if you had to set realative humdity to say 20% for -20 degrees, you would choke from it being to dry.
i Live in michigan, have my humidistat set at 40%, if i go any lower 37-38% me and my wife will wake up with dry throats choking.
My windows condensate pretty bad when the temperature is 15 degree F outside. but i can't go any lower on the humidity cause of the dry throats.
Outside Temperature Inside Humidity
20º to 40ºF Not over 40%
10º to 20ºF Not over 35%
0º to 10ºF Not over 30%
-10º to 0ºF Not over 25%
-20º to –10ºF Not over 20%
-20ºF or below Not over 15%