal performance of material tiles
made of the same construction material, surface colorand texture but different size, a number of some extra
concrete tiles sized 30 cm· 30 cm were studied. The
sampling tiles were placed on an especially modulated
platform covering a surface of 40 m2 (Fig. 3). The
platform was horizontal and insulated from below. The
heat transfer effects between the platform and the sam-
ple materials were eliminated because of the platform’s
insulation.
3.2. Thermograph imaging method
The surface temperatures of the sample materialswere measured with an IR camera, that is an infrared
condition monitoring system (AGEMA Thermovision
570, 7.5–13 lm wavelength). The IR-camera measures
and images the emitted infrared radiation from an object
(Fig. 4). The fact that radiation is a function of the
object’s corresponding surface temperature (Planck’s
law equation (1)) makes it possible for the thermal
camera to calculate and display this temperature(Gaussorgues, 1994; Thermovision, 1997). The mea-
sured infrared radiation is also function of the object’s
emissivity.
Q ¼ erT 4 ð1Þ
Q is the object’s long wave radiated energy (W/m2), e is
the object’s emissivity that is a function of wavelength,
the direction of observation relative to the surface
and the surface temperature (Gaussorgues, 1994), r is
the Stefan–Boltzmann’s constant (5.67 · 108 W/m2/K4)
and T is the object’s surface temperature (K) (Wolfe and
Zissis, 1997).
In the present study, the emissivity values given by
Gaussorgues (1994) and Wolfe and Zissis (1997) have
been used during the experimental procedure. The
emissivity of most of the materials has been also mea-
sured using hot plate techniques and no significant dif-
ferences have been found. In practice, the emissivity
values for the total number of the studied materials were
close to 0.9. (Fig. 5).
4. An analysis for the study of the thermal performance of
the tested materials
Due to lack of uniformity in the surface temperature
distribution at same studied materials, the measured
surface temperatures correspond to the average tem-
perature values of the total horizontal surface. The IR
monitoring system estimated these values automatically.Fig. 3. The site of the experimental campaign with the modu-
lated platform.
Fig. 4. Visible and infrared image of selected building materials.
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The most important reasons for the existence of non-
uniform surface temperature distribution are the color
contrast, the surface roughness and the tile’s heat
transfer effects (Fig. 6).
The estimated mean hourly surface temperature val-
ues for each material tile are given in Table 2 and in
statistical box plots (Figs. 7–20). Table 2 gives the mean
daily, the absolute maximum and the absolute minimum
surface temperatures for every material tile within 9:00
to 18:00. The box plots (Figs. 7–20) show the mean daily
surface temperature and the mean daily temperature
range of the material tiles. They are presented separately
Fig. 5. Radiation contributions to the general measurement situation.
Fig. 6. Same examples of material tiles characterized by non-uniform surface temperature distribution due to color contrast
(a), roughness (b) and heat transfer phenomena (the arrows shows the direction of the incident solar radiation) (c).
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for the total number of the materials according to their
construction material (Figs. 7–14) and their surface
color (Figs. 15–20). The lower and upper lines of the box
plots are the minimum and the maximum values of the
corresponding mean hourly surface temperature values.
