Every building material resists heat flow in some manner. Glass does it poorly, fluffed insulation does it really well.

The ability of a material to resist heat flow is called its R-value. When I recommend ugrades, such as adding insulation to the attic, I use software that generates a payback time for the upgrade, taking into account the R-value and cost of installation. It may cost a thousand dollars to insulate an attic, but if it saves a family $300 per year in energy costs, the payback is a trifle over three years.

Windows often have a long payback. They are expensive to remove and put in, and the R-value of the very best windows is still lower than the R-value of your average 2×4 wall with fiberglass batt insulation. Windows are given a U-value, which is simply 1 divided by the R-value. A U-value of .50 is an R-value of 2. An old single pane window might have a U of .60. A brand new triple pane vinyl framed xenon filled window may have a U-value of .20. So, for a thousand bucks a window you go from an R-1.6 to an R-5. Sure, that’s 3 times better, but the plain old wall on either side of the window is an R-11.

My recommendation is to spend money where it is the most effective. There is a place for new windows, and certainly replace them if they are cracked, or you are doing a major wall remodel anyways. I would lay it out thusly: First, focus on blower door guided airsealing, including the duct system. Second, insulate the attic. Third, insulate the walls. Fourth, insulate the floor. Fifth, replace the windows. Airsealing and insulation are not as sexy as a new window, but they are a much more effective use of your money.

So back to the original question, would I replace that window? Trick question! The window shown is on an unconditioned porch, outside of the pressure and thermal boundary. Note the 4′ on center posts, the tongue and groove ceiling, and the door trim just visible in the back of the room. I know, I’m a jerk.