Abstract: A method of reducing heat transmission through a window in a heated building, in a direction from the interior of the building to the building surroundings, while simultaneously maintaining complete visibility through the window both in, as well as against said direction, characterized in that a plastics film is applied to a window pane, said film being substantially fully transparent for visible light and which has substantially reduced transparency for a wavelength range corresponding to the heat radiation from the interior of the building. The film may comprise or consist of a polyethylene terephthalate film. Possibly a transparent polyvinyl fluoride film is also arranged in the window. Alternatively, a compound film comprising a polyethylene terephthalate film and a polyvinyl fluoride film is applied to the pane. Preferably the polyethylene terephthalate film is applied nearer to the interior of the building than the polyvinyl fluoride film.