Trouble With Ambient Temps

So I built my BRB enclosure, and humidity stays at a steady 91%. I have a rheostat and can set the surface temp at 88, but my thermometer still says the ambient temp is 70. I think it may be because I have a good 4 inches of substrate for the pothos I put in to help with humidity

Surface temps have little to do with ambient temps I am guessing you have a grow lamp inside the enclosure now? You have find that insulating the top and sides may help but it may take a secondary heat source to lift the ambient air temps up. Sometimes raising the room temp a few degrees can make a big difference.