Oops forgot a factor of 4
Radiative heat transfer for surface with an emissivity of 1 at 300K is
about 612uW/square cm/degree C
which is equivalent to about 25mm of styrofoam.
Which raises the question what's the emissivity of the isothermal
surface used when measuring the thermal resistance of a slab of styrofoam?
Bruce
Bruce Griffiths wrote:
Radiative heat transfer (for a surface with an emissivity of 1) is
around 150uW/square cm /degreeC at 300K.
That's equivalent to about 10cm of styrofoam.
It seems unlikely that the radiative heat transfer component is
included in the thermal resistance rating for Styrofoam.
The radiative component is independent of insulation thickness where
the insulation doesnt absorb in the 10-30um infrared region.
Adding carbon black to the foam appears to increase the thermal
resistance of 25mm thick foam by about 10%.
Bruce
Bob Camp wrote:
Hi
That raises the interesting question of weather radiant transfer is
already included in the rated thermal resistance. My guess is that
the 8" of foam is enough to cover any radiation issues and still get
you above 20 C / W.
Since you are probably starting with 1 or 2" slabs, including the
aluminum foil would be pretty easy. It can't hurt and it might help.
Bob
On Mar 10, 2010, at 7:29 PM, Neville Michie wrote:
I have been wondering about achieving the rated thermal resistance
from plastic foam,
the problem being that radiant transmission may be very strong
through the foam.
What happens if you interleave concentric sheets of foam plastic
with aluminium foil? (taking care to keep the foil
on isothermal surfaces)
Will this stop the radiant transfer and leave only the thermal
conduction of the plastic foam?
cheers, Neville Michie
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