I will preface this by saying that I'm certainly much less experienced at this orchid thing than you.
catts like to dry out between waterings at least some. I don't really know if sphagnum is the best choice for catts...all the ones I've seen in stores have been in bark mixes. two of mine are in s/h, the other one is in bark mix (until repotting time when it is going to make the move to s/h).
you might want to check out this thread:
click here