A sealed package is disclosed for a self-heating assembly that produces heat by virtue of a liquid reactant reacting exothermically with a solid reactant to heat a product. The sealed package includes an outer container, a lower support structure (e.g., of open cell foam) within the outer container, a layer of the solid reactant above the lower support structure and within the outer container, an upper structure (e.g., of open cell foam, as well) above the solid reactant and within the outer container, and an upper layer that is permeable to liquid and that extends across an otherwise open top of the outer container above the upper structure.