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Heat loss through conduction is analogous to osmosis, with heat gravitating from a warmer space to a cooler one as energy is transferred from one molecule to the next as the energy level looks for equilibrium. The term R value (or its converse, U value, which is used to make heat loss calculations) is a coefficient of a materials capability to resist heat loss through conduction. Heat loss through conduction, however is often the least important factor in residential heat loss, with other main factors being convection, air infiltration.
Convection is the process by which currents of air move around fueled by the engine of warm air rising and cool air falling. Heat loss through convection can be extreme in the case of a loose fill insulation such as blown in fiberglass.
Heat loss through air infiltration has to do with air moving into and out of a house. Most houses are full of holes, and it is typical for all of the air in a house to be exchanged with the outdoors every two hours, if not more often, which makes air infiltration sometimes the main vector for heat loss in a house. A loose pack insulation such as fiberglass batts will do little if anything to prevent air infiltration.