Question: is a window a viscous fluid? 

Answer: The answer to that is "no." The old "glass is a slow moving liquid" bit is an old wives tale. Or in this case, an old professors tale.\n\nThe essential reason is that in a fluid all the molecules are disordered and only loosely bound. While in glass the molecues are strongly bound, although disordered. \n\nThere is some wiggle room in the definition of solid and liquid, but it probably doesn't matter in this instance. The main evidence for glass being a liquid is the claim that glass in, say, old buildings has slowly settled and made the bottom thinker has never been shown to be due to glass being a liquid. In fact, the existence of perfectly fine glass from the time of the Roman Empire makes it unlikely.\n\nThe second link I've included is an extensive discussion of the whole "windows are flowing down" bit - the conclusion being, that it is more likely the panes were made with uneven thickness and the glass was inserted with the thicker end down - which  is the logical thing to do in that circumstance.
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