The line inside the box is the average surface tempera-
ture value. The materials with the smallest average sur-
face temperature are presented at the left part of each
graph, while the warmest materials are presented at the
right part. Because of the large number of the studied
Table 2
Mean daily and absolute maximum surface temperatures during the experimental period of August 2001 within 9:00 to 18:00
Material
number
Mean daily
surface tem-
perature (C)
Absolute
maximum
surface
temperature
Absolute
minimum
surface
temperature
Material
number
Mean daily
surface tem-
perature (C)
Absolute
maximum
surface
temperature
Absolute
minimum
surface
temperature
1 34.8 39.5 23.9 48 41.8 48.1 27.2
2 35.5 40.6 23.7 49 35.2 40.3 23.1
3 35.5 40.7 23.6 50 43.7 50.4 28.9
4 37.5 43.3 24.7 51 44.4 52.0 27.6
5 33.3 37.7 22.2 52 39.9 45.9 26.1
6 36.9 42.5 23.7 53 39.1 45.5 24.8
7 38.5 44.8 24.9 54 37.9 43.7 24.6
8 42.1 49.6 26.1 55 41.1 48.2 25.5
9 37.8 43.8 24.6 56 44.0 50.9 28.9
10 44.0 51.6 27.5 57 45.2 52.7 28.5
11 40.1 46.6 26.7 58 40.6 46.9 26.8
12 43.9 51.7 27.9 59 40.1 46.3 26.0
13 37.3 42.7 25.2 60 46.7 54.0 30.3
14 39.5 45.8 25.5 61 33.9 38.1 23.715 37.6 43.0 25.6 62 33.2 37.5 23.1
16 38.7 44.6 25.7 63 34.7 39.2 24.1
17 33.2 37.7 23.1 64 32.6 37.2 21.9
18 34.5 39.3 23.5 65 37.4 42.9 24.4
19 32.5 36.8 22.8 66 38.9 44.9 25.2
20 35.2 40.4 23.7 67 38.9 44.8 25.3
21 36.2 41.4 24.6 68 37.6 43.2 24.7
22 38.3 44.3 25.1 69 38.4 44.3 24.9
23 33.4 38.0 23.4 70 37.3 42.9 24.3
24 34.1 38.4 24.2 71 42.8 50.0 26.6
25 31.6 36.1 22.5 72 37.7 43.7 23.9
26 32.6 36.9 23.2 73 38.1 43.6 25.1
27 29.7 33.4 21.0 74 37.7 43.3 24.5
28 32.8 37.2 23.0 75 36.9 42.0 24.629 34.5 39.9 23.7 76 33.6 38.0 23.0
30 30.1 34.2 21.1 77 43.2 49.6 28.8
31 32.2 36.6 22.0 78 42.7 49.2 28.2
32 38.3 44.5 26.1 79 40.6 46.3 27.1
33 32.4 37.5 22.1 80 43.3 50.4 28.5
34 41.4 48.3 28.2 81 40.3 46.9 26.3
35 37.6 43.2 25.6 82 42.4 49.3 28.3
36 43.1 50.7 29.0 83 40.3 47.2 26.0
37 39.1 45.7 26.4 84 41.4 48.2 26.7
38 40.8 46.7 27.4 85 37.8 43.8 24.4
39 40.6 46.7 26.7 86 38.6 45.0 25.1
40 39.9 45.8 26.7 87 38.1 44.3 25.2
41 38.9 44.7 26.1 88 35.4 40.8 23.4
42 39.5 45.1 26.6 89 33.9 38.6 22.5
43 36.3 41.1 24.7 90 35.4 40.8 23.3
44 38.0 43.1 26.0 91 40.5 46.7 27.3
45 38.7 44.2 25.9 92 42.0 48.8 27.9
46 38.6 43.8 26.8 93 42.5 49.2 27.7
47 37.9 43.5 25.1
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materials a selection of the most commonly used ones
was done.
The minimum values of the mean daily and the
absolute maximum surface temperatures were observed
10
15
20
25
30
35
40
45
50
55
60
No 5
White
No 1
Green
No 2
White
Black
No 3 Light
Brown
No 6
Brown
No 4 Gray No 7 Red No 8 Black
Surface color
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 7. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the material tiles
made of mosaic.
10
15
20
25
30
35
40
45
50
55
60
No 19 Whi te No 20 Whi te-
Black
No 21 Green No 22 White-
Green
No 11 Red No 12 Black
Surface color
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 8. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the material tiles
made of granite.
10
15
20
25
30
35
40
45
50
55
60
No 17
White
No 18
White
No 13
Orange
No 15
Gray
No 9 Red No 16
Gray
No 14
Brown
No 10
Black
Surface color
T
e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 9. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the material tiles
made of concrete (30 cm ·30 cm).
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for the white colored material tiles. On the contrary the
maximum corresponding values were noticed in the dark
colored material tiles. Namely, the mean daily surface
temperatures ranged between 29.7 C (for the white
marble tile no. 27) and 46.7 C (for the asphalt tile no.
60). Furthermore the absolute maximum temperatures
varied from 33.4 and 54 C for the same corresponding
materials.
From a first point of view it seems that from the
white colored materials the ones made of marble pre-sented the lowest value of the mean daily surface tem-
peratures. The observed differences in the measured
surface temperatures for the white colored materials are
due to the surface texture. In general the smooth sur-
faced materials present lower surface temperatures than
the ones with rough surface or anaglyph schematics
(Fig. 15).
Continuously, the black colored materials had the
largest surface temperatures from the total number of
the material tiles. The material tile made of asphalt
presented the greatest surface temperature because of its
black and rough surface. Large surface temperatures
10
15
20
25
30
35
40
45
50
55
60
No 64
White
No 61
White
No 49
White
No 44
Gray
No 69
Green
No 66
Orange
No 52
Red
No 71
Dark
Green
No 51
Black
Surface color
T e m
p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 10. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the material
tiles made of concrete (40 cm ·40 cm).
