Typically, the thermal conductivity values for polyimides are significantly
less than for silicon dioxide, and all are thickness dependent (bulk versus
micron thin films).

So a rule of thumb is the following ordering;

silicon nitride: 1.9E-2 (Cal/cm.sec.C)
silicon dioxide: 3.4E-3 (Cal/cm.sec.C)
polyimide PI2556: 3.5E-4 (Cal/cm.sec.C).

(Another useful thermal conductivity unit is mW/cm.C).

The poor thermal conductivity of polyimide versus silicon dioxide is one more
challenge in substituting low-K polyimide materials as interlevel dielectrics
in complex integrated circuits.

---Rich Griffith