AliExpress: 3.5$

Lib: https://github.com/adafruit/Adafruit_Si7021

Demo: https://github.com/adafruit/Adafruit_Si7021/blob/master/examples/si7021/si7021.ino

Datasheet: https://www.silabs.com/documents/public/data-sheets/Si7021-A20.pdf

Sensors test examples: 

Test 1 with SD datalogging 2 mux 20 hours

Test 4 v5 High Humidity

Test 5 v5 saturated salt solution

Test 1 v6 balcony RH47-RH50

Test 4 v6 Saturated NaCl solution

Test 5 v6 above water

Test 2 v7 room RH12 low humidity

Test 3 v7 balcony RH15-92 changing humidity

Test 4 v8 RH12 to RH94#Si7021vsSHT85SensorsHumidity

Test 1 v8b Balcony high to mid RH

Test 1 v9 humidity absorber draft

Test 2 v9 steam in bathroom