10
15
20
25
30
35
40
45
50
55
60
No 27White
No 30White
No 25WhiteBlack
No 31White
No 33Light
Brown
No 26WhiteBlack
No 28White
No 23White
No 24WhiteBlackRed
No 29WhiteBlack
No 35WhiteBlack
No 32Pink
No 37Gray
No 34Red
No 36DarkGray
Surface color
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 11. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the materialtiles made of marble.
10
15
20
25
30
35
40
45
50
55
60
No 78
Gray
No 77
Red
No 79
Brown
No 60
Black
Surface color
T e m
p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 12. Mean daily surface temperature and temperature
range, within 9:00 to 18:00 for the period of August 2001, for
the material tiles made of pave stone.
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were also observed in tiles made of pebble with dark
gray and dark green surface color (no. 57, 44 C, no. 56,
45.2 C). This was caused by the existing surface
roughness at the pebble tiles (Fig. 17).
10
15
20
25
30
35
40
45
50
55
60
No 76
White
No 43
White
Brown
No 75
White
Gn
Red
No 41
Light
Brown
No 42
Red
No 40
Black
Brown
No 59
Light
Gray
No 58
Gray
No 39
Light
Brown
No 38
Brown
No 56
Green
No 57
Dark
Gray
Surface color
T e m
p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 13. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the material
tiles made of pebble.
10
15
20
25
30
35
40
45
50
55
60
No 89
Brown
No 88
Brown
No 90
Brown
No 85
Brown
No 87
Green
No 86
Brown
No 81
Brown
No 83
Green
No 91
Brown
No 84
Black
No 92
Red
No 82
Gray
No 93
Red
No 80
Black
Surface color
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 14. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the materialtiles made of stone.
10
15
20
25
30
35
40
45
50
55
60
No 27 Marb le No 19 Grani te No 5 Mosaic No 76 Pebb le No 61 Concrete
Construction material
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 15. Mean daily surface temperature and temperature
range, within 9:00 to 18:00 for the period of August 2001, for
the white colored material tiles.
10
15
20
25
30
35
40
45
50
55
60
No 4 Mosaic No 44
Concrete
No 37 Marble No 58 Pebble No 82 Stone No 78 Pave
stone
Construction material
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 16. Mean daily surface temperature and temperature
range, within 9:00 to 18:00 for the period of August 2001, for
the gray colored material tiles.
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10
15
20
25
30
35
40
45
50
55
60
No 84
Stone
No 8
Mosaic
No 36
Marble
No 80
Stone
No 12
Granite
No 51
Concrete
No 57
Pebble
No 60
Asphalt
Construction material
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 17. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the black
colored material tiles.
10
15
20
25
30
35
40
45
50
55
60
No 9
Concrete
No 7 Mosaic No 42
Pebble
No 11
Granite
No 34
Marble
No 93 Stone No 77 Pave
stone
Construction material
T e m p e r a t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 18. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the red colored
material tiles.
10
15
20
25
30
35
40
45
50
55
60
No 1 Mosaic No 21 Granite No 87 Stone No 69
Concrete
No 83 Stone No 56 Pebble
Construction material
T e m p e r a t u r e ( ° C
)
Mean surface temperature
Temperature range
Fig. 19. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the green
colored material tiles.
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In order to study the impact of the size of the
building materials on their thermal performance during
the daytime period a comparison analysis was per-
formed for material tiles made of concrete. Two different
groups of concrete tiles were measured and studied with
respect to their size (30 cm · 30 cm, 40 cm · 40 cm). The
two groups had the same surface color and texture. The
t -test (Livada and Asimakopoulos, 2002) was applied on
the mean daily surface temperatures and it was found
that the surface temperatures were statistical equal
(confidence level a ¼ 0:05). Table 3 gives the mean dailysurface temperatures of the two groups. From this
comparison, can be assumed that the size of the tiles
does not affect the thermal balance of the studied
materials during the day.
