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There exists a building energy performance gap between theoretical simulations and the actual energy usage as measured. One potential reason for this gap might be a mismatch between predicted and measured values of the heat flux q through the building envelope. There is therefore a need to develop accurate and more cost-efficient methods for measurement of q. The standard ISO 9869-1 states that, at the outdoor surface, q = ho(Ts − Tenv), where ho is the overall heat transfer coefficient, including both convective and radiative components, Tenv is the environmental temperature, and Ts is the temperature of the building surface. It has previously been shown that the sol-air thermometer (SAT) could be used for convenient measurement of Tenv under dark conditions. In the present work, two SAT units, one heated and the other unheated, were employed for accurate outdoor measurements of ho in cold winter climate. Validation was performed by comparison of results from the new method against measurements, where previously established methodology was used. With current operating conditions, the measurement uncertainty was estimated to be 3.0 and 4.4%, for ho equal to 13 and 29 Wm−2K−1, respectively. The new SAT steady-state method is more cost-effective compared to previous methodology, in that the former involves fewer input quantities (surface emissivity and infrared radiation temperature are unnecessary) to be measured, while giving the same ho results, without any sacrifice in accuracy. SAT methodology thus enables measurement of both Tenv and ho, which characterizes the building thermal environment, and supports estimation of q.