Field measurements of 21 volatile organic compounds (VOC) using diffusive samplers, formaldehyde, temperature, and humidity were performed from the time of building completion throughout the following one-year period in two new semi-detached twin apartments. One of these was occupied after six weeks. Headspace analyses from all building materials and products showed 120 different VOC. Formaldehyde concentrations were strongly seasonally dependent in the vacant apartment and increased to above 400 μg/m3 during the warm season. The formaldehyde concentration generally decreased in the occupied apartment but increased again during the fall season. VOC originating from building materials generally showed a decrease in emission, but strong seasonal variations were observed. It was shown that human activity introduces several VOC to the indoor environment. Storage of motorcycle parts in the crawl space of the occupied apartment resulted in migration and an infiltration of benzene and toluene into the apartment above and probably to a delayed peak concentration in the twin vacant apartment. Similarly, large VOC increases in one apartment were reflected by a later increase of the same VOC in the twin apartment. Hexanal increased during the warm season. TVOC, as the sum of 21 VOC, was generally approximately 50 % higher in the occupied apartment during the cold season. The results indicate the difficulties in interpreting long-term measurements. The “flushing period” recommended for this type of building has been estimated to be about 130 days.