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double glazing revisited, historic windows, Connecticut winter.

MKCF | Posted in Energy Efficiency and Durability on

The certain eventual failure, and high price tag of IGU’s makes them very unsavory to me. They also look very ugly compared to beautiful old wood windows, I think. Even the best commercially available insulated windows with divided lites do not have the right proportions to match the very narrow muntins of old windows, often as narrow as 5/8″. The depth is also too shallow from the top of the muntin to the face of glass- just doesn’t look right on an historic New England home.

If one happens to be a professional millworker, which I am, one can address the issue of the looks by making one’s own tall skinny muntins, hiding the wide and ugly glazing seal inside the perimeter of the sash so you don’t see it (just like this guy does: But that still does not address the issue of longevity. I don’t want to have to go through the whole process every 20 years or less, when the seal in my glazing fails, which we all know it will.

Why bother, you ask, when you could just add a storm window? I’m speaking here about divided lite glass doors (French doors). They are really beautiful and everyone loves them, but they are problematic. Storm doors are one solution, but they are without doubt a real pain when you’ve got armfuls of groceries: you have to hold one door open with your butt while you open the second door. Don’t you hate that?

Therefore: how about adding a second pane of glass with a spacer in between, but making no attempt to seal it? I can’t find any information on this system, how well it performs compared to single glazed windows with storm windows for example. And if we’re going to go that route, what’s the best way of stacking up the panes so you don’t get condensation?

When I was gutting my old house, I found some replacement windows which had amazing glazing: two layers of glass, fused together around the edge with yet more glass. The sightline was very narrow, compatible with shallow rabbets and skinny muntins. I looked it up, and found that this was the original Thermopane, the factory now shut down. Maybe we can bring this system back. I, for one, would gladly sacrifice a small amount of thermal performance in exchange for good looks and longevity. How do you make that stuff??

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  1. user-2310254 | | #1
  2. Expert Member
    Dana Dorsett | | #2

    Unsealed double low-E (coated on surfaces #3 & #4) double panes can have reasonably good (<U0.30) performance, but the tightness of the interior pane would have to be VERY good to prevent fogging.

    The Holy Grail solution is still vacuum insulated glass, which can be very thin and very high performance, with welded glass edges.

    A removable large low-E exterior pane applied over the entire exterior of the glazed section of the door may be a solution for someone with millworking skills to make it reasonably aesthetic, with the ability to remove it for cleaning. Tight low-E storm windows over typical clear glass wood sashed single panes typically raise performance to the U0.30-U0.35 range.

  3. MKCF | | #3

    Steve: thanks for that rant, I couldn't agree more with Mr. Buell! A note on one of the comments, "HD camera and 60" LED screen"- hilarious. But in all seriousness, a house needs windows, because of the beauty factor. Even Joe Lstiburek says beauty is important.

    Dana: Putting some numbers on this concept is a great place to start. I do question the aesthetics of a single large pane covering up all my pretty muntins, but you are right it would be easy to clean. Some questions: 1. Would condensation be an issue in this situation? 2. Why a single pane? Why not individual panes fitted into the divided lites, sealed to the weather on the outside but allowed to breathe on inside? Wood certainly has better insulating qualities than glass, so thermal bridging shouldn't be a factor here, correct? Would this change the numbers?

    Any info on Pella's "removable interior panel"? Is it basically an interior storm? Can't find a diagram.

    A while ago I made a cedar door for a sunroom. It was a divided lite door with single pane glazing and wood stops, very traditional. I think I used Dap caulk on all the wood stops, inside and out, to seal the glass. I followed this up with a wooden frame on the inside of the door which was held in place with brass screws, a rabbet held a large acrylic sheet against the inside face of the door, which covered up the entire glazed section. Dana I think this is what you are picturing.

    10 years later it was time to re-varnish the sunroom (originally done with Sikkens, don't get me started) and I revisited the door situation. I removed the frame of acrylic because it looked awful, an ugly sheet of plastic covering up the nice wood. So, I added a second layer of glazing within each lite. I took out the interior stops, then I fitted skinny wooden spacers in where the stops had been (3/16" square I think) and a second layer of glass, then put the old wood stops back in. Unfortunately, I sold the house a year later so I can't report on how this system performed, but I will say it looked very nice for about a year. My guess is that caulk in between all the layers wouldn't change the performance much since it's clearly not an airtight system. Maybe just a small bead, or caulk on three sides only? Has anyone else done this? Maybe Azek spacers in between the panes in case moisture does get in there? Weep holes in the bottom stop?

