Evaluating the phonic conductivity of glasses is difficult because of the combined mode of heat transfer. In this work, a model is developed for the flash experiment, and numerical results are reported. Simulations in the non-gray case have shown that heat transfer in the sample is completely free from any radiative contribution for conditions of small optical thicknesses and reflecting walls. Thus diffusivity identified from the rear-face thermogram is a quantity of phonic origin. Experimental values for phonic conductivity are presented. The results for float glass are in agreement with literature data. For silica, a new result has been obtained : the temperature dependence of the phonic conductivity is found to be similar to that of its specific heat, confirming microscopic models of thermal conductivity in disordered systems.