We investigate the physical properties of the interstellar gas in the nearby edge-on spiral galaxy NGC 891, using Herschel PACS/SPIRE observations of the most important far-infrared (FIR) cooling lines – [Cii] 158 μm, [Nii] 122, 205 μm, [Oi] 63, 145 μm, and [Oiii] 88 μm – obtained as part of the Very Nearby Galaxy Survey (P. I.: C. D. Wilson). We compare our observations to the predictions of a photo dissociation region (PDR) model to determine the gas density, n, and the strength of the incident FUV radiation field, G0, on a pixel-by-pixel basis. The majority of PDRs in NGC 891's disc exhibit properties similar to the physical conditions found in the spiral arm and inter-arm regions of the face-on M51 galaxy. We estimate a stronger FUV field in the far north-eastern side than compared to the rest of the disc.