Humidity in PVC enclousure?

My new 3x2x15” pvc enclosure has been up and running for a while. I have a 40 watt VE RHP, and temps are running perfect, but I am having trouble getting it to hold humidity. I have eco earth mixed with spragnum moss for substrate. I am spraying heavily once a day to keep humidity up. It still wants to drop. Substrate is not very deep. Would adding more substrate help with humidity, or is there anything else I should try?

I have 2 custom 3X2X1.5 wood cages that I use Reptile Prime in about 1.5-2" deep. I found I blocking off some of the air flow to help retain humidity from dropping too quickly. I did make it with extra holes so I could plug or expose as needed. Also the cage with a little more substrate seems to hold humidity a bit longer then the other one. There is a noticable difference with only half inch difference in depth.

I have a 4x2x1 from AP, and I have the same issue. I just try to spray it down daily. Wish I had a more "carefree" method, but oh well.

I rotate substrate between Reptile Prime and ReptiChip.

My AP T8s hold humidity well... my one with a 60 watt RHP is holding 60% with dry substrate, but I keep my boa in that one and his water bowl is pretty large in the center.

Adding a 2nd water bowl or moving the current one below the RHP will usually raise humidity as it evaporates the water more.

Adding thicker substrate would probably not hurt either since your heat is via RHP, thin is best for UTH so the heat penetrates more accessibly. I like to use thicker substrate and pour some water in and mix it around instead of misting. Mist settles at the surface and evaporates off faster than if it is evaporating from below a drier surface. (Kind of like gardens.. mulch is dry, dirt below is wet, and it is still more humid above it there than over the pavement)

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I do the same. When I change the water in the water bowl every other day I dump some and mix around the substrate. Don't really mist much

Thanks for all the advice. The thicker substrate is definitely helping. I am going to try a second water bowl directly under the RHP too.