A two-component on-site foam system for producing an intumescing fire protection foam is described with a density of less than 200 kg/m3 and an increased fire resistance endurance, with a polyol component (A), which contains at least one polyol, one catalyst for the reaction between the polyol and the polyisocyanate, water or a blowing agent based on a compressed or liquefied gas as foam-forming agent and at least one intumescing material based on an acid-forming agent, a carbon-supplying compound and a gas-forming agent, and a polyisocyanate component (B), which contains at least one polyisocyanate, wherein the polyol component (A) contains at least one polyester polyol, at least one aminopolyol, at least one halogen-containing polyol, at least one acid-forming agent, expanding graphite and at least one ash crust stabilizer, the quantitative ratios of the polyols to the polyisocyanate or polyisocyanates being matched so that, when the polyol component (A) is mixed with the polyisocyanate component (B) as specified, the molar ratio of isocyanate groups of the polyisocyanate to the hydroxyl groups of the polyol (NCO:OH ratio) is larger than 1:1, as well as to the use of this system for filling openings, cable and pipe leadthroughs in walls, floors and/or ceilings of buildings with foam for the purpose of fire protection.