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Studies of the spin filter properties of tunnel barriers consisting of the insulating ferromagnets, EuO and EuS have been conducted for many years, but detailed investigation and application of their properties has been restricted by difficulties associated with growth and stoichiometry. We have recently demonstrated a new insulating ferromagnetic material GdN and shown that GdN barriers are strongly spin-filtering and can be incorporated into superconducting tunnel junctions. S/Insulator/S tunnel junctions enable detailed investigation of the magnitude of non-tunnel (leakage) currents and the optimisation of the tunnel barrier. We show that our insulating ferromagnet devices have a very low zero-bias leakage, but that behaviour for higher bias voltages is strongly non-ideal. We will discuss the potential for such devices to be used as spin-sources for both conventional and superconducting spintronics.