How Insulation Works
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To understand how insulation works it helps to understand heat flow, which involves three basic mechanisms: conduction, convection, and radiation.
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Conduction is the way heat moves through materials: when a spoon is placed in a hot cup of coffee heat moves through the handle to your hand.
Convection is the way heat circulates through liquids and gases. This is why lighter, warmer air rises, and cooler, denser air sinks in your home.
Radiation is when heat travels in a straight line and heats anything solid in its path and absorbs the energy.
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Most common insulation materials work by slowing conductive 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 air space.
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Regardless of the mechanism, heat flows from warmer to cooler areas until there is no longer a temperature difference. This means that in the winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and especially outside. Wherever there is a difference in temperature heat will flow indirectly through interior ceilings, walls, and floors to those spaces. During the cooling season, heat flows from outside to the interior of a house.
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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.
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. The R-value of most insulations depends on temperature, aging, and moisture accumulation. With multilayered installation we calculate the R-value by adding the R-value of each layer.
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When homeowners install more insulation, they believe they are increasing the R-value and creating more resistance to heat flow. In most cases, homeowners are adding more loose-fill insulation to the current loose-fill base, not understanding that the base layer has become compressed over time. The compressed loose-fill base has a higher density, which means the R-value will not change due to the thickness of the insulation being disproportion.
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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 that 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.
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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.
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