The R-values of permeable insulation systems are generally determined in test apparatuses designed to assure one-dimensional heat transfer and to assure no air intrusion effects. Such classical R-values are then calculationally employed to help describe in situ heat-transfer rates through insulation systems.

The research reported herein reexamines the applicability of these classical R-values to commonly encountered insulation systems where air intrusion is physically possible. Several kinds of findings are reported: (a) Laboratory-determined R-values for several types and thicknesses of permeable insulators, under conditions of no externally imposed flows. (b) Laboratory-determined heat-transfer rates for several permeable insulators under conditions of externally imposed flows (air intrusion). (c) Theoretically calculated heat-transfer rates for permeable insulation systems which sustain infiltrative-exfiltrative air intrusion effects.

The results obtained show that the thermal insulation provided by a permeable insulation system may differ substantially from that which is predicted by simple application of classical R-values.