Different materials transfer heat by conduction at different rates - this is measured by the material's thermal conductivity . Measurement of thermal conductivity Let us now measure the flow of heat through the material over time. Conversely, materials with low thermal conductivities will transfer small amounts of heat over time - these materials, like concrete, are poor thermal conductors. This is why if you throw a piece of copper and a piece of concrete into a campfire, the copper will heat up much more quickly than the concrete. It is also why fiberglass insulation, and also feathers and fur, have air pockets - dead air is a poor thermal conductor, and so the air pockets aid in cutting back on the heat loss through the material.
Home insulation is thus a poor thermal conductor, which keeps as much heat in as possible.
Instead of being rated in terms of thermal conductivity, insulation is therefore usually rated in terms of its thermal resistance , which is defined as Materials which have a high thermal conductivity have, by definition, a low thermal resistance - they are poor heat insulators. On the other hand, materials with a low thermal conductivity have a high thermal resistance - they are good heat insulators. Good insulating materials therefore should have a high thermal resistance.