Aluminium-TiH2 powder mixtures have been cold compacted into precursors for foaming using uni-axial pressing. After heat treatment, foaming was only observed for precursors with a % of theoretical density above 94%. This density limit corresponds to the transition from interconnected to closed porosity, confirming that isolating porosity and enveloping the foaming agent in the matrix are vital to successful foaming. In Al-TiH2 samples compacted to low density, oxygen ingress into the structure occurs leading to reaction with TiH2 and additionally, oxidation of the Al powder. On melting, the molten Al fails to envelop the TiH2 particles and as the TiH2 decomposes the gas escapes through the network of connected porosity. By compacting Al-TiH2 samples to high density the porosity becomes isolated, preventing the exposure of TiH2 to oxygen in the environment and hindering the passage of the evolved gas from the compact. On melting, a continuous liquid is formed which envelops the TiH2 particles and facilitates foam evolution.