The width of the gap between the panes
Half an inch is the sweet spot
Every gas has a cavity width at which it performs best. Below that width, conduction straight across the gap dominates. Above it, the gas begins to circulate freely and carries heat across by convection instead. For argon the best-performing gap in a double-pane unit sits right around half an inch. Manufacturers know this, which is why so many standard units land on a 1/2 inch air space with 3 millimeter glass either side.
Why narrow gaps push you toward krypton
Krypton is denser again and holds its advantage in much tighter cavities, roughly a quarter to three-eighths of an inch. That is why it turns up in triple-pane units, where three panes have to fit inside a sash of normal depth. It is also considerably more expensive, which is why it is not the default anywhere. If a quote offers krypton in a wide double-pane cavity, that is money spent in a place where argon would have performed about the same.
Elevation is the other thing that changes cavity behavior, and it is worth raising only to dismiss it. Manufacturers shipping into the mountain states sometimes fit capillary tubes that vent the cavity so a unit survives the pressure change, and vented units cannot hold a gas fill at all. Indiana sits far too low for that to apply, so if it comes up in a sales call, someone is reading from a national script.
Which direction the window faces in Bayhill
The cavity gas works on heat conduction, which runs both ways and does not care about the sun. Solar gain is a separate problem handled by the low-E coating and reported as the SHGC number. On a west wall in July, late-afternoon sun through untreated glass will overwhelm anything the fill is doing. On a north wall in January, the fill and the coating are the entire story, because there is no sun to work with at all.
The practical consequence is that on sunny elevations you specify the coating carefully and treat the gas as a given, while on shaded ones the U-factor is what you are buying and the gas is part of how you get there.
Manufacturing quality you cannot see
Two units can leave two factories with identical spec sheets and behave very differently at year fifteen. The variables are the primary seal, the secondary seal, how consistently the cavity was filled at the line, and how well the assembly tolerates thermal cycling. There is a certification program for insulated units that tests exactly this through accelerated weathering, and a manufacturer who participates will normally say so on the spec sheet.
None of it can be inspected from a showroom floor. What you can do is choose product lines with a long, plainly written seal-failure warranty and read whether it is prorated. One free proxy is available to you as well: ask how long the exact series has been in production. A line shipping largely unchanged for a decade has a field record behind it, good or bad, and local installers will know which. A brand-new line has a spec sheet and nothing else.
Frame material and what it conducts
The edge of the glass is the weak band
A window gives up at its perimeter long before it fails in the middle. The spacer bar that holds the two panes apart is also a bridge between them, and a solid aluminum spacer conducts heat straight around the edge of the unit no matter how good the gas at the center is. That cold band is where condensation forms first, and in a humid Indiana winter it is where water sits and eventually damages the stool and the wall beneath it.
Warm-edge spacers made from foam, thin stainless, or a composite raise that edge temperature. On a whole-window basis, changing the spacer often matters as much as changing the fill, and it is rarely the thing anyone is talking about.
Two panes or three in a Bayhill winter
What you give up going to triple
Triple glazing gives you two cavities, and in this climate it does deliver a lower U-factor. The costs are not only financial. The sash is heavier, which matters on a large double-hung and on tired old frames. The extra pane cuts visible light slightly, which some people notice in a north-facing room. The unit is deeper, which complicates an insert into a shallow original opening. And there are now four interior glass surfaces available for coatings, so the specification gets more complicated and easier to get wrong.
For most houses in this region the honest position is that triple pane is a comfort purchase aimed at specific cold rooms rather than an energy purchase that pays for itself.
What it all adds up to for a Bayhill house
Line the variables up and a pattern appears. The conditions that make argon worth more, meaning long tenure, expensive heat, a well-built frame, a proper half-inch cavity and a warm-edge spacer, are the same conditions that make any window upgrade worth more. The conditions that blunt it, meaning a leaky opening, a conducting frame, a short stay, a shallow retrofit cavity, blunt everything else on the quote too.
So the useful conclusion is not really a verdict on the gas. It is that the fill works as a proxy for whether the rest of the window was specified by someone paying attention. On a quote from a company that answers the spacer question without hesitating, the argon has already been handled.
How the house is heated and what that fuel costs
Heat pumps change the math
The value of any reduction in heat loss is set by the price of the heat you are no longer losing. A house on natural gas has relatively cheap heat, so a small reduction returns a small number of dollars. A house on electric resistance heat, or one running a heat pump that leans on backup strips through the coldest weeks, pays much more per unit of heat and gets proportionally more back from the same window.
Air conditioning matters less here than people assume. The temperature difference across a window in summer is far smaller than in winter, so the same U-factor improvement buys much less in August than in January. Fuel prices also move, and they have moved hard in both directions in recent years, so treat any payback figure carried out to a decimal point as a sales exercise rather than a forecast.
How long you intend to stay
Whatever argon returns, it returns in small annual installments across the life of the seal. Twenty years of small numbers adds up to something real. Three years of small numbers does not. It is one of the few window features whose value is genuinely a function of how long you stay, and it is worth being honest with yourself about that before a salesperson does the arithmetic on your behalf.
The counterweight is that because the fill usually costs nothing, a short stay is not a reason to refuse it. It is a reason not to pay for it. Resale is the other half of the question: new windows do help a listing, but what a buyer or an inspector registers is the frame, the operation, and whether any glass is fogged. No appraisal or inspection report distinguishes an argon-filled unit from an air-filled one.