Abstract A testing program to investigate the thermal performance of concrete masonry wall assemblies has been developed and completed. The intent was to study the thermal contribution of both the air cells contained in concrete masonry blocks and the mortar joints holding the unit blocks together. This was accomplished by testing three individual walls. The first constituted a 6-in (0.15 m) conventional concrete masonry wall. This wall acted as the control specimen with which the other walls were compared. In the second wall, the mortar was eliminated and replaced by an adhesive. This allowed the measurement of the contribution of the mortar to the thermal resistance of the conventional prototype wall. In the third wall the air cells (along with the mortar joints) were eliminated to study the effect of the air cells on the thermal resistance of the conventional wall assembly. A temperature-controlled test plate, calibrated with fibrous glass board material of known thermal conductivity was used with heat flow sensors to determine periodically the thermal resistances of the test walls. Results indicated that the R-value of the 6-in conventional concrete masonry wall (including mortar joints) measured 2.05 (hr ft2 °F)/Btu. However, in the absence of mortar joints, the thermal resistance of the concrete masonry wall assembly decreased by approximately 8%. Thermal resistance of concrete masonry walls in which the air cells were eliminated was approximately 25% less than the conventional prototype wall, and 18% lower than the mortarless wall. This indicates that the air cells in the concrete blocks contribute significantly to the thermal resistance of masonry walls. Based on the testing conditions, it is concluded that both the containment of air cells and mortar joints in the masonry wall assembly represent an asset rather than a liability to the thermal integrity of masonry envelopes. Analysis and comparison of these results with published data are included.