What type of materials make a good insulators?

Answer 1

That rather depends on what you wish to insulate from.

For electricity, materials of low electrical conductivity, such as porcelain, and many plastics, are useful. Particularly if designed to have suitable environmental properties, such as corrosion resistance, heat resistance and so on.

For a magnetic field, then there are specially designed metal sheets, (mu-metal) that may be used to shield a part from magnetic fields.

For thermal isolation, it is hard to past a vacuum, for it has low conductivity. This is the basis of your thermos flask. In applications such as the walls of refrigeration containers, or houses, then one usually goes for a low-mass material of low (thermal) conductivity. Air is a common insulator, and may be entrained against conduction and convection by the use of closely packed fine fibres such as wool or fibreglass, or the closed-cell foam materials.

For Acoustic insulation, then we choose to have a material whose mass and resilience are as far as possible from those of air. Dense massive materials, such as concrete, or gypsum wallboards are very effective. It is vital that there are no penetrations or air passages through a sound insulator, for these would bypass the insulator. For sound waves are carried as vibrations through the air.