I think it's okay as long as it doesn't have pine, cedar, or fertilizers. However, I have no idea about Hemlock mulch, so I personally wouldn't take the risk. I would perhaps suggest bigger pieces of coconut husk, I've heard it holds humidity better. However, you can always buy cypress mulch online...
http://www.bigappleherp.com/Zoo-Med-...=2&category=29
You can also make a humidity hide, or a cheap humidity device. Basically, put some water in a plastic cup with a lid on it and cut a hole in the lid, some people do the same but with sphagnum moss. Maybe you could even put some moss around your terrarium and what not. For the humidity device, fill a cup with water and poke a few holes in the top. For both of these, they should be placed on the hot side of the terrarium. You could also invest in a automatic spraying device of sorts.
If you're using a tank with a screen top, cover over 70% of the top to keep humidity. You could also once a week simply pour a few cups of water (depending on how big your terrarium is), and stir it in well. I usually do this if the coconut husk becomes too dry.