Heat energy can flow by the three mechanisms you mention,
conduction, convection, and radiation. Conduction works when heat
energy flows from a hot object to a colder one when they touch. How
fast the heat flows by this mechanism depends on the temperature
difference between the objects and the thermal conductivity of the
materials and the interface, and the area of the interface. Convection
is an aid to heat conduction when some of the material flows from one
place to another, carrying the heat energy along with it. Hot stuff
typically expands, and becomes less dense and floats upwards, so
convection currents arise naturally when a liquid or gas is allowed to
flow and there is a temperature difference from one place to another.
Radiation is the carrying of heat energy from one place to another by
light waves (or their longer-wavelength cousins infrared waves).
All three cups should block light radiation from the inside to the
outside, although may emit infrared waves from their outer surface to
the environment. The insulation comes from preventing heat energy from
going from the inner surface to the outer surface.
Styrofoam is made mostly out of air, which conducts heat very
poorly, but convects well. Styrofoam traps the air in small pockets,
preventing it from flowing, and so it reduces conduction and convection
at the same time.
If your cup was made out of hard plastic polystyrene and not the
air-puffed version, it still may insulate well because the long polymer
chains in the plastic do not conduct heat well. Plastic cups are
typically thicker than the paper ones too, and a thicker cup makes a
The paper cup is a lot like the styrofoam cup I mentioned above,
with the dry wood fibers trapping air. Usually paper cups are waxed,
and wax is a poor conductor of heat (but better than air). It is probly
thin and lets heat through.
Porcelain is a dense material made out of clays and stony
materials. These conduct heat better than air or wax or plastic. When
some atoms jiggle on one side of a crystal of a rocky mineral, it is
easier for this motion to be conveyed through the crystal lattice of
the mineral than it is for plastic or wax, where the motion is damped
The metal conducts heat extremely well because electrons are free
to move through the material and can carry energy from one part to
(republished on 07/27/06)