It is crucial to be aware of heat transfer in buildings in order to understand the principles of good thermal insulation. Heat transfer modes are the following: heat can be transferred by conduction, convection, radiation, or a combination of all of them.  Heat always moves from the hottest areas towards the coldest, while looking for balance; this is known as the zero principle of thermodynamics. The greater the temperature difference, the faster heat flows to the cooler area.

The figure shows the amount of losses through floor, walls, roofs, windows, doors and gaps around them. It is necessary to insulate the building to avoid these energy losses. The thermal conditioning in buildings is based on radiation and convection. The most significant transmission mode is conduction through the opaque envelope of a building.

What is thermal insulating for a building?

A thermal insulator is a material characterized by its high thermal resistance. It establishes a transmission barrier between two habitats that would even in temperature otherwise. In general, every material offers resistance to heat transmission, but according to the international regulation, a material is considered thermal insulating when:

• Thermal conductivity coefficient (λ) < 0,6 W/(mK)
• Thermal resistance (R) > 0,5 m²K/W.

The following tables show some conductivity values for different insulating materials of inorganic and organic insulators, with animal or vegetable origin: the lower λ, the better insulating properties it has.