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Lifecycle Cost Difference Between Single-Pane and Double-Pane Windows

big__o | Posted in General Questions on

our house will have a wall of windows- south facing.
the goal is to put in site-built single pane fixed windows with very high shgc and visibility.

Ive seen houses with 40 yr old single pane windows.

My consultant thinks the energy hit is too great, even though Ive told him there will be movable insulation behind the windows. he thinks that double pane windows, even if they have to be replaced every 20 years due to fogging, is cheaper in the long run.

what say you?

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  1. charlie_sullivan | | #1

    Partly depends on climate, but I think double-pane will be worth it. Modern high-quality double pane should last a lot longer than than 20 years, and with low-e and argon, the comfort benefit alone will be worth it, nevermind the cost savings. You probably aren't going to want to put the moveable insulation in even on a cloudy day.

    1. big__o | | #2

      oh, I totally forgot to include climate. zone 3A?(humid) approx 2500 heating degree days and 2500 cooling degree days.

      How long are the double panes lasting now?

      1. Expert Member
        BILL WICHERS | | #5

        It won’t make any difference. Internal fogging of double pane windows is due to seal failure, nothing else. Modern seals hold up pretty well. If you’re really worried, I believe Anderson is offering their welded seam double pane windows again that have no seal — the glass is thermally welded around the perimeter so there is no seal to fail. Those welded glass windows will last until you physically break the glass.


        1. big__o | | #8

          That's great to know Bill. Thanks. It also sounds like it would be way out of my price range for more than 400s.f of fenestration.

          Maybe I could do an acrylic window? Better u value, and better vlt.

          1. Expert Member
            BILL WICHERS | | #9

            You could try acrylic, or polycarbonate (which is much stronger). Either of those could be built with a solvent weld around the perimeter so you wouldn’t have a conventional seal. I don’t think you’d see a huge difference in U values compared to glass since most of the insulating work is done by the air layer between the panels and not the material the panes themselves are made from.

            The downside to using acrylic or polycarbonate is that they are both MUCH easier to scratch, which makes it harder to clean them (and some cleaners, particularly those containing ammonia, will cause hazing). It is possible to polish out scratches though. There is also a risk of yellowing over the long term. I don’t think you’d be able to get any low-E coatings either.

            Personally, I’d just go with regular glass IGUs here. I doubt you’ll have any issues with seal failure. You could always shop around for the longest warranty you could find to limit your risk too.


  2. jackofalltrades777 | | #3

    Single Pane R-1 windows are not even code legal. If you are getting permits, the inspector will not allow single pane windows to be installed. If it comes to selling the home later, it will be a huge negative to the future homeowner/buyer.

    At night the heat loss during winter will be great on single pane windows and condensation issues on the interior will be bad on cold nights. When warm 68F interior air hits that 10F glass surface, instant condensation and a wet mess on the wall and floor underneath the windows every morning. Not to mention, the moisture on the windows will also freeze overnight and then in the morning you will be trying to look through a 1" thick chunk of thawing ice on the windows. What a disaster.

    Just install double pane windows with a high SHGC. No point in installing outdated single pane R-1 window technology.

    1. big__o | | #6

      You make good points. I don't know if condensation would be a problem so that's something I do have to look into.

      Another problem is it's very difficult to get high shgc double pane here. Everything is lo-e with .4x vlt and <.3 shgc and I'm specifically looking for the heat gain of high shgc windows.

  3. joshdurston | | #4

    Good windows are also about comfort. Not everything can be boiled down to lifecycle costs and payback unless that's all you care about.

    Unless the insulation is also a vapor barrier and air tight, insulating in front of the windows will be a condensation nightmare.

    Locally, people used to say it's not worth it to insulate your basement based on the energy savings, but ignored the fact you might actually want to spend time in your basement.

  4. big__o | | #7

    Good point. But I would argue that a single pane window with an r5 panel behind it would be superior in the comfort department to an r2.4 double pane window

    1. Trevor_Lambert | | #12

      The difference being, you can see thr0ugh the double pane window and you can't see through the R5 panel. You can do a lot better with with a double pane than R2.4, I would hope. Triple panes are typically R7 and up, which beats even your single+panel, plus you can see through them all the time.

      1. big__o | | #13

        once it gets dark(and cold) visibility is zero in either case.

        hmm. I havent seen r7 triple pane windows. even if they existed the cost would not be justifiable, im sure.

        I priced out windows from Europe and the cost would be about AT LEAST 10x the price of site built single panes

        1. Expert Member
          BILL WICHERS | | #18

          R7 is approximately equivalent to U 0.14, which IS attainable with a triple pane window in one of the larger (ideally 1-3/8") IGU thicknesses. They are more expensive windows, but not so expensive as to be completely out of reach of everyone. You don't need to get European windows to go this route, either, most of the Canadian manufacturers have no problem making full depth triple pane windows, and I'm sure some American manufacturers can too. Alpen certainly does, although they do it a slightly different way with their "heat mirror" films.

