HCA: Hermit Crab Association

Is a standard room humidifier (with dechlorinated water) near the tank safe for hermit crabs? Or do they have anything internally (the part that vaporizes the water) that could make the vapor harmful to hermit crabs? I need a quick temporary solution to low humidity and the humidifier is what I have on hand.

Increasing the humidity in the room won't do a lot for the humidity in the tank, IMO.
What do you have as a lid/ cover?
What and how deep is your sub?

I was planning on placing the humidifier right next to the tank so the vapor can go through the screen lid. I have a 5:1 mix of sand to EE, about 5 inches deep

Cover the screen with plastic wrap (ie- Saran Wrap). That will hold the humidity in the tank. If your sub is moistened to sand castle consistency, then that along with the plastic wrap may be all you need to do. My first two tanks did not have lids (10 and 20 gallons), only plastic wrap over the top. The humidity was always over 80%. I now have a 50gallon, which has a plexi glass lid. The sand and EE mix is what I use too. It will dry out if your tank humidity is too low. Do you have any crabs under? If not, you can dig down into your sub a bit to see if it has dried. If you have crabs down, you can take a chopstick or something similar, and slide it along the glass front of your tank to see how far down is dry or if it still holds shape.

I do have crabs under, I'll check using a chopstick I have plastic wrap on top of the tank, but my heating lamp is on the screen lid because I don't have anywhere to hang it, so there's an area of the lid not covered in plastic where the lamp is, which I think it's what's letting the humidity escape. I was thinking that using a humidifier with the water vapor going directly into the tank would have the same effect as spraying, I just wasn't sure if there's anything about that that would harm the crabs

A heat lamp and a partially open lid will zap your humidity. A room humidifier won't hurt, but probably won't be able to bring the humidity up to proper levels consistently. Can you get a UTH to mount on the side or back, instead of the heat lamp?

I have 2 uth's but they're not very strong which is why I had to get the heat lamp :/

Can you insulate? Depending on the type of UTH - some can be insulated over top and some can not. The ones you can't insulate over top of, you can insulate around them.

I'll try that! That sounds a lot better. Also, I decided to try using the humidifier and the humidity has gone up to 75%