Heat Basics- Why Insulation Isnt Enough

Claudette Reichel  |  6/25/2008 8:49:37 PM

Figure 1, Conduction

Figure 2, Convection

Figure 3, Radiant Heat

Heat moves three ways, so controlling it one way doesn’t necessarily affect the other two. It moves by:

  • Conduction, through materials, from warmer to cooler areas – rapidly through some materials (like metal) and very slowly through others (like insulation). (Figure 1, Conduction)
  • Convection, the transfer of heat by moving fluid, such as air. Although forces (natural wind, mechanical fans, etc.) can move air and the heat it contains in any direction, warmer air tends to rise. Rising warm air displaces sinking cooler air, leading to “convective loop” air currents.  (Figure 2, Convection)
  • Radiant heat energy, like the heat in sunshine and the heat you feel near hot pavement or a fire  (Figure 3, Radiant Heat)
    • Low emissivity (low-e) materials and coatings hinder radiant heat flow because they “emit” (give off) very little heat. In other words, they are lousy radiators. Low-e windows have an invisible low-e coating that reduces long wavelength radiant heat transfer between the layers of glass, resulting in less total heat transfer than for an insulated window without the coating.
    • Radiant barriers (usually shiny aluminum foil materials) have a low-e and a high radiant heat reflecting ability. They reduce radiant heat transfer from a hot surface, like a roof, to a heat-absorbing surface, like insulation. To work, an air space must be on a shiny side, but it does not have to face the heat source. In fact, under a roof, the shiny side should face down to prevent it from being covered by dust and losing effectiveness.

A radiant barrier’s resistance to heat flow is variable and depends on conditions, including direction of heat flow, dust accumulation and other factors. In general, it is more effective at reducing vertical heat flow (such as from a roof to the attic floor). In general, white and light-colored materials tend to reflect more radiant heat than dark materials, but other characteristics of the material can greatly affect this.

  • R-value is a standardized measure of a material’s resistance to heat flow (primarily by conduction).
    • The effective R-value of a wall is not the same as the R-value of the insulation in it, because of framing and other gaps in the insulation. Convection air currents within a wall, settling and thermal bridge materials that readily conduct heat through the wall system (such as steel framing without proper insulation or thermal beaks) reduce the effective R-value of the whole wall.
    • Air conditioners don’t really “create cool air.” They remove heat (and moisture) from the air in your home. This normally makes up the biggest part of an annual home energy bill in southern homes.
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