Fig. 21 shows the surface temperature distributionfor a number of representative building materials to-
gether with the air temperature during the hot day
period of the 7th August 2001. The selected material
consist of white colored tiles made of marble (no. 27)
and concrete (no. 49) and black colored tiles made of
concrete (no. 51) and asphalt (no. 60). The maximum
difference (22.5 C) between surface temperatures was
Table 3
Comparison of the measured surface temperatures in the case of
concrete tiles with the two different sizes (30 cm ·30 cm, 40
cm·40 cm)
Surface color Surface
texture
Mean daily surface tem-
perature (C)
(30· 30) (40· 40)
Black Smooth 44.0 44.4
Gray Smooth 38.7 38.7
Gray Rough 37.6 38.0
Red Smooth 37.8 37.9
Orange Smooth 37.3 37.4
White Smooth 34.5 33.9White Rough 33.2 33.2
10
15
20
25
30
35
40
45
50
55
60
No 33 Marble No 3 Mosaic No 85 Stone No 66
Concrete
No 79 Pave
stone
No 38 Pebble
Construction material
T e m p e r a
t u r e ( ° C )
Mean surface temperature
Temperature range
Fig. 20. Mean daily surface temperature and temperature range, within 9:00 to 18:00 for the period of August 2001, for the brown
colored material tiles.
20
25
30
35
40
45
50
55
60
09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
Time
T e m p e r a t u r e ( ° C )
No 60 Asphalt(Black)
No 51Concrete(Black)
No 49Concrete(White)
No 27Marble(White)
Air Temperature
Fig. 21. Distribution of surface temperatures within 9:00 to 18:00 of 7th August 2001, between selected material tiles.
242 L. Doulos et al. / Solar Energy 77 (2004) 231–249
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observed for the tiles made of marble and the asphalt at
14:00 LT.
It is often that in the urban environment the building
materials are not always chosen according to their sur-
face color. The most commonly used construction
materials are asphalt, concrete, pebble and pave stone.
However, from the experimental procedures it wasfound that the thermal performance of these materials is
not satisfactory. Figs. 15–17 and 20 show that tiles made
of concrete, pebble and pavestone were warmer than the
other materials. The marble tiles were the coldest from
the total number of the studied materials. Therefore, the
use of materials made of marble in the open urban areas
is thermally more efficient than the use of concrete,
pebble and pave stone materials.
5. A comparative analysis between the surface material
temperatures and the ambient air temperature
The concluding remarks, comparing the material
mean surface temperatures with the mean ambient air
temperature (31.2 C, Fig. 1) during the experimental
time period, are the following:
(A) The majority of the materials studied were
characterized by greater average surface temperatures
than the average air temperature. Only the white colored
tiles made of marble (no. 27 and no. 30) were cooler than
the ambient air. The surface temperatures varied from
29.7 to 30.1 C correspondingly for the two material
tiles.(B) The warmest (38.1 C) light-colored material tile
was the one made of pebble with white and green surface
color (no. 73). The maximum temperature difference
between the light colored materials and the ambient air
was estimated equal to 6.9 C.
(C) In the case of the dark colored materials the
maximum temperature difference was observed for the
one made of asphalt (no. 60) equal to 15.5 C. Besides,
the coldest dark colored material was that made of stone
(no. 84) with average surface temperature of 41.4 C and
a temperature difference with the ambient air of 10.2 C.
6. Statistical analysis of the material surface temperatures
on a 24-h basis
A further investigation of the thermal performance of
the studied materials is attempted through a statistical
analysis. The analysis was based on a number of mea-
surements performed on a 24-h basis during the period
from 9:00 of 14th August 2001 to 9:00 of 15th August
2001. The 24-h period has been divided in three different
subperiods, namely 9:00 to 3:00, 11:00 to 15:00 and
22:00 to 3:00.
6.1. Analysis of the mean surface temperature of building
materials within 9:00 to 3:00 (LT)
The total number of the studied building materials
was separated into nine different groups with respect to
their construction material. Namely each group was
consisted from material tiles made of the same con-struction material but different surface color. F -ANO-
VA test (Livada and Asimakopoulos, 2002) of the means
was applied on the mean surface temperatures within
9:00 to 3:00 LT, in order to study the statistical signifi-
cant differences that caused by the differences in the
surface color.
Table 4 shows that the F -values for each of the
studied group are smaller than the critical values F 0:05 at
a confidence level of 0.05 (a ¼ 0:05). It was found thatthe color of the building materials in each group does
not affect the estimated mean surface temperatures.
A similar methodology was applied on the meansurface temperatures of building materials categorized
into six different groups according to their surface color
(Table 5). Namely, in this case the F -values of mean
surface temperatures were estimated for same colored
materials but of different construction material.
