Why doesn’t the Generic Window Air Gap yield thermal properties?

I don’t understand. I see plenty of properties in your screenshot.

Top is “Wall”; bottom is “Window”

Sorry Chris I was out walking so a bit short with my response.

The upper algorithm decomposes the “Generic WALL Air Gap” material and does indeed display thermal properties.

The lower algorithm decomposes the “Generic WINDOW Air gap” material and does not display thermal properties.

If the Window material is just an oops and I can use the Wall material that’s fine. Its just that that the emissivity of materials surrounding an air gap can affect the final R-value and didn’t know if there would be something special baked into the Window Air Gap material to account for the fact that it would be sandwiched by glass.

In any event I have unusual circumstances that don’t really resemble a modern 1" IGU. I want to run an energy calc on my 1895 vintage house which has original single pane double hungs, over which I constructed traditional tilt out storm windows…so there’s something like a 5" gap and probably a decent amount of infiltration into the gap; definitely not still air.

This question is still a little bit unclear but I guess I’ll just assume that when you say “thermal properties” you are talking specifically about R-value and not other properties like the thickness of the window gap or the gasses within it.

You are correct that the R-value of a window air gap cannot be known without knowing the properties of the glass panes on either side of it (along with the temperature differential across these two panes). The emissivity of the glass panes on either side affects radiant heat transfer across the gap (as you mention) and a higher temperature differential between the two glass panes will result in more convection of the gasses within the gap.

So that’s why you can only get the R-value/U-Factor of an entire Window Construction and not an individual gas gap material. That’s also why the calculation of U-Factor for windows requires the specification of “standard wintertime conditions” since this is needed to ultimately determine the temperature differential across the gap(s).

That “Generic Wall Air Gap” is a no-mass opaque material that can only be used in opaque constructions and NOT window constructions. As you can see, this opaque material is a lot simpler than what is needed for window constructions since the insulation from gaps in opaque constructions makes up a lot less of the total wall R-Value.

Thank you for taking the time Chris.