          You'll have other issues with insulating panels inset into the windows at night. You're unlikely to ever get those well sealed, which means you're going to get freeze issues between the window and the foam as the mositure in the air condenses out on the single pane glass and freezes there.


  5. Patrick_OSullivan | | #10

    > the goal is to put in site-built single pane fixed windows with very high shgc and visibility.

    Why are you looking for high SHGC in CZ 3A?

    One article to take a look at:

    1. big__o | | #14

      thanks for the link.
      our feeling on the topic is high shgc gives the most heat gain possible, and can be insulated when it gets dark, to get the best of both worlds

  6. gusfhb | | #11

    Single pane glass is silly
    If you shop around you can probably get double pane for the same price as custom cut single pane.

    Modern seals last decades. The cheap replacement vinyl casements in my house are at least 20 years old and while one physically broke, none have leaked

    In cold weather you will be so uncomfortable you will not want to be near those single panes

    YOu know what is better than single pane and foam?

    R6 COG triple pane

    The Cardinal glass with the interior coating reflects heat back at you so that when you put your face near it on the coldest day it feels warm, like foam insulation does

    1. big__o | | #17

      well I'll be....
      you are right patio glass igu (CLEAR) is less than the cost of custom cut single pane.

      I cant find that price in my area but a 7 hour drive will get me $85 per unit in 34x74.
      I have a 36x93 RO but Im sure I can figure that out.


  7. walta100 | | #15

    If you want to understand the long term costs build a BEopt model and enter your costs per square foot of each flavor of window and you will get a real answer to the dollar question.

    The comfort question is a no brainer if it ever gets close to zero where you are. Your single pane window will condense tons of water and destroy your trim. Also the cool surface temp of the glass will take all the humidity out of the air in your house. Living at 25% humidity is not pleasant.


    1. big__o | | #16

      the condensation is something I havent looked into. design temp is 22 degrees but it rarely gets below freezing

      btw, thanks for the rdh link again.

      1. Expert Member
        BILL WICHERS | | #20

        Condensation will occur any time the interior surface of the glass (regardless of if it's a single or multi pane window) drops below the dew point of the indoor air. It's not difficult to calculate, so if you know your indoor temperature and humidity levels, you can calculate when you'll get condensation by knowing the type of window you're using and the outdoor air temperature. If you want to look into this, that's the only info you need to run the calculations.


  8. jackofalltrades777 | | #19

    You're in a Zone 3A climate. Why do you want the highest SHGC? You realize you will overheat the house in the spring and fall and maybe even summer if you don't have large overhangs to protect the south facing windows.

    I think you are trying to reinvent the wheel here when there is no need for it. Just get good quality double pane windows and call it a day. Some companies offer lifetime warranties to the original homeowner. You will be dead and gone, long before the window seals fail.

  9. big__o | | #21

    thanks Bill. Indoor rh stays at or around 40% in the winter, and indoor temps are usually 70 degrees. I looked up some charts and it looks like at an outdoor temp of 30 something degrees Ill start to see condensation?

  10. Expert Member
    Dana Dorsett | | #22

    If the existing windows are reasonably tight wood sash, adding a tight exterior LOW-E storm window would bring performance up to U0.35 or lower, pretty close to the IRC's current code-minimum of U0.32 for zones 3 & 4A or B (but not 4C, which requires U0.30 or better.)

    The upfront cost of a low-E storm is a FRACTION of the cost of a code min replacement window, but still more than the cost of a clear glass storm window. Despite the upcharge for the hard coat low-E, it will "pay off" more quickly in energy-dollar terms than clear glass, in both heating and cooling climates (and balanced climates such as 3A.) Larson's storm windows sold at box stores aren't the absolute tightest in the industry (that still goes to the northeastern regional player Harvey) but they're still not bad, and widely available. There are others.

    A single hard-coat low-E on surface #2 (such as the inward facing surface of the storm window), cuts the solar gain by slightly more than half, but it turns the window into a net energy gainer during the winter. From an energy use point of view this is more effective than moveable R5 panels, unless you're (actively removing them near dawn and reinstalling them after dusk.) For south facing windows with any sort of roof overhang (or awnings) to shade the mid-day midsummer sun) this is pretty optimal for energy savings. From a comfort point of view <U0.20 triple panes (or argon filled double low-E double panes with a second low-E coating on surface #4) still have a comfort advantage when standing next to the window on the coldest dark hours of the winter.

    Unless you're doing a serious deep energy retrofit or trying to hit more serious efficiency benchmarks it's hard to beat the comfort & economics of low-E exterior storm windows.

  11. big__o | | #23

    "A single hard-coat low-E on surface #2 (such as the inward facing surface of the storm window), cuts the solar gain by slightly more than half, but it turns the window into a net energy gainer during the winter."

    Dana, during my online searches Ive seen you mention this before. What resource are you using to come to this conclusion. Based on my rough calculations, the heat gain from a high shgc window will not be less than the heat loss, in my climate.

    Im really curious and anxious to see your resources as if what you state is the case, then it changes a lot for me(which is a good thing)

    This is new construction btw.


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