Mobile air conditioners produce warm exhaust air, which have to be outward blown off over an exhaust air hose. With mobile adoption and / or in a flat usually only the possibility remains of blowing the warm exhaust air by an open or tilted window. Thus the cooling becomes however uneconomic and uncomfortable, because warm air penetrates from the outside again and again into the cooled area. HotAirStop is a water-repellent, sturdy special foil with inserted zipper, which is installed by means of self adhesive hook and loop strips at windows and window frameworks. The plug of the exhaust air hose can be problem-free put by the small opened zipper and the current reversal of warm exhaust air from the outside is prevented. As side effect it does not keep away also from mosquitoes and other unwanted insects, cats can not get jammed in tilted windows any more. HotAirStop is also useable for vented dryers, both in the warm and in the cold season. Closing the windows is through removing the exhaust air hose and folding in the foil or complete removing from HotAirStop at any time possible. HotAirStop is suitable for all usual window sizes, whose entire rotating measure does not exceed 400 cm. The applicable window size results for example out: 1x window height left + 1x window height right + 1x window width. HotAirStop is washable at max. 40°C with mild detergent.