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Energy Solutions

Insulated Storm Windows?

Can top energy performance be achieved by combining fairly standard windows with really good storm windows or even a second set of prime windows?

A low-e storm window at the Brattleboro home of my colleague Peter Yost.
Image Credit: Peter Yost

I’ve done a lot of digging into window options in the past few months — not only for a special report on windows that BuildingGreen published, but also for the renovation of the early-19th-Century farmhouse that my wife and I recently purchased.

The state-of-the-art with windows in terms of energy performance and quality is clearly seen in the triple-glazed European windows that are certified by the Passivhaus Institut in Germany. You can now buy these wonderful windows with unit center-of-glass R-values above R-9 yet high-enough light transmission to work well for passive solar houses (solar heat-gain coefficient above 0.60).

The problem is that these windows are incredibly expensive — some over $100 per square foot, which comes to $1,500 for a typical 3′ x 5′ window. You get a lot for that price in the way of top-quality materials, construction detailing, durability, thermal breaks, air tightness, and energy performance, but the windows are simply way above the budget range for most projects. Furthermore, replacing existing windows is often very hard to justify unless the existing windows are in very poor shape.

Insulated-glass storm windows?

As I’ve thought about ways to obtain top-performance windows more affordably, I keep coming back to the idea of installing (or keeping) fairly standard, double-glazed windows — with low-e coating and argon gas-fill — and then installing a much-better-than-usual storm windows.

I’m considering three different options. The first option would be to install a single-glazed, low-e storm window on the exterior of the prime window. The storm would have to have a durable (hard-coat) low-e coating, since it wouldn’t be protected in a sealed air space, as most low-e coatings are. With this configuration I would end up with triple glazing and two low-e coatings in the full assembly. I’m not sure whether these would be old-style storm windows that are installed and removed seasonally, or more sophisticated (and convenient) triple-track or double-track storms with screens.

A second option would involve an insulated (double-glazed) storm window, so that I would end up with quadruple glazing — two insulating glass units (IGUs) separated by an air space. Because no storm window manufacturer (yet) makes such a product, this would likely necessitate custom-made storm windows. I know those can be made, because I’ve seem insulated-glass, low-e storm windows that J.S. Benson Woodworking & Design has produced in the shop next-door to our BuildingGreen office in Brattleboro.

A third option would be in install two sets of insulated-glass prime windows. I’m still working through how this would work — and very much open to suggestions. I could install a double-hung window on the interior and a casement window on the exterior, or I could install two sets of double-hung windows.

I’ll have to think about cleanability of windows with these options; that might be a little challenging with the two prime windows. If I’m going to have two low-e coatings in the window assembly, I also need to figure out whether the temperature reached in the interior IGU might get too high. By trapping a lot of heat it’s possible that the temperature reached by the glazing seal could be higher than the sealants are rated for — in which case I might need to find a window made using silicone seals (as I believe Andersen uses).

If you have experience with such a detail (particularly options two and three), let me know what you did. Or let me know what your thoughts are even if you haven’t tried it. You can post a comment here and share your experience with others or, if you prefer, e-mail me directly.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.


  1. Richard Baumgarten | | #1

    storms rule
    Storm windows are underappreciated. Instead of windows they should be thought of as transparent siding, a giant shingle that gets properly lapped into the cladding...No sealants required. Now that the storm window is redirecting water, you can use any type of window you want. Want to go green, go with wood double hungs, or prime aluminum windows without vinyl thermal breaks. Put in as many panes as you like depending on the zone. There's no need for vinyl, toxic paints, e-coatings, insulating gasses, carcinogenic self adhered flashings, or sealants. If it isn't compostable or recyclable, please don't do it.

  2. Matt Dirksen | | #2

    Two solutions to two areas?
    I don't know the configuration of your house, but is it possible that there are two solutions, depending on the use of the room and/or the exposure? I know on my home I currently have 50 year old original single pane double hung units with storms. Given their age and condition, I need to do something. I also live in a split story house (public on one half, bedrooms on the other), and I "think" I am now leaning toward looking at different window solutions for both sides. I am (sniff, sniff), convinced that I need to replace my windows, but not before I have a place to donate my old units.

