How does Felt work as an insulator?

Felt traps air. It is the air that is the actual thermal insulator. In homes, it has been superseded by fiberglass, which is better at the same thing and doesn’t degrade over time.

Vacuum is the best insulator, at least as far as conduction goes. It isn’t practical in walls and ceilings so trapped air insulators are used there. But it is great in coffee cups and chemistry dewars.

Felt traps air. The air is the actual thermal insulator.

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