Heat Transfer
The testing to determine R-Value is done in a very controlled
environment where heat transfer is measured in terms of conduction
and radiant heat properties only, not convection. 
Conduction
is the thermal transfer of heat through a material like the
handle of a pot when the pot is heated. Radiant transfer is
the transfer of heat through the air waves like the sun's radiant
heat in the summer. The third type of heat transfer is convection
which uses air movement to transfer heat like a fan-assisted
space heater. The R-value test does not test for convection
and only tests for radiant and conductive heat transfer. The
R value system's standard is based on 1 inch of yellow pine
which equals R-1.
You may ask if convection occurs in our homes since we have
doors and windows which appear to prevent air penetration from
outside. With windows, doors, dormers, utility entrances, overhangs,
sunrooms and decks; it is virtually impossible to seal the building
envelope completely. Air seeping in can set up convection currents,
and most houses built today use insulation which performs poorly
under convective conditions. Among these are fiberglass, cellulose,
mineral wool, perlite, and cotton. All of these products allow
warm or cold air to transfer though the insulation material.
In the winter, this may cause cold spots on your floors, walls,
ceilings or electrical boxes.