The MoWiTT calorimeter chambers are surrounded by a guard space through which temperature-controlled air is circulated.

The calorimeter control volume, defined as the inner surface of the chamber, is established by a layer of large-area heat flow sensors, shown at left during calibration of the calorimeters.

Measuring all of the heat flows into the control volume (except the fenestration sample) allows one to determine the dynamic net energy flow through the sample, shown as the measured points above. The calculated curve is derived from the measured air temperatures and incident solar radiation. Energy flow into the chamber is defined as positive. Even though the measurement is made in August in Reno, NV, the nighttime heat loss is clearly apparent, as is the difference in heat gain between a clear and a partly cloudy day. On the latter day variations in solar intensity due to changes in cloud cover are clearly visible in variations of both the calculated curve(which is based on measured temperatures and incident solar intensity) and the measured points.