- Abstract One of the uncertainties in the energy balance of buildings is associated with the envelope thermal transmittance which, based on blueprints, can be determined from handbooks and engineering calculations. The calculated value is of questionable accuracy due to variable construction practices, uncertainties in material properties, and sometimes even different interpretations of the blueprints. Thermal bridges and multidimensional heat flows are usually unaccounted for and add to the uncertainty. Therefore, it is sometimes necessary to determine the thermal transmittance experimentally. Electric co-heating, i.e., the replacement of the heating system by electric heaters, is a known experimental procedure for the determination of the overall thermal transmittance (losiness) of single-family buildings. However, for multifamily buildings this procedure is impractical due to the buildings's size and complexity. Instead, an equivalent procedure is suggested which permits the determination of an effective envelope thermal transmittance. Its calculation is based on a steady-state heat balance, averaged over many days, for a sample apartment of the building. Since apartments in multifamily buildings are usually not well thermally isolated from each other, heat transfer between the sample apartment and its neighbors has to be accounted for. This is solved by measuring the temperatures in the neighboring apartments and calculating the respective heat flows. To minimize the error introduced by the uncertainty in such heat flows, the average temperature in the sample apartment is controlled, with electric heaters, to be at a temperature that will, at least nominally, cancel the heat flows from the neighboring spaces. The procedure was tested in an apartment of a 60-unit multifamily building.