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His climate is very wet, with cool but not freezing winters. He specified that he added two layers of plaster: the first is perlite-heavy, the second is a weather protecting layer with sand.
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Craig Ray's cob house in New Zealand (below) boasts a mix of perlite and paper pulp in the plaster with splendid results. It hasn’t been tested much in earth plaster, and the R value would certainly reduce depending on the ratio of perlite to plaster. Perlite is a volcanic glass with an R value of 2.7 when used pure and compacted.
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Paper adobe (R2.5-3 per inch): Paper adobe, as suggested by the name, is where you mix re-pulped paper with clay and earth to create a paper-rich adobe.You can buy cork granules and mix them into your plaster instead of the sand. The cork tree’s bark can be harvested every 9 years. It has a very high insulating value, is also superb for sound-proofing, and, counter-intuitively, it’s relatively fire-resistant as well. Please note: That plaster needs to be pretty straw/sawdust heavy to have a decent effect. Sawdust/straw: If you mix a decent quantity of straw or sawdust into your plaster (instead of the sand), things are soon going get warmer in your mud world.But if you live in a temperate climate, or a place where you need a little extra oomph in the winter months, insulating plaster can work a treat. You’ll want to use the Other Ways to Insulate an Earthern Wall listed in the next section. First, let's be clear Mixing insulating materials into your earth plaster alone isn’t going to cut the mustard in seriously cold climates.