Similar results were obtained in this case. The mean
surface temperatures of the studied building materials
with the same color but of different construction
Table 4
F -ANOVA test of mean surface temperatures for nine groupsof materials, with respect to their construction material and of
different color
Building materials F F 0:05
Mosaic 0.405
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material were found statistically significant at the con-
fidence level of 0.05 (a ¼ 0:05).As a result it could be mentioned that the balance
between the absorbed and the emitted heat during a 24-h
day period appears to be the same for the total number
of the studied materials. However, examining the daily
temperature profile (Figs. 22 and 23) of the materialswith black surface color and with white surface color,
high temperature differences are observed especially
during midday hours.
6.2. Analysis of the mean surface building material
temperatures during the midday hours
In order to investigate the thermal behavior of the
building materials during the day the surface tempera-
tures were studied within the period of 11:00 to 15:00 of
14th August in the daytime. Namely, the mean surface
temperatures and their standard deviation were calcu-
lated and the F -ANOVA test was applied on the surface
temperature values considering the two different cate-
gories according to their construction material (Table 6)
and surface color (Table 7) (Livada and Asimakopoulos,
2002).
As far as the mean surface temperatures of same
construction materials are concerned, these were found
statistically significant different except for those materi-
10
15
20
25
30
35
40
45
50
55
60
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9
Time
T e m p e r a t u r e ( ° C )
No 12Black
No 11 Red
No 22WhiteGreen
No 21Green
No 19White
Fig. 22. Distribution of hourly surface temperatures, within 9:00 of 14th August 2001 to 9:00 of 15th August 2001, for material tiles
made of granite.
10
15
20
25
30
35
40
45
50
55
60
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 5 7 8 9
Time
T e m p e r a t u r e ( ° C )
No 51Black
No 71DarkGreen
No 52Red
No 44Gray
No 61White
Fig. 23. Distribution of hourly surface temperatures, within 9:00 of 14th August 2001 to 9:00 of 15th August 2001, for material tiles
made of concrete.
Table 6
F -ANOVA test of mean surface temperatures, within 11:00 to
15:00 of 14th August, for nine groups with respect to their
construction material and of different color
Building materials F F 0:05
Mosaic 6.91 >2.32
Granite 10.55 >2.62
Concrete (30· 30) 8.61 >2.32
Concrete (40· 40) 5.70 >1.63
Marble 15.81 >1.86
Pave stone 1.013 1.94
Stone 5.69 >1.94
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als made of pave stone (Table 6). From the pave stone
material group, only three different colors were studied
(red no. 77, gray no. 78 and brown no. 79) where simi-
lar mean surface temperatures were measured during
the day (varying from 40.7 to 43.8 C). Furthermore, the
differences of the mean surface temperatures for thesame colored materials are statistically significant for
each group (Table 7). As a result both the construction
material and color should be taken into account for the
lowest surface temperatures.
In an attempt to define ‘‘cold’’ and ‘‘warm’’ materials
the multiple statistical test of Tukey and Kramer were
applied on the mean surface temperature values (Livada
and Asimakopoulos, 2002). This was to examine the
significance of the mean surface temperatures differ-
ences, within the daytime hours, for each pair of the
studied materials.
The Tukey and Kramer test became more reliable
with the performance of the t -test of the mean surface
temperature differences for all the possible pairs of the
studied building materials as both of these statistical
tests are applied at the same confidence level (Livada
and Asimakopoulos, 2002).
The concluding remarks are as follows:
1. For building materials made of mosaic as ‘‘cold’’ can
be considered the white, the white–black, the green
and the brown surface colored materials (no. 5, no.
2, no. 1 and no. 6).
2. For those made of the granite as ‘‘cold’’ can be con-
sidered the white and the white–black surface coloredmaterials (no. 19 and no. 20).
3. For those made of the concrete as ‘‘cold’’ can be con-
sidered all the white colored materials (no. 17, no. 18,
no. 49, no. 61, no. 62, no. 63 and no. 64).
4. For those made of the pebble as ‘‘cold’’ can be con-
sidered the white, the white–brown and the white–
green–red surface colored materials (no. 76, no. 43
and no. 75).
5. For those made of the marble as ‘‘cold’’ can be con-
sidered the white, the brown (beige) and the white
with black shades surface colored materials (no. 27,
no. 33 and no. 29).
6. For those made of the stone as ‘‘cold’’ can be consid-
ered the brown surface colored materials (no. 88, no.
89 and no. 90).
The same statistical test was performed between the
different colored construction tiles. The corresponding
results are the following:
(A) For black surface colored materials the lowest tem-
peratures were observed at those made of mosaic,
concrete and marble (no. 8, no. 48 and no. 37).
