Why are summer and winter R-Values different for AIR-CELL® Insulation in roofs and floors?
Heat flow by radiation and conduction is unaffected by the heat flow direction, and AIR-CELL® is just as effective against these forms of heat flow in any orientation. However, in any air space, if the heat flow is in the upward direction, it will always be partially assisted by natural convection. Natural convection is the occurrence of air movement due to the warmer lighter air tending to rise and cooler heavier air tending to fall to replace the warm air, thus heat transfer by natural convection is always in the upward direction. Therefore, when considering the Total R-value for a system with a horizontal air space such as a roof space or sub-floor space, the system will always have a higher resistance to heat flow in the downward direction. Therefore, typically the resistance will always be higher for summer conditions. In walls, as the heat transfer by convection will be equivalent for heat flow into or out of the building, there is no difference in Total R-values for walls for su
