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Cob: Re: Thermal Mass and R-valuesMike Carter and Carol Cannon cobcrew at sprynet.comSun 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
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