Current research in building insulation is primarily focused on enhancing the performance of polyurethane foam or exploring alternatives with biobased materials, with particular attention placed on polyols. This study investigated the use of modified cellulose filaments as a polyol to enhance the environmental performance of polyurethane foam. Two distinct etherification methods were employed to modify cellulose filaments (CFs), yielding accessible and reactive ether functions from the hydroxyl (OH) groups of CFs. Polyols and the resulting polyurethane foam were characterized. Kinetics of foam formation, morphology, density, thermal conductivity, and mechanical properties in compression were studied. Analysis revealed reduced reactivity with modified CFs as a substitute of the petroleum-based polyol, affecting foaming. Impacts on the resulting properties of the foam were observed, such as the size of the cells (from 0.14 ± 0.06 mm