Why do they make mirrors the internal material of thermal insulators?

Question

how do all encompassing mirrors help keep out heat?

Answer 1

This is helpful only if there is an air or vacuum gap between the insulating wall and the object being insulated. It's to reduce the radiative heat transfer between the two. All objects with a finite temperature emit electromagnetic radiation. At room temperature and below, this is in the infrared The emissivity of a surface refers to how efficiently it reflects and emits radiation (it's always the same number for a given wavelength).

Answer 2

Mirrors, by definition, relect light, or electromagnetic radiation. Because heat is often transfered by electromagnetic radiation (infrared radiation), mirrors are ideally suited to reflect it. If you're trying to keep something insulated from a cooler ambient, the reflective material should be facing the inside. If the reverse is true, the reflective material should face the outside. Coating both the inside and the outside would be a way to thermally insulate regardless of the situation.