Argon fill is not a long-term solution. The argon actually sinks between
the glass panes and creates a negative area in the center madking plates
slightly closer together in the center than around the periphery. In cold
climates with high indoor himidity a circular condensation appears in the
middle of the glass pane. For me this is hard to believe, but I was told
the same story by several folks in the biz.

Experience: I just replaced 18 Anderson sashes, vintage 1990. Anderson
made changes to correct this problem, one of which, I am told, is to
eliminate the argon fill. Super-dry air-fill insulates just as well and
doesn't migrate. I also learned that the gas fill (air or argon) has
little to do with the insualation value; it is the distance between the
inner surfaces of the glass that does the job. Anderson installers have a
gage that measures this distance to check for possible sash insulation
failures.

DH - Illinois