1. Steam heating means comprising (i) a heating assembly comprising an exothermic layer, an exothermic composition having a water-retaining sheet, and wherein said exothermic layer and said water-retaining sheet arranged in layers, wherein said exothermic composition comprises oxidizable metal, water and a water-retaining agent, wherein said water-retaining sheet comprises a polymer and (ii) cover, wherein the cover is an at least partially air-permeable and able to contain said heater assembly, wherein (A) the water content in the vapor heating means is equal to or more than 40 wt. h. and equal to or less than 80 wt. h. to 100 wt. h oxidizable metal. (B) the content of said water-retaining agent in the exothermic composition is equal to or higher than 0.3 wt. h. and equal to or less than 20 wt. h. to 100 wt. h of said oxidizable metal. (C) water content of said exothermic layer is equal to or higher than 8 wt. h. and equal to or less than 45 wt. h. to 100 wt. h. said oxidizable metal and (D) the water content of said water-retaining sheet is from 15 to 30% by weight of the maximum water absorption The water retention listom.2. Steam heating means according to Claim. 1, wherein said water-retaining agent comprises one, two or more components of the carbon fibrous materials, water-absorbing polymers and water-absorbing poroshka.3. Steam heating means according to Claim. 1 or 2, wherein said water-retaining agent comprises a carbon component, and carbon content of said component in said water retaining agent is equal to or more than 90% by masse.4. steam heating1. Паровое нагревательное средство, содержащее (i) нагревательный узел, содержащий экзотермический слой, имеющий экзотермическую композицию и водоудерживающий лист, в котором указанный экзотермический слой и указанный водоудерживающий лист расположены слоями, при этом указанная экзотермическая композиция содержит окисляемый металл, воду и водоудерживающий агент, при этом указанный водоудерживающий лист содержит
