► SiN film hydrogen permeability at 400°C was determined as P=1×10−17molH2/msPa0.5. ► We presumably report the first quantitative data on H permeation through SiN. ► Permeation reduction factor of 2000 was achieved with only 500nm thick SiN layer. ► High SiN barrier efficiency was found only when H is incorporated in SiN forming strong bonds. ► SiN containing ∼7at.% H is ∼100 times less permeable than the film without H. Amorphous silicon nitride films, 500 and 700nm thick, were deposited on Eurofer substrates by applying reactive radio-frequency magnetron sputtering from pure Si targets in an argon/nitrogen atmosphere. The hydrogen permeation through such double-layered, 40mm diameter membranes at 400°C and 1bar upstream pressure involved the use of a conventional technique with enhanced sensitivity. The extremely high barrier efficiency for these films with respect to hydrogen, expressed as a permeation-reduction factor in excess of 2000, was only achieved with films containing 6–7at.% of hydrogen. The achieved permeation-reduction factor at 400°C corresponds to the permeability of silicon nitride, which is as low as P=1×10−17molH2/msPa0.5. The hydrogen concentration was determined with an Elastic Recoil Detection Analysis, which indicated that this high concentration represents only the strongly bound hydrogen that is not mobile at this low temperature, but impedes the mobility of the diffusive hydrogen. A silicon nitride film with a low hydrogen content is a far less efficient barrier, which supports the role of the strongly bound hydrogen.