The U-value (formerly known as the k-value) is the most important value for the thermal assessment of building components. With the new testo 635, it has never been easier to determine this value. To calculate the U-value, three temperature readings are required: the outside temperature, the surface temperature of the interior wall, and the room air temperature. With the new wireless probes, the outside temperature can be quickly and conveniently measured with the window closed. The probe is simply positioned outside and transmits the measurement data wirelessly to the device inside. With the new patented U-value probe, the other two required temperatures can be determined with just one sensor. To measure the surface temperature, the three wires of the U-value probe are attached to the interior wall using modeling clay. The air temperature is measured via a sensor at the probe connector. The three required temperatures are obtained through the connected temperature sensors and transmitted to the testo 635. The device then calculates the U-value and displays it directly on the screen.