The thermal conductivity, k, is the property of a material without reference to its dimensions. In practice, one often knows the material and the dimensions of a wall. for instance In this case it is convenient to use the specification, R-value, which labeled on insulation and other building materials and it represents the en incorporates both the wall thickness and the material The r- value is ofte thermal resistance of a material For any slab of material it is given by r=l/, where l is the thickness of the slab and k is its thermal conductivity. This number can be obtained from standard reference books. The units of R are commonly hr. Fft2/Btu For a wall or any other surface built up of a layered series of material each having thickness 11, I2, I3..and conductivity ki, k2, k3..., the total R-values, Rr=R+R+R+ ++→+ k 复旦大学环境科学与工程系 Department of Environmental Science and Engineering, Fudan UniversityThe thermal conductivity, k, is the property of a material without reference to its dimensions. In practice, one often knows the material and the dimensions of a wall, for instance. In this case it is convenient to use the specification, R-value, which incorporates both the wall thickness and the material. The R-value is often labeled on insulation and other building materials and it represents the thermal resistance of a material. For any slab of material it is given by R=l/k, where l is the thickness of the slab and k is its thermal conductivity. This number can be obtained from standard reference books. The units of R are commonly hr· ̊F·ft2/Btu For a wall or any other surface built up of a layered series of material each having thickness l1, l2 , l3 … and conductivity k1, k2 , k3 … , the total R-values, or RT = R1+R2+R3+… ... 3 3 2 2 1 1 = + + + k l k l k l