  4. jkstew | | #4

    I'm kinda liking the LED screen instead of glass idea. Not only is it safer, greener, and more private, but you could change your effective location with a different video feed.

  5. user-2310254 | | #5


    It would have to be 8K OLED. ;-)

  6. Expert Member
    Dana Dorsett | | #6

    If the double panes need to "breathe", let it breathe to the outdoors. Indoor air getting into the space between the panes will condense on the cold exterior pane.

    Doing individual exterior low-E panes for every light would work in theory, but it's a lot of detailing to get right. In theory practice makes perfect, but in practice... let's just say perfection doesn't really exist.

    Also note, the R-value of the muntin bars will be substantially less than air-filled double-pane with a low-E coating on surface #2 (the inward facing side of the exterior pane.)

    Interior storm windows exist from numerous vendors. A low E coating on surface #3 (the exterior facing side of the interior pane) helps, but for best performance you want the low-E coating on surface #2 (the exterior pane).

  7. MKCF | | #7

    Thanks again for the thoughtful response!

    If the panes are allowed to breathe towards the outdoors, how come you're not worried about moisture getting in between the panes from outside (rain, thick New England fog)? Wouldn't that be just as bad?

    I also suppose it follows that regular windows with interior storms suffer from condensation too, huh?

    And exterior storm windows would suffer from condensation on their inner face as well. They just happen to be easier to fix or replace than the windows themselves.

    Or, do "properly sealed" storm windows and "properly sealed" single pane windows work great without serious condensation issues? If this is the case, then it's also best to try and seal the glazing layers of individual panes. Right?

    What about failed IG units that have been "fixed"? My understanding of this is, someone comes along and drills a few tiny holes in the seal, edge-wise. The whole point of this exercise is to eliminate condensation, correct?

    Does it work?

    If so: what are the specifics. Are there secondary exit holes towards either the inside or the outside of the window, or not at all (drill a few tiny holes in the seal, seat the unit back in the sash, let the air find its own way out somehow)? In other words, a "mostly sealed" but not "all sealed" approach. What materials are used? What's the longest lasting gasketry or caulk? Poly felt?

  8. morganparis | | #8

    I recall a conversation some years ago with a National Park Service architect, she told me their standard of practice at the time for thermal upgrades to old windows in historic buildings was a single sheet of glass over the whole interior. Least obtrusive, easily reversible should restoration to original condition be desirable in the future. Of course this makes the window inoperable, a nonstarter in rooms with egress requirements (this is also rather a problem with the OLED approach!). It may be worth contacting your state historic preservation organization for advice on their current recommendations.
    As far as replacement sash are concerned, we do a fair amount of Historic District work, and often our clients (and sometimes the regulating authority) will ask for true divided lite IG units expecting them to be the most authentic looking. I point out the ugly wide flat muntins and steer them toward SDL (simulated divided lite) which is available with classic 5/8 muntins and having only one pane unit per sash instead of nine or more reduces the opportunity for future seal failure significantly. These units have the muntin pattern adhered to both the outer and inner faces of the unit as well as between the panes, making the look about as close as you can get to a traditional window in IG.
    Of course there’s also a case for simply rehabbing old rattley windows with upgraded seals without any glass 'improvements'. There was an excellent article detailing this approach in Fine Homebuilding a few years ago.

  9. Brad_S | | #9

    "I don't want to have to go through the whole process every 20 years or less, when the seal in my glazing fails, which we all know it will."

    My plan for everything I do in my house is limited by 20 years from 2017. That's as long as anything I do needs to last because it's going to get replaced.

