| Topic: |
Science > Physics |
| User: |
"CWatters" |
| Date: |
04 Feb 2008 03:34:19 AM |
| Object: |
Radaited heat loss |
I understand that a shiny/polished surface is a good reflector of thermal
radiation and indeed there are several foil based insulation products on the
market that use this principle. However...What happens if you paint the
shiny surface or coat it in some way (eg wall paper). How does that effect
the ability of the foil to reflect radiated heat?
I imagine any _radaited_ heat that gets through the wallpaper is reflected
back into the wallpaper where it's absorbed and then it gets conducted
through the foil. Is that correct? Do foil based insulation products require
an air gap to be effective?
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| User: "Androcles" |
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| Title: Re: Radaited heat loss |
04 Feb 2008 06:02:30 AM |
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"CWatters" <colin.watters@NOturnersoakSPAM.plus.com> wrote in message
news:13qdn0isl9rl60@corp.supernews.com...
|I understand that a shiny/polished surface is a good reflector of thermal
| radiation and indeed there are several foil based insulation products on
the
| market that use this principle. However...What happens if you paint the
| shiny surface or coat it in some way (eg wall paper). How does that effect
| the ability of the foil to reflect radiated heat?
|
| I imagine any _radaited_ heat that gets through the wallpaper is reflected
| back into the wallpaper where it's absorbed and then it gets conducted
| through the foil. Is that correct? Do foil based insulation products
require
| an air gap to be effective?
|
Think of a mirror. The glass is transparent, the reflective coating
works fine and sends the radiation (light) back to your face.
If you place an absorbent coating on the glass then that can't
work as a mirror, but any light that does get through will be
reflected back to the coating.
.
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| User: "Bruce Scott TOK" |
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| Title: Re: Radaited heat loss |
04 Feb 2008 08:50:56 AM |
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CWatters wrote:
I understand that a shiny/polished surface is a good reflector of
thermal radiation and indeed there are several foil based insulation
products on the market that use this principle. However...What happens
if you paint the shiny surface or coat it in some way (eg wall
paper). How does that effect the ability of the foil to reflect
radiated heat?
Here, "radiated heat" is IR (infrared) while the solar radiation the
material receives is mostly visible (due to the fact that the
photosphere is at 5600K while the radiating material is perhaps 300K or
a little more).
I imagine any _radaited_ heat that gets through the wallpaper is
reflected back into the wallpaper where it's absorbed and then it gets
conducted through the foil. Is that correct? Do foil based insulation
products require an air gap to be effective?
Wall paper may or may not be a different story; it is easier if you
consider dark paint. Then the shiny surface is mostly irrelevant, as
apsorption or reflection is decided in a quite narrow surface layer (how
narrow, depends on the details of the solid state physics).
Now if you consider wallpaper the way you want then
you have a multilayer system: the wallpaper and then the layer under it.
What happens is that each layer comes into thermal equilibrium
separately. For the wallpaper you have incoming and outgoing radiation
at the outer surface, and also at the inner surface. For the shiny
material underneath you have just the single surface. At that layer you
have a relation between absorption (as a sink, i.e., the reservoir
underneath is not affected), reflection, and incoming radiation. All
are visible. The temperature of the object is determined by ambient
heat if the reservoir is considered infinite. The energy balance of the
material surface is given by absorption (visible) and rerediation (IR).
For the wallpaper you have a balance like this at each side. If you
take into account the effect of all these radiation balances on the
temperature of the materials (as you would for a rediative atmosphere,
for example) you have to treat the _total energy_ balances of the
materials including models for their energy content.
In the real world most of these temperatures are set by ambient air, not
by radiation. A clear exception is a metal in cold air which radiates
on its own very well and is therefore colder than the ambient air. The
balance is given by heating via ambient air set equal to cooling by
radiation. On a summer day in the sun it is the other way around:
heating by radiation is strong enough that ambient air cools the
material so that in balance it is warmer than the ambient air.
Hope this helps. If it is not at all the sort of explanation you want,
please note what you want to assume regarding the properties of the
wallpaper :-)
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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| User: "Ian Parker" |
|
| Title: Re: Radaited heat loss |
04 Feb 2008 05:58:04 AM |
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On 4 Feb, 09:34, "CWatters" <colin.watt...@NOturnersoakSPAM.plus.com>
wrote:
I understand that a shiny/polished surface is a good reflector of thermal
radiation and indeed there are several foil based insulation products on the
market that use this principle. However...What happens if you paint the
shiny surface or coat it in some way (eg wall paper). How does that effect
the ability of the foil to reflect radiated heat?
You get the IR radiance characteristic of wallpaper.
I imagine any _radaited_ heat that gets through the wallpaper is reflected
back into the wallpaper where it's absorbed and then it gets conducted
through the foil. Is that correct? Do foil based insulation products require
an air gap to be effective?
You need to ensure that heat cannot flow ny conduction - Yes.
- Ian Parker
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| User: "Timo A. Nieminen" |
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| Title: Re: Radaited heat loss |
04 Feb 2008 11:39:04 AM |
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On Mon, 4 Feb 2008, CWatters wrote:
I understand that a shiny/polished surface is a good reflector of thermal
radiation and indeed there are several foil based insulation products on the
market that use this principle. However...What happens if you paint the
shiny surface or coat it in some way (eg wall paper). How does that effect
the ability of the foil to reflect radiated heat?
I imagine any _radaited_ heat that gets through the wallpaper is reflected
back into the wallpaper where it's absorbed and then it gets conducted
through the foil. Is that correct? Do foil based insulation products require
an air gap to be effective?
You don't need the air gap. A gap won't change the emissivity. With no
gap, the foil/paint will be at the same temperature as the foiled/painted
object. Leave some nice shiny metal, reflecting 90% of the incoming light,
out in the sun, and see how hot it gets as a result of IR emissivity of
0.05.
An air gap is good. If you can let the roof/foil/ceiling cool by
convection, then you won't get temperatures determined by radiative
equilibrium. But this is more than just an air gap, you need ventilation
(which can be passive).
The usual usage of foil around here has very little gap, with the foil
going on top of the roof framing, and tiles (or other roofing) right on
top of it. Mine is attached under the rafters and works very well - the
difference between the two halves of the house on a sunny day when the
work was half done was very noticeable. The usual method of retro-fitting
foil into uninsulated houses is to put it on top of the ceiling joists.
All work.
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
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