Couple of thoughts:

- The other factor to consider here is the use of insulation to protect against moisture in the structure, which will deteriorate materials and decrease the thermal resistance of the structure. Looks like the comment section gets into this.

- I think it's mistake to not insulate the floor. Rule of thumb is soil underneath building is 2C less then average indoor space temperature[1] - for commercial buildings, probably more residential since there's less equipment. So there will be heat loss. More importantly, the ground consists of wet soil with moisture that wants to evaporate up into the structure. Insulation is one of the crucial components of preventing this (in addition to proper site drainage, and air/vapor barriers).

- I think this kind of steady-state calculation is great for a quick back-of-envelope calculation like this. But, for the sake of completeness, I want to mention that heating is a dynamic process with multiple modes of transfer, and not modeling it that way means we're neglecting the following impacts: air infiltration/ventilation that changes with pressure differences, solar radiation, heat generated from people and office equipment, latent energy impact of humid air, and the changing resistance associated with air films on surfaces. Thermal mass is also one, but is already mentioned in the original post.

[1] https://bigladdersoftware.com/epx/docs/8-3/auxiliary-program...

ETA: Now that I've thought about it some more, I think there is one possible source of error here that can really throw off even back-of-envelope calculations like this: accurately calculating the thermal resistance of the windows.

Specifically, one underlying assumption in this calculation is that the R-value for each material is uniform, even though in reality a wall (for example) consists of different materials with different heat transfer rates (i.e think of heat through a cross-section of the wall cut through a stud versus a non-stud area). The manufacturer=provided R-value for assemblies will incorporate the thermal bridging of studs, and fasteners, but won't account for the impact of thermal bridging at wall/roof/floor intersections and at windows. The thermal bridging impact is going to be negligible for wall intersections for a simple rectangular prism-type building, but the impact of window thermal bridging should really change the window thermal resistance.

Which in turn may really change the ROI impact of insulation calculated here, since windows are basically open holes in your thermal envelope, and even a slight reduction in its performance can reduce the efficiency impact of wall insulation. In my experience it's more common to go for higher-performance windows (double/triple-pane) before going for wall insulation increases in energy retrofits.

Assuming this isn't already incorporated, it can be approximated by looking it up in the window manufacturer specifications. They usually provide edge-of-window and center-of-window U-values (reciprocal of R-value, more appropriate for heat transfer in parallel constructions) that can be used to calculate a new area-weighted R-value for the window (i.e assume 2 inches offset from window frame for edge-of-window U-value).

I would also look up window air film coefficients, since there's a stagnant film of air on surfaces that can add about about 1/3 increased resistance to double-pane windows. R-values typically don't include film coefficients.