伝統技能で施工された土壁（木舞・貫入り）の熱物性測定

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  • MEASUREMENT OF THE THERMOPHYSICAL PROPERTIES OF TSUCHIKABE (WITH WATTLES AND NOGS EMBEDDED) CONSTRUCTED WITH TRADITIONAL SKILLS
  • デントウ ギノウ デ セコウ サレタ ドヘキ(コマイ ・ カンイリ)ノ ネツ ブッセイ ソクテイ

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 Tsuchikabe (wall made of a mixture of soil, water, sand, and straw) made of natural materials does not induce sick building syndrome. While being built, tsuchikabe requires no VOC-containing materials. A building with tsuchikabe produces fewer CO2 emissions. It therefore holds promise for reducing LCC (CO2). Buildings with tsuchikabe, however, are on the decrease because of the long work period and high labor costs. The authors intend to verify whether the indoor environmental control performance of the tsuchikabe, which uses natural materials, is worth its building price. To begin with, the thermal insulation performance of the tsuchikabe (composite material) built by a plasterer using traditional skills was measured as reported in this paper.<br> Two types of specimens were used: one measuring 900 mm on each side that allows measurement of equivalent thermal conductivity with the calibrated hot box method (JIS A1420 Annex B) (hereinafter Specimen A) and the other measuring 300 mm on each side that allows measurement of direct thermal conductivity with the heat flow meter method (JIS A1412-2) (hereinafter Specimen B). Specimen A has the same shape and size as the actual material. Specimen A was prepared over a foundation of two nogs, six bamboo laths, and 28 wattles, with plasterers applying a covering of ara tsuchi (a mixture of paddy field earth, rice straw and water) (55 mm in thickness) and a browning coat of nakanuri tsuchi (mixture of river sand, mountain earth, water and rice straw) (both sides: 10 mm in thickness). Specimen B was prepared over a foundation of 12 wattles and a browning coat of nakanuri tsuchi (60 mm in thickness). As it turned out in our case, the interval of wattles used in the specimens ranged from 22 to 24 mm. This spacing is smaller than that of the standard building procedure.<br> The authors confirmed the occurrence of dry shrinkage of tsuchikabe as reported in this paper. The volumetric shrinkage (average) under the steady state (air dry condition) is 4.7% for Specimen A and 5.5% for Specimen B. Plasterers generally estimate dry shrinkage of tsuchikabe prior to building one. The equivalent thermal conductivity of Specimen A as a composite material was 0.392 W/m·K. The thermal conductivity of Specimen B was 0.344 W/m·K. The equivalent thermal conductivity of Specimen A was estimated using the thermal conductivity of a single material (wattle, earth, and nog (cedar)). The thermal conductivity of the wattle part (wattles and earth) is 0.394 W/m·K, and the thermal conductivity of the nog part (wattles, earth and nogs) is 0.336 W/m·K. The authors consider the difference between both measurement values is attributable to nogs. Assuming the wattle part and the nog part conduct heat in parallel, the apparent thermal conductivity of the composite material can explain the measurement value at a sufficient accuracy.<br> The thermophysical property of tsuchikabe built by a plasterer was measured as reported in this paper. The thermal insulation performance of the tsuchikabe is no larger than that of other building materials when compared in terms of equivalent thermal conductivity. However, analysis confirmed that sufficient thermal insulation performance is duly expected to occur because of tsuchikabe's thermal capacity. When evaluation of the humidity adjusting performance of the wall progresses, it will be able to evaluate the indoor environmental control performance and the validity of the construction price.

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