Doug Miller wrote:

... Emissions from an IR source inside the building will be mostly absorbed
by the skylight, instead of passing through, no?

I think so, for wavelengths longer than 3 microns.

Meanwhile, the skylight will radiate IR as well.

Sure, but we were talking about sky vs skylights, no? A cheap IR thermometer,
eg Raytek's $99 version, will ignore water vapor absorption and read a lower
sky temp than an expensive one, eg a $1000 Exeltech, which will read a lower
sky temp on a cloudy or humid day.

Nick