We all know windows are cold in winter and do a poor job of keeping the heat in. Unfortunately, the calculations that show the potential savings for new windows are dismal, with an extremely long payback period.
We are also aware that all too often an audit will over estimate the potential saving, all be it, for many reasons. Well, my window question is, are we adding to this problem by over stating the initial heat loss through those windows?
I'm sure whole house heat loss calculations have been verified against actual energy bills, but that is essentially averaging the highs against the lows. If we look at just the windows and apply the typical heat loss calculation along with the typical HDD, we are assuming an inside temp of 70° (base 65°). But those windows are actually much colder, thus, the actual heat flowing through them is much less. To add insult to this over estimate of heat loss, once the new windows are installed the surface temp increases. Now our over stated savings for new windows shrinks as the new higher surface temp actually increases the delta T. Despite modestly increasing the r-value of the window we now lose a portion to the increased temperature drive. That take back actually occurs throughout the house anywhere insulation levels have been increased.
The bottom line is, we over estimate the heat loss from our windows from the start and then the window temps increase to steal a significant portion of our savings. R-value goes up 50% while the delta T increases (just guessing) 25%, that would be significant.
The numbers are just a guess of course, but they illustrate the race condition between improving the r-value while increasing the temperature drive. But surface temps of windows during cold spells can easily drop to 40° or lower while our calculations are still using 70°.
Here are some numbers. I will post the related math if someone is interested.
18 windows in my 7500 HDD climate, U= 0.5 and total area = 180 ft²
The typical calculated heat loss would be 16.2 mmbtus (million BTU's).
Replace those windows with U= 0.25 windows and the heat loss is cut in half to 8.1 mmbtus with an apparent savings of 8.1 mmbtus.
However, the initial heat loss was never that high. From this link, https://www.energyguide.com/info/window2.asp , at 0° outside we should expect an inside surface temp of about 44° on a simple double pane unit and 56° on a more high tech unit.
Factoring in the lower surface temp on the simple double pane window it would cut the heat loss approximately in half. Therefore, the initial heat loss est should have been 8.1 mmbtus. Now recalculating with the new windows but allowing for the increase in surface temp, the new heat loss becomes about 5.9 mmbtus. Thus the savings are closer to 2.2 mmbtus and not the 8.1 mmbtus typically estimated.
Ouch! Those windows that were hard to justify on a savings basis just became impossible. Comfort, yes. but payback will take a very long time.
This take back due to changes in surface temp occurs throughout the house as we improve the insulation. It will take a sharper pencil than mine, but it does account for part of the shortfall when projected savings do not meet expectations.