The invention relates to a flame-retardant coating which forms an insulating layer and is based on substances which carbonize and which form a foam layer in the event of a fire, on film-forming binders, on blowing agents and on customary auxiliaries and additives, wherein the blowing agent and foam layer forming substance present comprises melamine polyphosphate. The novel flame-retardant coating which forms an insulating layer is particularly stable under tropical conditions (at up to 100% rel. humidity, about 75.degree. C.), i.e. it releases only an extremely small amount of NH.sub.3.