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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Thermal transmittance ： ウィキペディア英語版 Thermal transmittance Thermal transmittance, also known as U-value, is the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure. It is expressed in watts per metres squared kelvin, or W/m²K. Well-insulated parts of a building have a low thermal transmittance whereas poorly insulated parts of a building have a high thermal transmittance. Losses due to thermal radiation, thermal convection and thermal conduction are taken into account in the U-value. Although it has the same units as heat transfer coefficient, thermal transmittance is different in that the heat transfer coefficient is used to soley describe heat transfer in fluids while thermal transmittance is used to simplify an equation that has several different forms of thermal resistances. It is described by the equation: :Φ = A × U × (T1 - T2) where Φ is the heat transfer in watts, U is the thermal transmittance, T1 is the temperature on one side of the structure, T2 is the temperature on the other side of the structure and A is the area in square metres. Thermal transmittances of most walls and roofs can be calculated using ISO 6946, unless there is metal bridging the insulation in which case it can be calculated using ISO 10211. For most ground floors it can be calculated using ISO 13370. For most windows the thermal transmittance can be calculated using ISO 10077 or ISO 15099. ISO 9869 describes how to measure the thermal transmittance of a structure experimentally. *triple glazed windows, with advanced coatings and frames: 0.8 W/m²·K;〔Passivhaus Institute's thermal testing results for Rehau Geneo 'PHZ' triple glazed window ()〕 *well-insulated roofs: 0.15 W/m²·K; *poorly insulated roofs: 1.0 W/m²·K; *well-insulated walls: 0.25 W/m²·K; *poorly insulated walls: 1.5 W/m²·K; *well-insulated floors: 0.2 W/m²·K; *poorly insulated floors: 1.0 W/m²·K; In practice the thermal transmittance is strongly affected by the quality of workmanship and if insulation is fitted poorly, the thermal transmittance can be considerably higher than if insulation is fitted well〔Field investigations of the thermal performance (U-values) of construction elements as built ()〕 ==Calculating thermal transmittance== When calculating a thermal transmittance it is helpful to consider the building's construction in terms of its different layers. For instance a cavity wall might be described as in the following table: In this example the total resistance is 1.64 K·m²/W. The thermal transmittance of the structure is the reciprocal of the total thermal resistance. The thermal transmittance of this structure is therefore 0.61 W/m²·K. (Note that this example is simplified as it does not take into account any metal connectors, air gaps interrupting the insulation or mortar joints between the bricks and concrete blocks.) It is possible to allow for mortar joints in calculating the thermal transmittance of a wall, as in the following table. Since the mortar joints allow heat to pass more easily than the light concrete blocks the mortar is said to "bridge" the light concrete blocks. The average thermal resistance of the "bridged" layer depends upon the fraction of the area taken up by the mortar in comparison with the fraction of the area taken up by the light concrete blocks. To calculate thermal transmittance when there are "bridging" mortar joints it is necessary to calculate two quantities, known as "Rmax" and "Rmin". Rmax can be thought of as the total thermal resistance obtained if it is assumed that there is no lateral flow of heat and Rmin can be thought of as the total thermal resistance obtained if it is assumed that there is no resistance to the lateral flow of heat. The U-value of the above construction is approximately equal to 2 / (Rmax + Rmin) Further information about how to deal with "bridging" is given in ISO 6946. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Thermal transmittance」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.