[MN Renovator]

I was walking around the house and measuring all of the windows because I want to install white single-cell shades for my windows. The weather has been mild and I made a pizza, french fries, and brownies in the gas oven early morning yesterday, so the humidity in the house was high. I found that multiple windows in the house had condensation between the panes from the indoor humidity. I'm not sure if it is just a leak from the inside of the house to the air gap or if there is a separate leak on the outside too that might be leaving an air bypass.

I realize that windows are the worst part of the envelope as far as convective heat loss/gain since their insulation value is so low. I've been doing a bunch of retrofits to my house such as air sealing, capping a vertical knee wall(rigid foam sprayed foamed in), adding insulation to that same knee wall, and will be pumping more cellulose onto the ceiling facing the attic as soon as it is warm enough to do that. The heat loss in the house was dramatically lower last year and this year with the mild winter my January 2012 heating bill is 1/3 of what it was in 2010. So far my cost to insulate and air seal (roughly $300) has seemed to already paid for itself during the heating seasons.

The windows in question include the following:
4 foot wide by 4.5 tall picture window(non-
2 foot wide by 4.5 tall casement(swing out) windows to the sides of the picture window
The two octagon windows in the master bedrooms, 2 feet I think, didn't measure those though.

The rest of the windows in my house are sliders and none of those seem to be leaking into their air gap. Whether these are losing more heat or not, I'm not sure if it is worth replacing them as they aren't rotting out yet(27 years old but seem to have remained well painted so far). My house is able to remain passive on sunny days where the inside and the outside temperatures are about 15 degrees difference for the high temperature of the day and about 30 degrees between setpoint of the overnight low temperature. So if the setpoint is 65 degrees outside and the nightly low is 35 and the day high is 50, the furnace wouldn't run at all and the inside temperature will be 70 with the sun and drop 5 degrees overnight. I keep my house on the cold side and wear a hoodie inside and on I work 10 hour shift and usually I get home and don't bother to turn up the deep setback, so even if its colder the furnace often doesn't run. I can comfortably sleep at very low temperatures under the covers. December and January heating bills were under $75(combined) this winter(about 15 therms so far February), last winter the highest methane gas bill was a little under $70. 2200 sq ft quad-split.

I'm trying to do a load test(on paper) and compare it with my actual load tests and be able to set up hypothetical situations on how much fuel would be saved and do more cost predictions on things like heat pumps, or a decision of going for an 80% or a 90%+ 2-pipe condensing setup. It's a little odd to think about but at this point hanging on to an old 76% atmospherically vented gas furnace and I'm having a hard to justify replacing for a 90+ unless this ones fails. Usually a conventional double-pane is considered 0.5U. My question is, what sort of U-value should I expect now that the windows seem to be breathing the indoor air?