We examine the in-plane thermal conductivity (K-ab) for different clas
ses of insulating cuprate crystals, and present measurements for YBa2C
u3O6+x and PTBa2CU3O6+x (x less than or equal to 0.11). An unusual, no
nmonotonic temperature (T) dependence of K-ab, previously observed in
La2CuO4+delta, is demonstrated to be a characteristic of the 123 compo
unds as well. This feature signals the onset of anomalous damping for
in-plane, heat-carrying phonons at T less than or equal to 250 K that
is highly sensitive to light oxygen doping in YBa2Cu3O6+x. Its absence
in Nd-2 CuO4+delta and similarity to the behavior of SrTiO3 suggests
that the phenomenon in the cuprates is connected with tilt distortions
of the CuO polyhedra. We discuss the implications of these results fo
r interpretations of the superconducting-state enhancement of K-ab, wi
dely observed in superconducting compounds.