As an insulator for electricity it would be a poor insulator because it conducts electricity well. Aluminum insulator is usually used in our lives. For example porcelain line post insulator can be made from ceramic, aluminum materials.

Why Is Aluminum foil a Better Insulator than cotton or paper?
Adding insulation to your home is an easy way to save on energy costs. Aluminum foil, also called tin foil, makes an excellent insulator and also called aluminum insulator, and in some situations, it works better than materials like cotton or paper. Aluminum foil is not best for every situation, though, so using it correctly is an important part of saving energy.
But we commonly used porcelain ceramic or polymer materials to make electrical insulator, such as porcelain line post insulators

Is aluminum foiling a good insulator?
It would depend on what type of insulator you have in mind. It was used as a conductor in wires for a while until it was discovered to become resistive over time. It would also be a poor insulator from heat. A lot of cookware is made of aluminum. Plastic and paper both would probably be considered as good insulators from both. Ceramic line post insulator is one of good insulators for electrical lines.  

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