"James Waldby" wrote in message
...
On Fri, 29 Jul 2011 12:39:46 -0400, Ed Huntress wrote:
wrote ...
On Jul 28, 11:49 pm, Larry Fishel wrote:
On Jul 27, 10:20 am, "Existential Angst" fit... wrote:

It would seem that if air itself was so good, solid insulation
wouldn't be so high a priority

Neither fiberglass nor Styrofoam are "solid". They are basically air
with a tiny bit of structure to keep that air from moving (much)...

Exactly. And they rely on the fact that stationary air is in fact a
poor conductor of heat.

Another way to explain it is that there are two variables that control
how effective trapped air can be as an insulator. One is preventing
mixing and convection -- movement of the air, for practical purposes.
Therefore, small pockets are more efficient than big ones. The other is
the relative conductivity and relative volume of the entrapping medium:
Plastics usually are better than glass, which is better than metal, etc.

I contend that as a practical matter, "small pockets are more efficient
than big ones" does not remain true as cell size shrinks to zero.
Instead (for any given medium and filler gas) efficiency improves as
size decreases to some point, after which efficiency gets worse, due
to increased importance of heat conduction relative to convection as
size decreases.
With an ideal medium, the ratio need not change,
because ideally the ratio of gas volume to medium volume can remain
constant as cell size shrinks and as constant strength (or, at least,
constant cross section of medium) is maintained. But as a practical
matter, after cell wall thickness reaches some minimal amount, it
cannot shrink further as cell size decreases.

Sure. But the R value of polyurethane foam, for example, increases as the
pore size diminishes to the practical minimum you can obtain and still have
continuous foam.

But that practical limit diminishes further with aerogels, including
nanogel, because the thickness of the entraining walls can be vanishingly
small. And then you get even better R values.


[snip re aerogels & nanogel]

Makers of "cenospheres" are quite proud of their products -- see
eg http://www.isbu-info.org/all_about_ceramic_insulation.htm and
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=115x89276

--
jiw