Glass is actually not quite as bad a thermal conductor as I thought. While metals like copper are hundreds of times more conductive, glass is more conductive than water, wood, and even salmon

There does not seem to be much difference, conductivity-wise, between ordinary plate glass and borosilicate glass. The thermal expansion of the boro, however, is only around 1/3 that of plate glass.

Glass is not quite as conductive as a thin sheet of rock, like slate, would be. But it is pretty close.

Jim