Humidex measures the relationship between temperature and humidity and gives a comfort level score. If you have a high temp but low humidity, your comfort is greater than a high temp and high humidity.

Windchill is a measurement of temperature minus some degrees due to the speed of the wind. The windier it is, the colder it feels. Even though the thermometer says one temp, the wind makes it feel several degrees cooler.

These are two different things so they are not comparable.

I found this link It says that humidex is calculated for temperatures above 80 degrees Fahrenheit, which is about 27 degrees Celsius. Wind chill is done when the temperature is below 50 Fahrenheit, or 10 degrees Celsius. The link also gives linear equations for computing the two values.