| In general terms, heat transfer is quantified by
Newton's Law of Cooling,
where h is the heat transfer coefficient. For conduction,
h is a function of the thermal conductivity and the material
thickness,
In words, h represents the heat flow per unit area per
unit temperature difference. The larger h is, the larger the
heat transfer Q.
The inverse of h is commonly defined as the
R-value,
The R-value is used to describe the effectiveness of
insulations, since as the inverse of h, it represents the
resistance to heat flow. The larger the R, the less the heat
flow  .
R is often expressed in imperial units when listed in
tables. Conversion to SI-units is provided in the Units
Section.
To convert R into a thermal conductivity k, we must
divide the thickness of the insulation by the R value (or
just solve for k from the above equation),
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