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*How Insulation Works*

Insulation works in three different ways: conduction, convection and radiation.

Conduction is the reason a frying-pan handle gets hot. The handle is not in the flame, but it is attached to the pan, which is in the flame, and the heat is transferred from molecule to molecule up the handle.

Convection is the transport of heat by the molecules of a fluid. In the illustration, air molecules warmed by contact with the warm surface rise (warm air rises), flow across to the cooler surface, and warm the cooler surface, thus giving up their extra heat.

Radiation is the transport of heat by electromagnetic waves and is the method by which the sun warms your skin.
Recognizing these three processes helps to explain how insulation works but helps little in computing heat loss. To keep things simple, engineers pretend that all heat flow through building surfaces is conductive and so can be calculated using the formula for conduction.

Ordinary unmoving air has a surprisingly high R-value of 5.7 per inch of thickness. Because of this, building insulation is a seeming paradox. In a real sense, you are paying for what you are not getting. Most insulations consist basically of the minimum amount of material required to stop air from moving. Look at most insulations under a magnifying glass, and you'll see thousands of tiny air pockets, between fibers, between particles or within cells.

But unmoving air isn't so ordinary. Warm air is buoyant and wants to rise, while cooler air wants to fall. To capitalize on the insulating property of dead air, we have to trap it within tiny spaces so that it doesn't travel very far. However, in stopping air movement with even minimal material, we add the conductivity of the material to the conductivity of the air. Building insulations are thus trade-offs between the amount of conductive material and the sizes of the air spaces created.

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