I live in Canada and it's been very dry, so dry that house and room humidifiers are not cutting it. It would probably be easier and more effective for the snake to have it's own humid hide when needed.
I use one when mine is in shed, simply out of a plastic deli cup with moistened paper towel. You could definately use sphagnum moss isntead. Depending on how much the humidity you need, you can leave the lid off or on (or partway), and the location of the hide in the tank will also have effect (placing the hide on the warm side of the tank of course makes it the most humid). I only use the humid hide when the snake is about to shed. You could mist with a spray bottle to keep the humidity at a more usual percentage if it's very dry. Be sure to keep in mind though that a humid environment is not neccessary for a corn snake, even detrimental to it's health if it's high all the time, so it might be wise to get a reptile hygrometer for monitoring the humidity.
I keep my reptiles in rubbermaid and sterilite containers, so they hold moisture more than something like a glass tank with a mesh top. If you're not using a storage container, you may want to switch as that will also help with the relative humidity in the animal's enclosure. This way even having just the water bowl in the tank should suffice. Alternatively, you could put plexiglass or saran over part of the lid to help hold in moisture.