I would start by using cardboard to block and tape off the area of the screen...assuming you have the typical glass terrarium thing going on. Cover about 90% of the screens area, allowing some area for air to go in and out. Don't cover it all the way.
Next, try adding another source of water in the cage, i.e a bigger water bowl, another bowl of water, or some moisture releasing substance like moss or perlite/vermiculite.
And if you don't have one already, get a humidity gauge.