    In your situation, is it possible the insulated storms make the most sense in one "zone" while a different solution is good for the so-called 2nd zone? I find that this thought process sometimes helps with articulating pro's and con's of potential solutions.

    Just thought I'd ask.


  3. Lloyd Alter | | #3

    Nooo, don't change those windows!
    I have to wonder whether this isn't overkill. The National Trust for Historic Preservation has done a lot of work on window replacement which I am sure you are well aware of, but the bottom line is that the conductive heat loss through the existing windows is minor compared to the air leakage, and that new windows have a short lifespan compared to new ones. I can't imagine removing original 19th century windows.

    What proportion of the wall do those windows actually cover? in a house of that vintage, maybe 15-20%? will moving the R-value from 3, that you can probably get with a storm on a repaired old window, to say 8 that you might get out of your proposed assembly, be worth the cost? You wrote in your own window guide:

    "People concerned about historic preservation hate to hear about windows be- ing replaced; they view windows as a key architectural feature that should be preserved whenever possible."

    I have hundred year old double hung windows on my house, and am removing the hideous exterior aluminum sliding storms and replacing them with magnetic removable interior storms that I will then caulk every winter with strippable caulk. The old windows are part of the character of the house and it wouldn't be the same without it.

    I have written a bit on replacement windows, in my role as President of the Architectural Conservancy of Ontario:

  4. Alex Wilson | | #4

    Replacing older windows
    I guess I didn't mention that the windows I'll be replacing on the old farmhouse are really lousy replacement windows installed probably in the 1970s. In our current house we have gorgeous 12-over-12 windows with hand-blown (wavy) glass panes. I treasure those historic windows; the windows that will be removed in the new (also old) house have almost no redeeming values.

    That said, I really like the idea of protecting old, historic windows with durable exterior storm windows. Those storms keep the prime windows dry and, with low-e storms, will significantly reduce UV exposure on the historic frames. On our current house I have wooden exterior storms that I had made about 30 years ago (as I recall for about $40 each). Those show off the historic windows very well. Were I buying storms to protect those 12-over-12 windows today I would look for higher-performance glazings, but otherwise follow the same approach.

  5. Lloyd Alter | | #5

    that changes everything
    I didn't realize you were replacing replacements. In the immortal words of Emily Litella, "Never mind"

  6. James Morgan | | #6

    Living well south of the Mason-Dixon line I don't know much about storm windows. Curious to know how to ensure compliance with egress codes.

  7. GBA Editor
    Martin Holladay | | #7

    Response to James Morgan
    The most common type of storm window is the aluminum triple-track window. These are operable storms that act like single-hung windows. The lower sash can be slid upward for ventilation or egress. Of course, during a fire emergency, an occupant may find that the storm window is hard to slide. The same problem (difficult operation) occurs with old primary windows, of course. If they are painted shut, or if the occupants are elderly, it's hard to open them in a fire emergency.

    Old wooden storm windows attached from the exterior with metal turn-buttons are another issue entirely. Many of these cannot be opened or removed by occupants during a fire. The only emergency solution is to break the glass.

  8. Robert Swinburne | | #8

    similar situation
    I'm a huge fan of good storms. I have single glazed 1970 windows in my Brattleboro area home which are quite tight. for a total of about $120 from Brown and Roberts hardware I added a sheet of glass to the inside of the lower sash and outside of the upper sash on all windows (double hungs) which make them a bit heavy to operate. The house already had cheapo aluminum storms on the outside. The end result is three layers of glass and a fairly tight window. - Low hanging fruit - the bigger issue will be stripping sheetrock, evicting mice and insulating the wall cavities.

  9. Ice Rabbit | | #9

    Single storm double cellular shade
    Location of the property? Insulation of the house?

    Very expensive windows may be complete overkill.

    A couple things I've learned with our >100yr old house in New England, which goes through about 5 months of winter every year.

    The low-e argon whatever window upgrade - told to me to be the only option at the time due to energy regulations - that we did on our south facing side porch / entry room; which has more glazing than wall space; completely killed the solar heat gain.

