[Stoves] Sand as Insulation

Sat Aug 27 13:06:23 CDT 2011

Dear Xavier

The thermal conductivity of quartz is not relevant to the thermal
conductivity of sand because the tiny air gaps between the grains of sand
are far more important than the thermal conductivity of the sand itself.
Sand tends to be sharp-edged broken crystals which only touch at their tips
(points). 

Yes it has thermal mass - lots of it - which still says nothing about its
insulation properties.

It boils down to its thermal conductivity. Saying something has a low heat
conductivity is the same as saying it is an insulation - it's an arbitrary
choice.

So, looking at the numbers quoted for sand (which as you note varies with
grain size, and in fact with grain size distribution) the values are from
0.06 to maybe 0.5 or 0.1 depending on who you ask. The conductivity is not
completely irrelevant in that if you had aluminum grains, it would not be as
good an insulator as sand, but I hope you get the point about the spaces
being dominant, not the material.

Xenospheres collected from coal combustion are made of silicon dioxide but
are phenomenal insulators because they are mostly a vacuum (inside) or air
(between them). Ash Resources in Johannesburg sells them graded into sizes
like 80 to 100 microns. The only thermal contact is between the spherical
surfaces.

I expect sand to be widely variable, and single grain sizes to outperform
multiple ones. It is pretty easy to clean sand using a bucket and water so
there is nearly no clay in it. 

It is of course widely available!

Regards
Crispin

-----Original Message-----
From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Xavier Brandao
Sent: 27 August 2011 04:29
To: stoves at lists.bioenergylists.org
Subject: Re: [Stoves] Sand as Insulation

Crispin,

I was wondering also how good/bad an insulator it was. I couldn't find
figures about its thermal conductivity. On forums, one or two persons were
wondering the same and couldn't find info on the web. There seemed to be a
consensus from house builders about sand thermal inertia, and about the fact
is was a bad insulator. It can be used in the walls of house next to an
insulator, to keep warmth inside.
Here is a insulation for wooden houses, with recycled glass and sand:
http://www.maison-bois-passive.net/isolation-sable-verre-recycle-pure-one.ht
ml

It is said on Wikipedia quartz is the main material to be found in sand, and
it has a conductivity of 6,8-12 W/(m·K) for a temperature of 20 °C:
http://fr.wikipedia.org/wiki/Conductivit%C3%A9_thermique 

Sandstone has a high conductivity also:
http://en.wikipedia.org/wiki/Thermal_conductivity
http://en.wikipedia.org/wiki/Sandstone

I read in another study that sand conductivity decreased as its size
increased.

If you find more info about that, I'm interested.

Regards,

Xavier

-----Message d'origine-----
------------------------------

Message: 7
Date: Wed, 24 Aug 2011 23:09:28 -0400
From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
To: "Stoves" <stoves at lists.bioenergylists.org>
Subject: [Stoves] Sand as Insulation
Message-ID: <0b2b01cc62d4$67a35d10$36ea1730$@gmail.com>
Content-Type: text/plain; charset="us-ascii"

Dear Friends

You know how I like to look up the numbers to see what the truth looks
like..

So I was looking up some heat conduction values for materials for a stove
and came across the following:

Heat conduction constants (k). The value is expressed as 

Watts/m.K which is derived from Watts/m2/m/K and reads as Watts conducted
per square meter of area, per metre of thickness, per degree Kelvin. The
first formula has the m2/m reduced to m.

So here goes: values for these materials at 225 C:

Sand                                      0.06 Watts/m.K

Sawdust                               0.06 

Rock wool                           0.045 

Polystyrene expanded 0.03

Wood                                    0.13

Brick dense                         1.6

Brick work                           0.5

Cement                                1.01

Cotton Wool insulation 0.029

Felt insulation                    0.04

Magnesia insulation        0.07

Paper                                    0.05

PVC                                        0.19

Straw insulation                0.09

So the big surprise is sand. We are often asked if sand can be used as
insulation and the answer is 'yes', apparently. In fact it is better than
magnesia insulation.

Maybe this is a typographical error. Confirmations, anyone?

Regards

Crispin

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