Polyurethane foams deteriorate rapidly; the effects of degradation can appear after 20-30 years. Conservation issues mainly related to the loss of polyurethane foam mechanical properties frequently affect museum artifacts. Many studies dealt with degradation of polyurethane, but no one correlated the polyurethane chemical changes with the mechanical properties loss of the foam structure. In order to find a degradation indicator permitting to obtain simultaneously information on the chemical and mechanical condition of the foam with a non-invasive method, we performed accelerated degradation at different relative humidity conditions. Chemical modifications have been observed by Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). Differential Scanning Calorimetry (DSC) has been used to follow polyurethane glass transition temperature variations. Mechanical properties of degraded and undegraded samples have been studied by Compression Force Deflection Test (CFDT) and Scanning Electron Microscopy (SEM) allowed visualizing foam surface modifications during the entire degradation period. ATR-FTIR spectra analysis of artificially degraded samples allowed identifying the ester/hydroxyl band ratio as a good indicator of polyurethane ester foam degradation. A correspondence was found between the decrease of the ester/hydroxyl ratio value and the loss of elasticity of the foam structure. The reliability of this indicator (ester/hydroxyl band ratio) has been validated by ATR-FTIR analysis performed on naturally degraded TDI based polyurethane ester foams.