Substrate and ambient temperature questions

[TMacie]

Hi All,
The fiance and I just brought home our new ball friend this weekend from the New England Reptile show. His name is Spot. This is our fist BP, we have a baby corn as well.
My question has to do with substrate and ambient temps. He is in a 48Wx24Dx18H wooden viv that we built. It is sealed inside and out, has a plexiglass front which is also the door.
We are using cypress mulch as substrate. I have 11" flexwatt under plexiglass for heat. This is one strip wide and goes the depth of the viv (11x24). This is just about 1/4 of the viv. I have a Alife thermostat controling the flexwatt, the sensor is secured to the plexiglass right in the center of its area. The temp there is 91, 88 around the edges of the plexi. We have about 1" of cypress on top of the heater setup. The substrate itself right on top of the heater is only measuring about 77. The substrate measures 68ish on the cold side.
Is this an issue? If so, any suggestion on how to fix it?
Also, ambient temps in the viv are about 68 as well. Is this an issue? Any suggestion on fixing that too?
As a note, I think these temps are too low, but I want to hear what people have to say and their solutions.

On a better note, with the cypress we are having no issues with humidity!

Thanks,
Travis