Fractional difference of the mean thermal dust SEDs as derived by cross-correlation with the GASS H i survey data at high Galactic latitudes, updated with the latest Planck 2015 temperature sky maps, (Planck Collaboration Int. XVII 2014) and by Commander in this paper. The dotted horizontal lines indicate fractional differences of ± 10 %. For comparison purposes, we also show the extrapolation to the 100, 140, and 240μm DIRBE frequencies. These observations are not included in the fits performed in this paper; see Sect. 5.5.2 for further discussion.