RHP not heating enclosure

[DiaCer0]

I built a 4x2x18" enclosure for my retic out of 3/4" plywood. Bottom is coated with drylok, walls and ceiling painted white. Plexiglass sliding doors. I purchased a VE 80 watt rhp and mounted it on the right half of the enclosure. I let the rhp run for 24 hours to get rid of the offgassing.

I have a 10"x6" adjustable vent 2" above the substrate on the left side of the cage. The problem I'm having is that even with the vent closed, I can't get the temperature above 83 degrees on the hot side of the enclosure. I put two black granite tiles underneath the rhp and I'm only getting around 83 degrees with my laser thermometer. The cool side is around 74 degrees. Any suggestions?