Does the humidex rating have an effect on plants? I thought the index was to show how the temp and humidity “feel” to us. Sort of like a wind chill factor for cold.
Anyway. You can add some water in a container to the room or tent to raise. Maybe put a propagation tray with a heating pad under it. I slow my fan down as well to raise but you have to monitor temp.
To lower, I increase exhaust fan speed, don’t leave excess water in the room or trays and crack a door open. I find just turning up the exhaust fan does the trick, though.
There are also products you can buy for short money, however, I’ve never used any. But for example
Hope this helps a bit. I’m sure others will have ideas.