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
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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