Sol–gel coatings containing diethylphosphite can efficiently protect polyurethane (PU) foam from fire. However, diethylphosphite can cause an allergic skin reaction and produce toxic fumes during combustion. The purpose of this study was to screen alternative additives to replace diethylphosphite. The screened flame-retardant molecules were phosphorus donors of low toxicity compared with halogenated compounds, such as resorcinol bis(diphenyl phosphate) (RDP) and bisphenol A bis(diphenyl phosphate), and bio-based additives with potential flame-retardant functional groups such as chitosan, adenosine triphosphate, cysteine and tyrosine. UL 94 and mass-loss calorimetry tests were used to examine the reaction to fire of the different formulations, and the most promising formulations were characterized using scanning electron microscopy, strain–stress tests and thermogravimetric analyses coupled with Fourier transform infrared spectrometry. It was observed that RDP is as effective as diethylphosphite in producing flame-retardant coatings. Chitosan and cysteine can also be used, but they need to be mixed with RDP to provide acceptable flame-retardant properties. Mechanical tests showed that the addition of chitosan and cysteine in the coatings increased the Young’s modulus of the PU foam and decreased the tensile strength.