dome entry - as heat sink or insulating?

[Tigi]

Hi,

I'm planning on casting a dome oven, and see that typically the entrance 'tunnel' is cast as part of, or at least of the same material as the dome I.e. a heat sink refractory. Is there any reason why this couldn't be an insulating material instead? Say a mix of perlite/refractory cement, or a proprietary insulating refractory?

I guess my question naturally extends to the floor of the entrance. If the hearth within the dome is firebrick, could the entrance likewise be an insulating material?

Any advice would be much appreciated!