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EMISSIVITY EXPLAINED IN LAYMAN'S
TERMS
Quick question:
Two identical cars are sitting in the sun in a
parking lot for four hours in the summer. One car has a black bumper, the
other, a chrome bumper. Which one is hotter?
Black? Makes
sense, chrome reflects heat and black absorbs heat, everyone will agree on
this.
Now, think about touching the chrome seat belt buckle in your
car, the ignition switch, or leaving a toolbox open in the sun and trying to
pick up a chrome tool. Want to change your answer?
Reality: The chrome bumper, tools and seat belt buckles are MUCH
hotter (over time), than an identical surface painted black. Why?
Emissivity
Emissivity is the scientific measurement of the ability for heat to
radiate (leave), an object. While it's true black paint will heat up
faster and chrome does reflect heat, the emissivity of these two surfaces are on
opposite ends of the scale as seen below.
Chrome has an emissivity of
.04 while black paint emits radiant heat at a rate of .95. The black
bumper will heat up much faster but 95% of the heat that is absorbed is free to
re-radiate at a rapid rate.
The chrome bumper reflects 96% of the heat
and only absorbs 4% but this heat is "trapped" in the bumper as only 4% can emit
from the surface. Immersed in a bath of infrared (the sun), the steel
under the low emissivity surface of the chrome bumper will soon exceed that of
the one painted flat black.
A good example of this is the surface of solar
panels used to heat water. There are two choices: flat black or black
chrome. You will pay more for the black chrome panel but you will
also get hotter water as 95% of the heat collected by the absorber is trapped
and only 5% is free to re-radiate out through the
glazing.
Webster's definition:
Main Entry: emis·siv·i·ty
Pronunciation: "e-m-i si-v-i tee
Function: noun
Inflected Form(s): plural -ties
Date: 1880
: the relative power of a surface to emit heat by radiation : the ratio
of
the radiant energy emitted by a surface to that emitted by a
blackbody
at the same temperature
Main Entry:
black·body
Pronunciation: 'blak-'bä-dE
Function: noun
Date: 1710
: an ideal body or surface that completely absorbs all radiant
energy
falling upon it with no reflection and that radiates at all frequencies
with
a spectral energy distribution dependent on its absolute
temperature
(for more data
on emissivity and heat movement, do a search for "Sir James
Dewar")
Ok...in layman's
terms. Emissivity is the ability for radiant heat to leave the surface of
an object. It matters not what the density, mass or thickness of the
object, only the surface. As can be seen below, emissivity (E
factor), plays a significant role in how heat moves into or out of our
homes, our cars, our bodies.
Fig
1.2 EMISSIVITY OF
VARIOUS COMMON MATERIALS
Material
Emissivity value
Gold, polished .03
Metalized Film Radiant Barrier .04
Silver, polished .04
Chrome .05
Aluminum,
polished .04
oxidized .78
Brass, polished .04
oxidized .61
Iron, polished .21
oxidized .69
Copper, polished .05
oxidized .78
Human skin .98
EMISSIVITY OF BUILDING MATERIALS
Wood .95
Glass .94
Paint,
average of 16 colors .94
Brick, common red
.93
Concrete .92
Plaster, rough coat .91
Source: Handbook of Chemistry
Based on the emissivity
chart above, it is clear to see that we are designing, building and living in
solar hot boxes. Time to change this so visit the rest of our site
and see how our low emissivity, high performance films can help you increase the
comfort of your home and reduce rising energy costs. Click here.
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