Fire retardant foam systems comprise a first part comprising at least one ingredient having NCO functionality; and a second part comprising at least one ingredient having an active hydrogen functionality that is co-reactive with the NCO, wherein the first part and the second part are formulated so that when the parts are mixed together they form a cured foam. The foam system comprises a first fire retardant ingredient that is a phosphorus-based compound, a second fire retardant ingredient that is an intumescent material, and a third fire retardant ingredient that is a brominated ingredient additionally having an active hydrogen functionality that is co-reactive with the NCO of the first part. Methods preparing a fire retardant foam comprise providing a fire retardant foam system as described above and mixing the first part and the second part together so that they form a cured foam. The foam made by this process and methods of protecting structures using the present foam system are also provided.