(B) For white surface colored materials the lowest tem-
peratures were observed at those made of mosaic,
concrete, granite, pebble and marble (no. 5, no.
64, no. 19, no. 76 and no. 27).
(C) For gray surface colored materials all of them ex-
cept for those made of pebble (no. 58) and pave
stone (no. 78) presented low temperatures.
(D) For green surface colored materials the lowest tem-peratures were observed at those made of mosaic
and granite (no. 1 and no. 21).
(E) For brown surface colored materials the lowest tem-
peratures were observed at those made of mosaic
and stone (no. 6 and no. 89).
From the combination of the above comparisons
turned up seven tiles (Table 8), which can be considered
the tiles with the lowest temperatures.
The same multiply comparison by Tukey and Kra-
mer statistical test was applied for the 21 sampling pairs
of materials that turned up from the above table (Livada
and Asimakopoulos, 2002). The corresponding results
show that the mean surface temperatures for the green
colored mosaic tile, the brown colored mosaic tile and
the brown colored stone tile (marked with * in Table 8)
are statistically significant higher than the others at a
confidence level of 0.05 (a ¼ 0:05).Finally as ‘‘cold’’ materials can be considered, from
the colder to warmer, the white marble, the white
Table 7
F -ANOVA test of mean surface temperatures, within 11:00 to
15:00 of 14th August, for the six groups of building materials,
according to their color and of different construction material
Surface material color F F 0:05
White 2.16 >1.90
Gray 4.02 >1.94
Black 2.90 >2.12
Red 2.50 >2.05
Green 7.18 >2.44
Brown 4.00 >2.12
Table 8
Mean surface temperatures and deviation for ‘‘cold’’ materials
as they assumed by statistical tests with respect to their con-struction material and surface color within 11:00 to 15:00 of
14th August
Material–surface color Surface tem-
perature (C)
Deviation
Mosaic–white (no. 5) 32.32 5.162
Mosaic–green (no. 1) 33.92 5.812
Mosaic–brown (no. 6) 34.98 6.932
Granite–white (no. 19) 31.46 5.353
Concrete (40·40) and White
with marble inlay (no. 64)
32.36 6.523
Marble–white (no. 27) 29.1 4.625
Stone–brown (no. 89) 33.6 7.655
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granite, the white mosaic and finally the white concrete
with marble inlay. In this comparison the surface texture
was not taken into account.
6.3. The impact of the surface texture on the thermal
performance of the materials
In order to investigate the impact of the surface
texture on the mean surface temperatures, 24 different
pairs of same colored and construction building mate-
rials but of different texture were studied for the period
of 11:00 to 15:00 within the 14th August.
The F -test of the variance differences was applied for
each of the studied pairs in order to define a proper
equation of t -test of the differences of the mean values
(Table 9) (Livada and Asimakopoulos, 2002).
The standard deviations for each of the studied pair
of building material were considered statistically signif-
icant equal at a confidence level of 0.05 (a ¼ 0:
05).
Afterwards the statistical test of the means (t -test)
was applied for all the samples with the same size and
with statistical equal standard. For all the cases the jt jvalues were smaller from the critical value t 0:05 at the
0.05 confidence level in the two tailed test.
Therefore the material’s surface texture does not af-
fect statistically the measured surface temperature during
the daytime (9:00 to 15:00). ‘‘Cold’’ building materials
can be considered independently of the surface texture.
6.4. Analysis of the mean surface building material temperatures during the night period
In order to investigate the thermal behavior of the
building materials during the night the surface temper-
atures were studied within the period of 22:00 of 14th
August to 3:00 of 15th August in the nighttime. Simi-
larly as above, the mean surface temperatures and their
standard deviation were calculated and the F -ANOVA
test was applied, considering the two different categories
according to the construction material (Table 10) and
the color (Table 11) (Livada and Asimakopoulos, 2002).
According to the construction material the mean
nocturnal surface temperatures were found statistically
significant different only for marble and stone (confi-
dence level a ¼ 0:05) (Table 10). According to the samecolored materials, the differences of the mean surface
temperatures are statistically significant for every group
(confidence level a ¼ 0:05) (Table 11).
