A method for sealing a door or window opening of a building under construction. A door or window is mounted in the door or window opening so as to seal the opening during a further construction phase of the building. The door or window is the component which is actually intended for permanent installation in the door or window opening. Further, a removable knockproof panel, such as a transparent or semi-transparent polymeric panel, is mounted in front of the door or window, such as covering the entire front of the door or window. Hereby, the door or window front is protected in the building construction phase. This eliminates the need for interim sealing solutions using wooden structures and plastic foils and the like. Compared to existing methods, energy is saved to heating of the interior of the building during the construction phase, because the high thermal insulation effect of the permanent door or window is provided at an early stage. The knockproof panel may be a transparent 2-20 mm thick polycarbonatic plate. One or more further panels may be arranged to protect sides of the door or window. A combined fitting means may be used to both mount the door or window in the opening, and to fasten the knockproof panel in a removable way so as to allow easy removal from the knockproof panels. This allows the protective panels to be mounted by the door or window manufacture for easy installation at the building site.