A solar collector (10) is disclosed which is tiltable about a horizontal axis so as to vary the angle at which solar radiation is received by the collector (10). The solar collector (10), which uses air as the heat transfer medium, has connected to it a pair of fixed, well-insulated air transfer passages (12, 14) which penetrate through into the interior of the collector (10) at the lateral sides thereof aligned with the horizontal axis about which the collector (10) is pivoted. The air transfer passages (12, 14) are insulated and are gasketed to the sides of the collector (10) so as to improve the efficiency of the solar energy system by avoiding losses of heat from the heat transfer fluid during transfer of the fluid from the collector (10) to the space being heated.