I am planning an addition and trying to compute the insulation design, and it seems like windows are where all the heat is lost. For example, thermal triple paned glass only has an R-value of about 2.3 compared to 18 for 6 inches of mineral wool. If you consider that that is just the R-Value for the GLASS, and that double hung windows seem to lose most of their heat at the seams, the value could actually be a lot lower than 2.3.
Other than making the addition windowless or reducing the size of the windows, is there anything I can do?
On a related note, I am planning to use commercial-style sealed windows, so I have no issues concerning heat leaking past the cracks in the window.