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Kiko Denzer on Art



Cob: Re: Thermal Mass and R-values

Mike Carter and Carol Cannon cobcrew at sprynet.com
Sun Jul 25 11:05:39 CDT 1999


From: alexander neumann <alex at privat.kkf.net>
Date: Sunday, July 25, 1999 8:22 AM

[snip]
>1), Vermiculite/Perlite/whatever in cob makes so-called "Leichtlehm", in
>contrast to straw-lightclay it is called mineral lightclay, but that
>doesn´t change the given R-values, which are defined by the density/ weight
>per volume. Lightclay is not supposed to be loadbearing.
>Wellerlehm, which is somewhat similar to cob has a density of about 1500 to
>1800 kg/m³, which gives an insulation value of 0.7 to 0.9 W/mK, Lightclay
>of 600 kg/m³ which is ,believing earth-guru Gernot Minke about the best you
>can get in real life, has 0.17 W/mK.
>In comparison, regular fibreglass or cellulose has 0.04W/mK, so, an 8inch
>insulated stick frame wall is more insulating than a 3feet cob wall.
>So, personally, I don´t think additives to cob or socalled leichtlehm are
>getting us anywhere.
[snip]

The equation that was developed for computing the thermal conductivity of
clay-straw mixes was derived by looking at data over a wide range of mix
densities.  There was over a 10:1 difference between extremes, so there is
obviously something worth talking about there.  Perlite/Vermiculite/pumice
do not share the same thermal properties as straw or each other.   Consider
also two samples that have the same density but in one, the insulating
material is in a solid layer in the middle instead of being dispersed evenly
throughout.  People have also been discussing the use of these insulating
materials in a non-structural outer coating or as an infill.  I will put
forth that the Minke straw-clay equation is not relevant to that layer
(unless it is made of straw and clay!) and that people should not be
discouraged from trying "natural" insulation alternatives on the basis of
that equation.  One might find that you reach a point where heat transfer
through the material is no longer primarily by conduction through clay and
sand; then increased densities of the insulating material could actually
increase thermal resistance.

Fiberglass and cellulose have enough drawbacks to warrant considering
alternatives, even if you get a thicker wall as a result.

Mike