Five series of rigid polyurethane-polyisocyanurate (RPU/PIR) foams were obtained. They were modified by ashes from burning paper (P) and wood: conifers (pine-S, spruce-S') and deciduous trees (oak-D, birch-B). The ash was added to rigid polyurethane-polyisocyanurate foams (PU/PIR). In this way, five series of foams with different ash contents (from 1 to 9% wt.) were obtained: PP, PS, PD, PS', PB. The model foam (reference-W) was obtained without filler. The basic properties, physico-mechanical, and thermal properties of the ashes and obtained foams were examined. It was specified, among other things, the cellular structure by scanning electron microscopy (SEM), and changes in chemical structure by Fourier-transform infrared spectroscopy (FTIR) were compared. The obtained foams were also subjected to thermostating in a circulating air dryer in increased temperature (120 °C) for 48 h. Ash tests showed that their skeletal density is about 2.9 g/cm