There are two main purposes for insulation. Insulation helps keep heat in during the winter and helps keep heat out during the summer. Whenever there is a temperature difference between the inside and outside of a building, heat tends to flow from the warmer to the cooler space. Insulation reduces or slows the heat transfer through the building envelope.
By understanding how heat moves, it is easier to understand how insulation works. There are three modes of heat transfer: conduction, convection, and radiation.
CONDUCTION is direct heat flow through matter. It is caused by fast moving molecules on the hot side colliding with and transferring energy to slower moving molecules on the cold side. It results from actual physical contact of one part of the same body with another part, or of one body with another. An example of conduction through contact is a cooking pot on the solid surface of a hot stove.
COVECTION is the transport of heat within air or liquid, caused by the actual flow of the material itself. Warm air rises and cold air falls to create a convection loop. The moving air either enters or exits a building during this process. Up to 45% of heat loss in winter happens through convection through the roof.
RADIATION is the transmission of electromagnetic rays through space. The radiant heat rays of the sun do not become heat until they strike an object such as the roof of a building. As the roof surface heats up, the heat energy is transferred by conduction throughout the rest of the roof mass. Infrared radiation from the sun is the source of 93% of the summer heat gain through a roof.
Thermal insulation does not stop the transfer of heat into or out of a building; it only slows down the transfer. R values are a means of showing the thermal value of an insulating material. R value is a measure of resistance to heat transfer by conduction and does not apply to other methods of heat transfer. Insulative materials act to hinder the flow of energy by using a gas and randomness of material to reduce direct contact (conduction) and air flow (convection).
R value has no utility to measure the reflective capability of a material. Highly reflective materials act to keep a surface cool by reducing the amount of the sun’s energy that is absorbed (radiation). The energy that is not reflected is either absorbed or emitted by the surface. The amount of radiation that is emitted is a function of the emissivity factor of the material. The most effective cool roofing materials then, are those with both high reflectivity (sun’s energy bounces off) and high emittance (easily sheds energy that is absorbed). A good cool roofing system combines reflective membrane with an optimum amount of insulation to reduce heat loss from convection in winter.