Confidence in the validity of heat flux transducer (HFT) measurements is sufficiently high that ASTM is preparing a standard practice for the use of HFTs on buildings. A key issue the standard practice will address is how to adjust the calibration of the HFT to the thermal environment of the measurement. Confidence in the use of HFTs is based in part on a propagation of errors analysis of key thermal influences on the accuracy of measurement. The user can expect the HFT to render a standard deviation of ±10% of the heat flux measured. Field measurements confirm this expectation. However, the variety of heat flux mechanisms inherent in building construction requires that the investigator choose the measuring situation carefully. Convection, even in “fully insulated” spaces, can cause unexpected lateral heat flux and results that are difficult to interpret. More work should be done with HFTs to investigate convection in walls and attics, as well as to investigate other lateral heat flux transfer mechanisms.