Lattice thermal transport in metals is traditionally described under the assumption that a highly mobile electron gas efficiently screens ionic displacements, resulting in a predominantly harmonic interatomic potential. Here, we show that strong electron-electron correlations can profoundly disrupt this screening mechanism, promoting intrinsic phonon scattering in the kagome metal ${\mathrm{CaCu}}_5$. By comparing orbital-resolved band calculations with high-resolution angle-resolved photoemission spectroscopy, we identify a strongly correlated Cu $3d$ flat band and extract an effective Hubbard interaction of $U\approx 3\mathrm{eV}$. We demonstrate that this on-site Coulomb interaction localizes the Cu $3d$ valence charge, indicating a reduced local charge response to ionic displacements. As a consequence of the reduced electronic screening, the lattice potential and phonon spectrum are strongly renormalized, substantially expanding the three-phonon scattering phase space. Consequently, electronic correlations alone intrinsically suppress the room-temperature lattice thermal conductivity by approximately 53%. These findings identify correlation-driven anharmonicity as a significant, nonstructural factor for suppressing thermal conductivity, thereby providing a possible route to mitigate the conventional trade-off between enhanced phonon scattering and high electronic mobility.

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• Flat bands
• Angle-resolved photoemission spectroscopy