You might be surprised to know that thermal imagers often fail to detect through glass.
From a physics point of view, it is difficult to explain the technical reasons for this problem, but its principle is very simple. Essentially, visible light passes through glass, but glass acts like a mirror for infrared wavelengths (which is why thermal imaging camera lenses are often made of germanium or zinc selenide rather than glass).
If you point the thermal imaging camera at a window, the screen won't show the image on the other side clearly, you'll most likely see a blur, and it's likely a blurry reflection of the lens you're holding.
But this is not absolute, some infrared frequencies can pass through glass, and certain types and configurations of glass allow different degrees of infrared light to pass through. For example, car windshields tend to yield better results than standard household glass.
In most cases though, most of the image will be obscured by infrared reflections from the "wrong" side of the glass, showing varying degrees of opacity. At least the objects viewed will lack noticeable detail and contrast.
In short, you cannot get accurate readings through glass (or various other types of highly reflective surfaces) with a thermal camera.