Abstract: A heat exchanger for ventilating insulated dwellings with minimum internal heat loss, is disclosed. The heat exchanger is characterized by its very low manufacturing cost, its excellent efficiency and its minimum and easy maintenance. The heat exchanger consists of an elongated box-like casing forming two opposite end chambers interconnected by a plurality of longitudinally-extending, thin walled glass tubes. Warm stale and humid air is expelled through the glass tubes from one to the other chamber, while cool outdoor drier air is circulated within the casing along a generally sinusoidal path about the glass tubes, so that a heat transfer occurs through the walls of the glass tubes. The casing is made of molded foam plastic for good heat insulation and in two half-parts having inner edges releasably and sealingly joined by a tongue-and-groove arrangement. Each half-part has a tongue and a groove, each extending along half the periphery of its inner edge, so that the half-parts can be made in the same mold.