Abstract: A heat exchanger made of plastic has flow channels for guiding two fluids countercurrently. The heat exchanger includes a cover part; an intermediate layer that at least partially forms the flow channels; and a trough part. The intermediate layer comprises (1) a flow channel for a first fluid comprising channel sections that are spaced apart from one another and (2) a second flow channel comprising opposite flow sections on back of the channel sections for a second fluid guided in an opposite direction from the first fluid. The wall areas that separate the channel sections and opposite flow sections are heat transfer membranes. The cover part and the trough part are connected tightly with the intermediate part, and the intermediate layer is a single piece. The heat exchanger can be used for room ventilation with heat recovery or air conditioning.