Heat travels in three ways:
1) Conduction a. Providing the conductivity and temperature difference is the same the amount of heat will travel equally in all directions. Up, down, sideways or any other direction in between 2) Convection a. This is raising the temperature of a fluid (air is a fluid). As the fluid it heated the molecules get further apart and thus making it less dense. As a result it rises. Ie. Heat does NOT rise, only the fluid it heats. 3) Radiation a. Radiation will travel in all directions equally when the surface properties (Texture, emissivity, and temperature) are equal.More than most would want to know about heat transfer
formatting link
As for the insulation and rodents. The first line of defense is get rid of the rodents. Find the holes and plug them. I understand they do not like copper mesh (no experience in this). 1/4 inch hard ware cloth will stop most rats and mice. A roof rat can get in through a hole the size of a quarter and mouse the size of a dime.
I've not seen an insulation a rodent will not chew through with the exception of vermiculite or Perlite. Good luck holding these in place. There is a 1" vacuum panel used in refrigerators. I can only assume they would not fit and be very expensive.
So this brings us back to go after the rodents. If you cannot find their entry point go to your favorite pet store and get a UV light. IT is use to show fresh urine and since rodents (mice and rats) urinate everywhere the travel you should be able to track down their entry points as well as determine if you have gotten the holes plugged. Fresh urine glows under a uv light.