R-Value is the rating system used to grade insulation products or a material’s insulating properties. The “R” is for “resistance” and refers to the resistance a material has to heat flow, or temperature conduction. When a product or home has a high R-Value, it means that it is insulated very well.
The main purpose for insulation is to stop the movement of heat. The problem with using R-Value as the sole measure of insulation effectiveness is that heat moves in and out of a home in four ways: by conduction (which R-Value measures), and by convection, radiation, and air infiltration (R-value doesn’t measure any of these).
Exposure to wind, humidity, and temperature changes create pressure differences between the interior and the exterior of a building. Conditions like wind pressure, hot air rising, and HVAC systems pushing air through every tiny little opening allows the air to make its way to the interior or exterior, or to unconditioned areas of the home
A good insulation system will have good R-Value (preventing heat loss due to conduction), will be pneumatically or spray applied, fully filling the building cavity (preventing heat loss due to convection), and will be packed pretty thick (preventing heat loss due to air infiltration and radiation).