You can get a humidifier that would be the best thing you can do to get your humidity high. Make sure also that is one that you can regulate. It will take some trial and error before you find at what rate of your humidifier will you get the adequate humidity. From what I know is that low humidity will make your plants perspire more meaning that you might have K deficiencies in the future. So Just letting you know that.
My plants have K def right now and a one has some Iron def =/ but they are getting better! They developed the problem because they were in around 10-20% humidity for like a bit over a week. Didn't realize that that could also play a part in my problem right now.
So get the humidifier and meanwhile you can sprinkle around the floor doors, all over so that that water can evaporate fast and raise your humidity. Also keep in mind that you will have to do this every hour or two else your humidity levels will drop right back in no time!
hope it helps~