Abstract The thermal Hall effect has been observed in a wide variety of magnetic insulators1,2,3,4,5,6,7,8,9, yet its origins remain controversial. Although some studies attribute the effect to intrinsic mechanisms10,11,12,13,14, such as heat carriers with Berry curvature, others propose extrinsic mechanisms15,16,17, such as heat carriers scattering off crystal defects. Even the nature of the heat carriers is unknown: magnons, phonons and fractionalized spin excitations have all been proposed.