Are all double pane/insulated windows filled with gas or Argon?
No. Most dual pane windows are simply filled with plain air. This provides a dry sealed air space which reduces heat transfer. Argon gas is less conductive than air and nitrogen and in extremely cold climates can result in a noticeable difference in heat transfer. In warmer climates like Seattle, however, the additional cost of Argon gas is not as beneficial or even necessary.
