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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.

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    There is also Heat gain from the Sun during the day even when it is cold outside potentially heating up floor concrete which gives back its energy back over time. All this has to be considered as well. – Ride Sun Nov 25 '19 at 17:34
  • Additionally, heat gain can be greatly mitigated by use of awnings and/or functional shutters. – peinal Nov 25 '19 at 18:36
  • There's R-10 windows - architectmagazine.com. Then you'd just have to compute the RoI on a window that's +40% more efficient and probably ten times the cost and needs more than a month's notice... Windows the big heat loss? Until you've air-sealed the house, no matter what it is, R-value is a negligible part of the equation. – Mazura Dec 5 '19 at 3:56
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Using the most energy efficient type of window will help.

Double hung and sliding windows are made to fit a little loose so sashes can slide past each other.

Use an awning or casement window that has a latch that pulls the sash in tight AND locks it tight will have less air infiltration, etc.

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One option is to use honeycomb-style insulating blinds. The downside is that they block the sun during the day so you don't get any heat gain. If you use them on the dark side (s) of the home they definitely help with heat loss. Depending on the amount of sun and quality of the windows, if you open and close them regularly as needed on the sunny side(s) they can help there too.

I've found these blinds to be helpful with older windows that were either single pane or had bad seals. Blocking drafts is the first priority, though.

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You may not be able to get rid of the windows entirely as they serve as egress (fire escape); you need at least two.

If the sun shines through the window, you wouldn't have net heat loss during the daylight hours -- it would be adding heat to the home (assuming a properly sealed/draftless window).

At night, insulating blinds or heavy insulating curtains would help. Insulating curtains need to close all the way to the ceiling (or box), floor, and sides. Often they have some weights sewn into the bottom and magnets in the sides to hold the two halves together. The purpose of all that is to minimize air currents (i.e. convection).

To minimize unwanted heat gain in the summer, you could install an awning outside. An appropriately sized and placed awning would allow the sun to shine inside in the winter but not the summer, as the sun in higher in the sky in summer.

You could also install insulating shutters outside.

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