The Thermal Resistance of Mineral Wool Products with Density Gradients: Theory and Experimental Procedures

Some standardized experimental procedures for the characterization of most common homogeneous insulating materials, in particular, low-density high- thickness mineral wool products, are based on some analytical models (two-flux model) which take into account combined heat transfer by conduction and radiation in homogeneous media. Interlaboratory comparisons and experimental validation of the models during some years has now covered most commercial products and proved that agreement is far better than testing accuracy. However, the above proce dures can no longer be applied when a density gradient occurs along the thickness of the specimen and hence a gradient of the radiative extinction coefficients is orig inated. The gradient occurs due to the weight of the upper layers of the product on the lower layers during binder polymerization. The system of differential equations which described the above models was therefore improved to take this effect into ac count. The solution was only possible by splitting the insulation into three layers, two facing the bounding surfaces of the product and a third forming a core. The solution is then used as the interpolating function of measured data in a procedure to characterize mineral wool products with density gradients, exceeding the maximum specimen thickness for the apparatus to be used, and such that the homogeneity assumption of standard models does not supply acceptable accuracy levels.