    The house has vinyl replacement windows with double glazing which I factor to be R2. The house had some blown insulation, but some cavities were missed / skipped and there was plenty of settling. We corrected this during a complete inside out renovation and insulated with fiberglass bats. For window coverings we installed standard size, off the shelf, double cellular shades, inside the window frame with a super tight fit, thanks to planed filler strips (invisible as I re-trimmed the windows) on the size that modified the opening width (had to remove the plastic end caps on the rail). When you open the shades in the morning you can feel the cool air pocket between the shade and the window.

    Some of the windows do get a thin film of heat shrink clear window film, which acts as a little air barrier.

    By opening the cellular shades during the daytime that face daylight / sun we benefit from solar heat gain on sunny days.

    Oh, and we use 1/4 of the heating oil - without other heat source in use - compared to our neighbors with similar age properties because of our improvements and aggressive settings on the programmable thermostat (as in lower heating temp than energy star proposes).

    Conservation can be done and energy savings can be achieved without retrofitting triple glazing.

  10. Roger Anthony | | #10

    double treble quad glazing
    I have taken the route of having two lots of double glazed windows.
    They are separated by a four inch gap.
    In the middle I have aluminium slated venetian blinds.
    The outer windows are European tilt and turn opening inwards, opening through the inner windows.
    The inner plastic frames have windows opening inwards, spaced to enable my wife to clean the insides, should that ever be required.
    This arrangement is for comfort mainly as I never enjoyed sitting by large double glazed windows in winter, feeling the cold dropping round me, nor in summer being cooked....the inner Venetians nicely block the sun and help keep the rooms cool.
    In winter with an outside ground temperature of minus 18-23C the room side centre of the windows reads 21C
    With a room air temperature of 22C.
    I recomend this system.

  11. Carl Mezoff | | #11

    Storm Window option
    With double glazed windows (Eagle, Pella) in my 30 year old house, I was looking around the shop for a way to add a storm window. My eye fell on some large 1/4" acrylic sheets scavenged from another job, so I cut these to friction fit an existing rabbet on the inside of the window. To hold them in place I ran a few small screws into the wood jambs. This provides an additional dead air space.

    Although almost all the windows were installed as operable units, we have found that after years of living in the house, we typically open only a few of them for air in the fall and spring. In heavy humidity and heat of summer, use of AC keeps all the windows closed. As a result, it is not necessary to remove all the panels during the summer.

  12. Joshua Lloyd | | #12

    I have wondered the same
    I have wondered the same thing if it would benefit the heating bill to add a storm to the Pella replacements that I installed about 6 years ago. Whether it is something I purchase or build myself. Of course this wouldn't happen until I re-side my house and add continuous insulation.

  13. David Guenette | | #13

    Heat build-up between doubled up glazing?
    I'm interested in the potential problem of storm windows for south-facing deep-set insulated windows (such as a buck build out when adding external insulation sheeting), where the distance between the new exterior plane placed storm window and the "innie" existing double-glazed window might form a thermal trap. Have you heard of any cases where insulated vinyl windows, or the between windows and storm trim (imagine using composite trim material) gets damaged by the heat that can build up in the dead air space between these two glazing planes on good sun days?

    Is this a realistic problem, or am I simply mistrusting that the existing window units (or trim) can take whatever heat build up may occur? If heat build up can be a problem, is there a solution? (I know that there are temperature calibrated openers, such as those used for greenhouse venting, but this solution would be expensive to implement, as it would require hinging the outside storm panel.

  14. Alex Wilson | | #14

    Heat build-up
    This is something I wonder about. I think we don't know enough about temperatures that can be reached in insulating glass units (IGUs) or the space between IGUs. My colleague Peter Yost has measured temperatures of 160°F between an insulated low-e IGU and an interior insulating blind, which is higher than the rated temperature for the IGU sealants used in most windows. This has made me wonder whether silicone sealants should be used rather than the more conventional butyl products. (I have heard that Andersen uses silicone exclusively, but have not confirmed that.)

