The heat capacities in the solid state, below the glass transition, of Bombyx mori silk fibroin with and without water have been determined based on the contribution of vibrational motions of the components: poly(amino acid)s and water. These vibrational heat capacities were constructed using the Advanced Thermal Analysis System (ATHAS) Data Bank. The heat capacities, C-p, of dry silk and silk-water were linked to their vibrational spectra based on the group and skeletal vibration contributions. For dry silk, the experimental and calculated C-p agree to better than +/- 3% between 200 and 435 K. The heat capacity of the solid silk-water system, below the glass transition, was estimated from a sum of linear combinations of the molar fractions of the vibrational heat capacities of dry silk and glassy water. The approach presented allows one to predict the low-temperature vibrational heat capacity for dry silk and for the silk-water system down to 0 K and, together with an extension to higher temperatures, above the glass transition. This can be used as a reference baseline for quantitative thermal analysis of this biomaterial.