I use MH's in my greenhouse which reaches about 80% humidity at times, and over open water on aquariums with no problems whatsoever. But you should always use a GFCI outlet when electricity is going to be used in a potentially wet environment. The GFCI will instantly cut the power if there is a short. I once dropped a lit fluorescent light into an aquarium and all I heard was a click from the GFCI. It scared me, but I was glad I had that GFCI.

I'd say the real concern is whether your tent can handle the heat generated by a 400w bulb. I'm assuming you'll have to ventilate, but then you might not have the humidity you're talking about.

Thanks, penfold. My chamber is 5 ft. x 5 ft. x 6.5 ft. high. I don't know how much heat will be generated, but I wasnt planning on ventilation. I think I'll try the setup and see how much it heats up, before moving any plants inside.

Any notes on what kind of heat you get out of an MH lamp in confined spaces? I was thinking of doing this outside where the ambient temps are very cold. I am not sure if I'm going to be having to do supplemental heat, or cool, or possibly both. I know there are dual stage temp controllers that will either heat or cool(bring in cold ambient air) depending on temp range.

It depends on several factors like the outdoor temperature, the size of the space, the insulation of the space, and whether the space gets any heat from the sun. If you're talking about a garage or shed in a freezing environment, you're probably going to need heat day and night. If you're talking about a popup greenhouse, you may need to vent during the day. It all depends.