You're probably going to have a hard time maintaining humidity because of your location.

You could try running a humidifier in the room the cage is in. (I would suggest a warm mist style unit)

What are you using to heat the cage? What is the temperature of the room that the cage is in? What is the lowest temperature the room gets during an average year?

What side of the cage is your hygrometer on? (hot side/cool side)