i dont have a rack but i did use cypress mulch for a short time last year. i bought a big bag from lowes and then found out that the blends have cedar in 'em.

i was glad to have to do away with it though cuz it really wasnt holding the humidity very well at all. i was having to spray it several times a day just to maintain the normal daily 50-60% humidity, even more to get it up to 60-70% when my girl went into shed. that was way too many intrusions IMHO. i ended up going to a 1/2 mulch, 1/2 newspaper (mulch on the warm side) then finally got to switch it all out when i got the forest bark i'm currently using, which is 100% fir bark.
the rest of the mulch got spread out in my flower garden so i was able to make some use of it.
the fir bark needs to be moistened very well (but i make sure it's not soaking wet) since its in chunks but once moistened like that it holds the humidity for practically 1/2 the day but i also have a very wet (but not dripping) hand towel over 3/4 of the screen on top and 2 water dishes (one of which is directly over the heat pad). so theres a few elements creating the humidity.