    Robotics will outperform humans in every physical task within twenty years and they'll be preferred -- i.e. significantly less expensive. Similar to Teslas, custom homes will be built in factories by robots. Those homes will then be shipped and assembled by robots (think pre-cut framing, but for everything.) In terms of custom construction and quirkiness, a victorian built in 2037 will surpass a pristine original victorian in every regard. They won't have old growth lumber, but they will have LVL or some type of composite material. People won't think about replacing windows, they'll think about replacing their entire house.

    If you can make something you like last for 20 years do it and don't worry about ever doing it again.

  10. Expert Member
    Dana Dorsett | | #10

    "If the panes are allowed to breathe towards the outdoors, how come you're not worried about moisture getting in between the panes from outside (rain, thick New England fog)? Wouldn't that be just as bad?"

    Absolutely NOT!

    As a rule it's only foggy out when the outdoor temperature & dew point are the same. Instances of fog when the outdoor temperatures are ABOVE the indoor temperature is extremely rare.

    Condensation only occurs when the temperature of the glass is at or below the dew point of the air between the panes.

    The exterior side of the interior pane facing the trapped air tracks the indoor room temperature since the other side of the pane is pretty much at room temperature. Similarly, the interior side of the exterior pane tracks the outdoor temperature.

    In the winter the dew point of the conditioned space air doesn't go over 35-40F (unless the house is actively humidified, or under-ventilated). If the air between the panes is "breathing" indoor air the exterior pane will often drop below the dew point, and condensation will form on the exterior pane. If the space is breathing outdoor air, the outdoor dew point is ALWAYS below the outdoor temperature (or at most exactly the outdoor temperature, if it's foggy), but both panes will be slightly above the outdoor temperature in winter (even during rime-icing fog) due to the heat loss moving through the glass, and condensation doesn't form. There are MANY hours of the winter when the temperature of the exterior pane will be below the indoor air dew point, and significant moisture can accumulate if indoor air is leaking into that space.

    During the summer the outdoor dew points rise, but days/hours with an outdoor dew point above 75F are few in CT, and even if you air condition the house to 75F the exterior side of the interior pane (and the entire exterior pane), will be above 75F due to the heat flow from the outdoors. You'd only be at risk of summertime condensation in the trapped air space is if the house is air conditioned to <<70F. The average outdoor dew point during the 8 most humid weeks of summer in CT is ~65F, so moisture accumulation from air leaks to the outdoors simply won't happen unless the indoors is kept downright chilly in summer.

  11. peaceonearth | | #11


    I have had (still do) some of the Pella windows you asked about. I'd say it really is an inside storm sash, clipped in against a rubber or vinyl gasket all around. I had a large non-opening window that always had condensation issues when cold. I eventually drilled additional weep holes thinking this would help vent and clear, but did not solve the problem. The weep holes are angled to the outside and are really not susceptible to water entry due to the way they are located. The double hung I have did not suffer from condensation much (not sure why the difference), but they are ready to be replaced for other reasons.

  12. MKCF | | #12

    More good information, as always, many thanks!!
    Howard: thanks for the report, and the construction details. Dana, it sounds like Howard's information confirms that you are totally right: the venting in a dual-glazed system should be towards the exterior. His fixed window, being sealed to the exterior, had condensation issues, while the movable window, which probably allowed air to exit due to leaks in the weatherstripping, did not. Apparently the weep holes he added to the fixed window were not sufficient. Any idea why? Should we take this to mean that exterior glazing in non-IGU dual-pane systems should be super breathable, and not just moderately breathable?

    What about doors? I seriously doubt that even the best weatherstripping will prevent all air movement between interior and exterior. Therefore it follows that a storm door would suffer from condensation on its interior face. Correct? Especially on a door that is not used often, given that most people tend to use just one of the doors in their house most of the time. True for me at least. Or, trust that the air sealing on doors is bad enough that condensation won't be a problem? Double glaze the interior door and add a storm door? Double glaze both doors?

    A separate topic: is there a weatherstripping system that everyone agrees is the absolute best? I saw a video in which Tom Silva installed a bottom seal on a door which extended when the door was closed, and retracted when it was opened. That looked pretty slick...

    Thanks and more thanks...

  13. Jon_R | | #13

    Note that walls and ceilings have similar issues - effecting where and how much moisture is deposited by ex-filtrating air (when heating). Where the air barrier is/are matters.

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