I have plugged in my new si7020 for the first time, directly into the tessel 2, but it reports a temperature that is very considerably too low. The temperature in the room is currently 23.5 C (about 74 F, measured by another thermometer and my own skin), while the tessel reports about 6 C (43 F). This is clearly way off, and I can be confident that the tessel is not refrigerating the sensor.
Breathing on it repeatedly raises it to 110C (232 F). Both C and F are as reported by tessel, and appear consistent, so the unit conversion appears correct.
Throughout, humidity appears correct (compared with weather forecast).
In the node module, climate-si7020, I see TEMPERATURE_OFFSET and TEMPERATURE_SLOPE. The sensor details are not currently available online, so I can't check the correctness of these values.