Civil Engineering Reference
In-Depth Information
Black
materials
get much hotter than white materials, and a
black surface may experience a very large temperature swing
between night and day (more than 50°C for a lightweight
black roof)
White materials
are cool. They do not
absorb much heat from the sun, and they are
good radiators
Black
1.0
Roofing
tar and
gravel
Ice
Black
asphalt
White
paint
Red
tile
0.9
Light
green
paint
Snow
Cream
paint
Light
concrete
Dark
concrete
Black
paint
0.8
Whitewashed
galvanised
roofing sheet
Man-made
fibre
reinforced
slate
0.7
0.6
Old
aluminium
paint
0.5
Bronze
paint
New
aluminium
paint
Tarnished
copper
0.4
0.3
Oxidised
galvanised
sheet
Stainless
steel No
301, 316
0.2
New
galvanised
sheet
0.1
Polished
copper
Aluminium
sheet
Polished
aluminium
foil
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.0
0.9
Solar radiation absorptance
Polished metal surfaces
get warmer than white
surfaces because, though they absorb a similar
amount of heat, they re-radiate less of it
Dull
metal surfaces
may get hotter than black
surfaces. They absorb less heat from the sun
but re-radiate little of it
Figure 1.51
Calculated extreme surface temperatures for air temperature of 30
°
C
Prediction of temperatures
Bright metal sheet materials (that is to
say, not coated) also tend to have high
reflectance but in their case long wave
emittance is low. This leads to the
perhaps surprising
result that they can experience
significant solar heating.
Figure 1.51 illustrates the
temperatures for different surface
finishes on a hot summer's day in
the UK (eg an air temperature of
30
are used, Figure 1.51
is likely to be equally valid for roofs
and walls. It is also likely to apply to
most locations in temperate or
equatorial zones.
There is a certain amount of
experimental data on measured
surface temperatures. For example, in
measurements on wall sandwich
C). It shows the large difference
which could be expected between,
say, two highly insulated walling
panels; one with a white and one with
a black finish. Because maximum
values rather than long term averages
°
