Whole house humidifier when furnace is in unconditioned space
It does not need to be installed in the unconditioned space near the furnace. It should be in the duct that feeds all the others. Look for a powered humidifier. It will need a 120V outlet, water supply, water drain and it will need low voltage wires connecting it to the humidistat and the furnace. If the duct that feeds all the branch ducts is in an unconditioned space then you may have an issue.