Table 9
Various material pairs comparison to examine the surface texture impact in the measured surface temperatures
Construction material Surface color–texture Surface mean
temperatures (C)
Deviation jt -testj
Concrete (30 cm·30 cm) Gray, smooth without schematic (no. 15) 36.74 6.97 0.98
Gray, rough without schematic (no. 16) 38.44 8.39
White, smooth without schematic (no. 17) 32.06 5.93 1.05
White, rough without schematic (no. 18) 33.76 7.25
Concrete (40 cm·40 cm) Gray, smooth without schematic (no. 44) 38.14 6.70 1.86
Gray, rough without schematic (no. 45) 39.2 8.72
Red, smooth without schematic (no. 53) 39.86 11.85 0.144
Red, rough without schematic (no. 52) 40.16 9.99
White, smooth without schematic (no. 62) 32.88 5.93 0.406
White, rough without schematic (no. 61) 33.52 6.48
White, smooth with inlay (no. 64) 32.36 6.52 1.246
White, rough with schematic (no. 63) 34.36 6.37
Orange, smooth with schematic and inlay (no. 65) 37.7 9.37 0.943Orange, rough without schematic (no. 66) 39.6 10.94
Green, smooth with inlay (no. 72) 38.66 13.93 2.23
Green, rough with schematic (no. 71) 44.28 17.69
Orange, rough with schematic (no. 70) 37.88 11.56 0.811
Orange, rough without schematic (no. 66) 39.6 10.94
Black, smooth with schematic (no. 50) 44.26 12.05 0.855
Black, smooth without schematic (no. 51) 46.28 15.86
Marble White, smooth with black shades (no. 25) 31.38 5.78 1.667
White, rough with black shades (no. 24) 33.76 4.41
White, smooth (no. 27) 29.1 4.63 1.363
White, anaglyph (no. 28) 32.0 6.31
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The Tukey–Kramer test was applied again on the
mean nocturnal surface temperature values for those
materials made of marble and stone. The concluding
remarks are the following:
1. For building materials made of marble, the white
with black and red shades (no. 24) and the white
(no. 28) surface colored materials could be character-
ized as ‘‘warm’’ materials, while the light brown (no.
33) surface colored material could be characterized as
‘‘cold’’ material.
2. For building materials made of stone, the brown (no.89, no. 91) and the red (no. 93) surface colored mate-
rials could be characterized as ‘‘warm’’ materials,
while the brown (no. 88) and the red (no. 92) surface
colored materials could be characterized as ‘‘cold’’
materials.
As a result it could be mentioned that the construc-
tion material determines the thermal balance during the
night (by affecting the emissivity), while the surface color
determines significant the thermal balance only during
the day (by affecting the albedo).
The same statistical test was performed between the
different colored construction tiles. The corresponding
results are the following:
(A) For black surface colored materials low tempera-
tures were observed for all the construction materi-
als, except for those made of asphalt (no. 60). Theconstruction material made of asphalt defined as
the ‘‘warmest’’ of all.
(B) For white surface colored materials, the rough with
schematics concrete (no. 63) could be characterized
as ‘‘warm’’ material, and the light brown marble
(no. 33) as ‘‘cold’’. The light brown marble was con-
sidered in the group with the white surface colored
materials.
(C) For gray surface colored materials, the cement
with the smooth surface (no. 46) and the pave
stone (no. 78) could be characterized as ‘‘warm’’
materials. For the rest gray surface colored materi-als, the mean surface nocturnal temperatures were
similar, so no material could be characterized as
‘‘cold’’.
(D) For green surface colored materials, the pebble (no.
56) could be characterized as ‘‘warm’’ material.
(E) For brown surface colored materials, the pebble
(no. 38) could be characterized as ‘‘warm’’ material.
6.5. The impact of the surface texture on the thermal
performance during the night period
In order to investigate the impact of the surfacetexture on the mean surface temperatures during the
night, the same pairs (as given in Table 9) of same col-
ored and construction building materials, but of differ-
ent texture, were studied for the period within the 22:00
of 14th August to 3:00 of 15th August.
The F -test of the variance differences was applied for
each of the studied pairs in order to define a proper
equation for the t -test of the differences of the mean
values (Table 12) (Livada and Asimakopoulos, 2002).
The mean nocturnal surface temperature comparison
according to the texture of the materials indicates,
showed in some cases statistically significant differences(confidence level a ¼ 0:05) (Table 12). In particular, forboth concrete and marble, the differences were caused
due to the low surface temperatures measured in the
smooth surface materials during the night, while the
corresponding tiles with rough surface had warmer
surface.
Generally the smooth surface materials appear to be
colder than the rough materials during the night. For 24-
h time period the light brown marble (no. 33) and the
brown stone (no. 90) could be characterized as ‘‘cold’’
materials.
