Heater problems

[Lost_in_the_woods]

The conditions: I have a 50 watt aquarium heater in an open top quart jar of water with 10% h2o2. It is set at 86 deg and the water in the jar reads about 90. this heater jar is in a 12"X12" cardboard box lined with 3/4" styrofoam on all six sides. This box is a well sealed and insulated box for shipping temp sensitive medicine.
One quart jar of inoculated rye is also in the box, nothing else.

The Problem: The air temperature in the box (two thermometers) will not go above 76.1 degrees.

What am I not doing right? and um... how is it even possible?

[anno]

>The air temperature in the box (two thermometers) will not go above 76.1 degrees.

The box gives off heat to the surroundings.
One solution is to raise the temperature in the jar a bit, or to build a tub in tub incubator, as seen  here
http://www.fungifun.org/English/Pftek#a5

I wouldn't worry too much though, 76°F is OK too for incubation.