A Problem with the Humidity

I have my boy BCI in a big enclosure 7,20 long, 2,60 deep, 4,60 high. It s hard to maintain a high humidity in such a big enclosure specially in winter. I have the humidity around 45%. This won,t cause health problems. What would cause respiratory problems is that if ventilation is not good and the air got stagnated. I do raise the humidity to 70% though when he is shedding. I achieve that through misting several times a day and reducing the ventilation in the last 2 to 3 days of the shedding process. I have also live plants in the enclosure. I have them in plastic pots hanging not on the floor so my BCI won,t crush them