Table 10
F -ANOVA test of mean surface temperatures, within 22:00 of
14th August to 3:00 of 15th August, for nine groups with re-
spect to their construction material and of different color
Building materials F F 0:05
Mosaic 2.03 2.22
Green 10.97 >2.49
Brown 11.72 >2.30
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7. Conclusions
The materials’ thermal balance is determined mainly
by their reflectivity to solar radiation and their emis-
sivity to the long wave radiation during the daytime. As
the emissivity values for the total number of the studied
materials were close to 0.9, the observed differences in
the mean daily surface temperatures are mainly caused
by the different albedo factors of the total number of the
studied materials. The physical characteristics of the
material tiles that affect their albedo are the color,
the surface texture and the construction material. The
rough and dark colored surfaces tend to absorb more
solar radiation than the smooth, light colored and flat
surfaces. Therefore the dark colored surfaces are warmer
than the light colored.
From the study of the total number of the pavement
materials according to their surface color material it was
found that the light colored tiles were cooler than the
others. As expected the white colored tiles were the
coldest, while the black colored were the warmest.
Afterwards from the analysis of the building materials
according to their construction material it was found
that tiles made of marble, mosaic and stone were cooler
than the other ones. Besides, from the analysis based on
the material textures, the tiles with smooth and flat
surface were cooler than the tiles with rough and ana-
glyph surface. Finally studying the impact of sizing it
Table 12
Various material pairs comparison to examine the surface texture impact in the measured nocturnal surface temperatures
Construction material Surface color–texture Surface mean
temperatures C
Deviation jt -testj
Concrete (30 cm· 30 cm) Gray, smooth without schematic (no. 15) 17.87 0.539 0.211
Gray, rough without schematic (no. 16) 17.95 0.319
White, smooth without schematic (no. 17) 17.33 0.578 1.531
White, rough without schematic (no. 18) 17.93 0.343
Concrete (40 cm· 40 cm) Gray, smooth without schematic (no. 44) 18.22 1.001 0.321
Gray, rough without schematic (no. 45) 18.4 0.884
Red, smooth without schematic (no. 53) 17.42 0.57 2.51
Red, rough without schematic (no. 52) 18.51 0.562
White, smooth without schematic (no. 62) 17.78 0.622 1.27
White, rough without schematic (no. 61) 18.33 0.503
White, smooth with inlay (no. 64) 17.08 0.662 3.82
White, rough with schematic (no. 63) 18.73 0.455
Orange, smooth with schematic and inlay (no. 65) 17.83 0.707 0.76
Orange, rough without schematic (no. 66) 18.2 0.724
Green, smooth with inlay (no. 72) 17.13 0.679 1.43
Green, rough with schematic (no. 71) 17.82 0.718
Orange, rough with schematic (no. 70) 18.02 0.722 0.37
Orange, rough without schematic (no. 66) 18.2 0.724
Black, smooth with schematic (no. 50) 18.62 0.822 3.38
Black, smooth without schematic (no. 51) 16.85 0.827
Marble White, smooth with black shades (no. 25) 15.6 1.452 2.19
White, rough with black shades (no. 24) 16.93 0.759
White, smooth (no. 27) 16.18 0.722 3.72
White, anaglyph (no. 28) 18 0.711
Fig. 24. Definitions of ‘cold’ and ‘warm’ materials.
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was concluded that for the material tiles with the same
construction material, surface color and texture but
different geometry characteristics (surface size and
thickness) the differences in the surface temperatures are
not statistical significant during the daytime period. The
observed non-uniform temperature distribution on some
materials’ surfaces was caused by their surface colorcontrast; the surface roughness and the heat transfer
phenomena. Therefore as ‘‘cold’’ materials can be
characterized those having a smooth and light colored
surface and construction materials made of marble,
mosaic and stone. Similarly as ‘‘warm’’ materials could
be defined those having a rough and dark colored sur-
face and construction materials made of pebble, pave
stone and asphalt (Fig. 24).
The use of ‘‘cold’’ materials is important in the urban
environment and especially in cities with hot climate.
The use of ‘‘cold’’ materials contributes to the reduction
of the air temperature due to heat transfer phenomena.However ‘‘warm’’ materials instead of ‘‘cold’’ are used
to the urban environments structure. This use is caused
either due to economic and esthetic reasons, or by bad
environmental planning. As a result, the temperature in
the urban environment is raised and the demand for
cooling load in the buildings is getting greater.
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