Nowadays, the best thermal insulators are aerogel-based materials. However, their industrial application is constrained by a complex synthesis route (requiring supercritical drying) and by their fragility. Moreover, the most common aerogels, based on amorphous SiO2, have a poor thermal stability. In this work, a fast and simple synthesis route to ultra-highly-insulating Si3N4 nanofelts is developed. The process is based on a specific treatment of SiOC polymer-derived ceramics in N-2 atmosphere. The obtained nanofelts possess porosity as high as 99.7% (10 kg m -3) and thermal conductivity down to 11 mW m(-1) K-1 in Ar atmosphere, which is among the lowest ever measured. The new material surpasses aerogels in terms of flexibility and temperature resistance. Moreover, it can be easily shaped in physical objects of indus trial interest. Thus, it offers a unique combination of intriguing properties such as thermal insulation, lightweight, temperature resistance and flexibility. These, combined with the simple manufacturing process, could lead to far-reaching implications in multiple technological fields. (c) 2020 Elsevier Ltd. All rights reserved.