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The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area per unit time, ) through it or . Thermal resistance varies with temperature but it is common practice in construction to treat it as a constant value. An R-value is a unit of thermal resistance for a particular material or assembly of materials (such as an insulation panel). The R-value depends on a solid material's resistance to conductive heat transfer. For loose or porous material, the R-value accounts for convective and radiative heat transfer through the material. However it does not account for the radiative or convective properties of the material's surface, which may be an important factor for some applications. R is expressed as the thickness of the material normalized to the thermal conductivity. The unit thermal conductance of a material is the reciprocal of the unit thermal resistance. This can also be called the unit surface conductance.〔McQuiston, Parker, Spitler. Heating, Ventilation, and Air Conditioning: Analysis and Design, Sixth Edition. Hoboken NJ: John Wiley and Sons Inc., 2005.〕 The higher the value of R, the better the building insulation's theoretical effectiveness. R-value is the reciprocal of U-factor. ==U-factor/U-Value== The ''U-factor'' or ''U-value'', is the overall heat transfer coefficient that describes how well a building element conducts heat or the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure.〔(【引用サイトリンク】title=U-Value Measurement Case Study )〕 The elements are commonly assemblies of many layers of components such as those that make up walls/floors/roofs etc. It measures the rate of heat transfer through a building element over a given area under standardised conditions. The usual standard is at a temperature gradient of 24 °C (75 °F), at 50% humidity with no wind〔(P2000 Insulation System ), (R-value Testing )〕 (a smaller ''U-factor'' is better at reducing heat transfer). It is expressed in watts per meter squared kelvin ( W/m²K ). This means that the higher the U value the worse the thermal performance of the building envelope. A low U value usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behavior of an entire building element rather than relying on the properties of individual materials. In much of the world the properties of specific materials (such as insulation) are indicated by the thermal conductivity, sometimes called a k-value or lambda-value (lowercase λ). The thermal conductivity (K-value) is the ability of a material to conduct heat ; hence, The lower the K-value, the better the material is for insulation. Expanded polystyrene (EPS), has a K-value of around 0.033 W/m-K.〔(Polystyrene insulation )〕 For comparison, phenolic foam insulation has a K-value of around 0.018 W/m-K,〔(European phenolic foam association: Properties of phenolic foam )〕 while wood varies anywhere from 0.15- 0.75 W/m-K and steel has a K-Value of approximately 50.0 W/m-K. These figures vary from product to product, so the UK & EU have established a 90/90 standard which means that 90% of the product will conform to the stated k value with a 90% confidence level so long as the figure quoted is stated as the 90/90 lambda value. ''U'' is the inverse of ''R'' with SI units of W/(m2K) and US units of BTU/(h °F ft2); :: where ''k'' is the material's thermal conductivity and ''L'' is its thickness. See also: tog (unit) or Thermal Overall Grade (where 1 tog = 0.1 m2·K/W), used for duvet rating. Note that the phrase "U-Factor" (which redirects here) is used in the US to express the insulation value of windows only, R-value is used for insulation in most other parts of the building envelope (walls, floors, roofs). Other areas of the world generally use U-Value/U-Factor for elements of the entire building envelope including windows, doors, walls, roof, and ground slabs. 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「The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area per unit time, \dot Q_A) through it or R = \Delta T/\dot Q_A. Thermal resistance varies with temperature but it is common practice in construction to treat it as a constant value.An R-value is a unit of thermal resistance for a particular material or assembly of materials (such as an insulation panel). The R-value depends on a solid material's resistance to conductive heat transfer. For loose or porous material, the R-value accounts for convective and radiative heat transfer through the material. However it does not account for the radiative or convective properties of the material's surface, which may be an important factor for some applications.R is expressed as the thickness of the material normalized to the thermal conductivity. The unit thermal conductance of a material is the reciprocal of the unit thermal resistance. This can also be called the unit surface conductance.McQuiston, Parker, Spitler. Heating, Ventilation, and Air Conditioning: Analysis and Design, Sixth Edition. Hoboken NJ: John Wiley and Sons Inc., 2005. The higher the value of R, the better the building insulation's theoretical effectiveness.R-value is the reciprocal of U-factor.==U-factor/U-Value==The ''U-factor'' or ''U-value'', is the overall heat transfer coefficient that describes how well a building element conducts heat or the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure.(【引用サイトリンク】title=U-Value Measurement Case Study ) The elements are commonly assemblies of many layers of components such as those that make up walls/floors/roofs etc. It measures the rate of heat transfer through a building element over a given area under standardised conditions. The usual standard is at a temperature gradient of 24 °C (75 °F), at 50% humidity with no wind(P2000 Insulation System ), (R-value Testing ) (a smaller ''U-factor'' is better at reducing heat transfer). It is expressed in watts per meter squared kelvin ( W/m²K ). This means that the higher the U value the worse the thermal performance of the building envelope. A low U value usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behavior of an entire building element rather than relying on the properties of individual materials.In much of the world the properties of specific materials (such as insulation) are indicated by the thermal conductivity, sometimes called a k-value or lambda-value (lowercase λ). The thermal conductivity (K-value) is the ability of a material to conduct heat ; hence, The lower the K-value, the better the material is for insulation. Expanded polystyrene (EPS), has a K-value of around 0.033 W/m-K.(Polystyrene insulation ) For comparison, phenolic foam insulation has a K-value of around 0.018 W/m-K,(European phenolic foam association: Properties of phenolic foam ) while wood varies anywhere from 0.15- 0.75 W/m-K and steel has a K-Value of approximately 50.0 W/m-K. These figures vary from product to product, so the UK & EU have established a 90/90 standard which means that 90% of the product will conform to the stated k value with a 90% confidence level so long as the figure quoted is stated as the 90/90 lambda value.''U'' is the inverse of ''R'' with SI units of W/(m2K) and US units of BTU/(h °F ft2);:: U=\frac=\frac=\fracwhere ''k'' is the material's thermal conductivity and ''L'' is its thickness.See also: tog (unit) or Thermal Overall Grade (where 1 tog = 0.1 m2·K/W), used for duvet rating.Note that the phrase "U-Factor" (which redirects here) is used in the US to express the insulation value of windows only, R-value is used for insulation in most other parts of the building envelope (walls, floors, roofs). Other areas of the world generally use U-Value/U-Factor for elements of the entire building envelope including windows, doors, walls, roof, and ground slabs.」の詳細全文を読む スポンサード リンク
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