I’m retrofitting 100-year-old wooden sashes with insulated glass units. The below chart shows insulating U-values improve as the gap between layers of glass increases, for air or argon fill, peaking at 7/16″ cavity gap. In my retrofit, the wider my glazing bead, the thinner the IGU has to be. What is the thinnest glazing bead that will keep the glass into the wood sash? I was thinking an acrylic glazing putty and glazing points? Interestingly the chart also shows that krypton gas has basically same U-value for all cavity gaps.
  

Center of Glass U-Values for Double and Triple Glazed Insulating Glass Units with Solarban 60 Low-e Glass with 100% Air, Argon, or Krypton, or Mixtures of These Gases (vitroglazings.com)