Sheathing panels are produced by methods which do not require natural resources such as wood and use significantly reduced embodied energy when compared with the energy used to fabricate gypsum sheathing panels. A novel binder, consisting in one embodiment of monopotassium phosphate and magnesium oxide, and combined with various fillers, is used to provide a controlled exothermic reaction to create a gypsum board-like core which can be formed into a suitable sheathing panel handled and installed in a typical manner. The panel is manufactured to have a desirable shear resistance and water vapor permeability, important performance elements in building envelope design. The manufacturing process results in a panel that does not require mature trees as source material, does not off gas, and involves much lower greenhouse gas emissions than the processes used to make traditional wood or gypsum-based sheathing panels.