Your greenhouse should be great for humidity. Every couple of weeks seems like not enough for a fern that likes to stay moist, though. I'd go for lighter waterings more often, for that guy, I think.
Try hefting the pot to see if it's gotten light, after a few days. With all this super warm weather, that plant still thinks it's summer, maybe. It also may have dried out to the point that the potting medium doesn't get entirely wet when you do water it. Have you soaked it in a bucket recently? Again, lifting the pot will tell you if the medium has dried out to a 'brick'.
