HOW INSULATION WORKS

To understand how insulation works it helps to understand heat flow, which involves three basic mechanisms -- conduction, convection, and radiation. Conduction is the way heat moves through materials, such as when a spoon placed in a hot cup of coffee conducts heat through its handle to your hand. Convection is the way heat circulates through liquids and gases, and is why lighter, warmer air rises, and cooler, denser air sinks in your home. Radiant heat travels in a straight line and heats anything solid in its path that absorbs its energy.

Most common insulation materials work by slowing conductive heat flow and convective heat flow. Radiant barriers and reflective insulation systems work by reducing radiant heat gain. To be effective, the reflective surface must be in contact with an air space.

Regardless of the mechanism, heat flows from warmer to cooler areas until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and especially to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors--wherever there is a difference in temperature. During the cooling season, heat flows from the outdoors to the interior of a house.

To maintain comfort, the heat lost in the winter must be replaced by your heating system and the heat gained in the summer must be removed by your cooling system. Properly insulating your home will decrease this heat flow by providing an effective resistance to the flow of heat. 

U.S. Department of Energy

R-VALUES

An insulating material’s resistance to conductive heat flow is measured or rated in terms of its thermal resistance or R-value -- the higher the R-value, the greater the insulating effectiveness. The R-value depends on the type of insulation, its thickness, and its density.

Installing more insulation in your home increases the R-value and the resistance to heat flow. In general, increased insulation thickness will proportionally increase the R-value. However, as the installed thickness increases for loose-fill insulation, the settled density of the product increases due to compression of the insulation under its own weight. Because of this compression, loose-fill insulation R-value does not change proportionately with thickness. To determine how much insulation you need for your climate, consult a local insulation contractor.

The effectiveness of an insulation material’s resistance to heat flow also depends on how and where the insulation is installed. For example, insulation that is compressed will not provide its full rated R-value. The overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because heat flows more readily through studs, joists, and other building materials, in a phenomenon known as thermal bridging. In addition, insulation that fills building cavities reduces airflow or leakage and saves energy.

The amount of insulation or R-value you'll need depends on your climate, type of heating and cooling system, and the part of the house you plan to insulate.

U.S. Department of Energy