Hero999 said:
But isn't more heat lost from windows by conduction rather than radiation?
True, but not related to the question of whether window glass transmits (enough) IR for images.
From:
https://en.wikipedia.org/wiki/Infrared_radiation
Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium. Image intensifiers are sensitive to this area of the spectrum. Examples include night vision devices such as night vision goggles.
Short-wavelength infrared (SWIR, IR-B DIN): 1.4-3 µm, water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications
See: Attached transmission spectrum of soda lime glass (window glass, untreated).
The basic problem, as noted in the Wikipedia reference is absorption by water. Related substances, such as sapphire are better at transmission of IR and are used in heat-seeking missiles for that reason, but glass still a good proportion of transmits IR heat. John