    If one were to install two low-e IGUs (an inner conventional window and an outer insulated low-e storm window) I don't know what sort of temperatures would be reached. But I'd like to understand that before deciding to go that route. In addition to potential degradation of the IGU sealants is concern about damage to other materials in the assembly.

  15. Kurt Kiley | | #15

    Laminated safety glass
    Is another option to look at, another article discussing restoration of historical casement windows recomended laminated glass for its better thermal and acoustical insulating properties.

  16. Brian Driscoll | | #16

    Experience so far
    I have stayed away from retrofitting more than one low e surface in my glazing arrangements, due to heat build-up concerns, but I too, am intereasted others' experience. In my 90 year old dutch colonial, I have done one of the following : 1) routered-in a sealed low e/ argon insulated glass unit into the existing original Anderson double hung primary sash, as I refinshed it. Then left the existing clear glass, aluminum combination storm on the outside. Or, 2) Replaced the clear glass in my existing aluminum storm window sash panels with hard coat low e glass. In either case, I totally insulate and air seal the original counterweight pockets on the primary window. And as I am residing and adding rigid insulation on the outside, I remove the storm window frames, lay a bead of clear removeable caulk on the stop, reinstall, and seal the bottom weep gap also with clear removeable caulk. ( We always have our storms down when rain is imminent)
    I have come to prefer option #2 because it eliminates the router work and you have less leakage into the air gap.

  17. David Jones | | #17

    Primary and secoundary double pane window
    I too have considered using 2 windows within the same R.O. (in new construction). I am frustrated by the high cost of triple pane windows.

    I believe there may be a small efficiency gain in having two argon filled spaces along with the additional larger (and therefore much more prone to convective heat losses) air space between the units.

    I suppose having two sash frames would contribute the the R-Value/Ufactor of the assembly. I have been trying to determine how to estimate the U-factor for the entire assembly.

    I believe there would be additional benefits in the assembly's ability to block sound and redundancy to resist some degree of impact damage.

    Those benefits seem minor. Beyond the cost benefit I believe the primary benefit might be in how the assembly resists air infiltration. With two sets of air sealing there is redundancy with may help overcome a deficient weather stripping. The air space between the two windows would also act as a cushion to absorb some gusts that get past the first weather stripping but doesn't make it past the 2nd weather stripping because of the momentary pressure drop between the two units. I think this might be an area where two units have a clear advantage over a triple pane unit.

    I am not sure there is any benefit to having a 2nd low E coating? Anyone know if the having the Low E coating on both windows would add to the performance vs. just one low e coating? Any idea how to quantify that?

    Assuming just one window gets low E, I have wondered about the best location for the Low Window, (inner or outer). i am inclined to think the outer window is the better location since it would block solar gain at the exterior and reduce heat build up between the two windows.

    There was a case being made about not using Low E at all in heating climates because the net effect of Low might be to block more solar gain in the winter than it saves in heat loss - even on northern facing windows. (Yes, i know this is difficult to believe).

    I believe condensation between the windows in unavoidable, but that it would be temporary as it is with traditional storm windows, (forming on the coldest nights, and disappearing during the next day). I would suggest that the outer window does need provisions for a small drain back the the outside. This would not help the air sealing, but is important.

    I have some minor concerns about the temperature between the windows and/ or between the panes of glass. The conditions are probably very similar to a storm door over a sold inner door. (Although the storm door is single pane and loses more heat back to the outdoors, the solid inner door transfers less energy so the net effect might turn out that the temperatures are similar with double windows as compared to a storm door.

    One solution to heat build up might be to leave one of the windows open (inner or outer) when the heating season winds down. I have considered that I would prefer to even remove the inner window during the cooling season altogether.

    Finally I have considered adding solar blocking exterior screen, (they replace the standard insect screen). These could be installed seasonally and block the solar gain during the cooling season even when the windows are open to let the breeze in!

  18. GBA Editor
    Martin Holladay | | #18

    Response to David Jones
    You have lots of questions. I'll try to tackle a couple of them.

    Q. "I am not sure there is any benefit to having a 2nd low E coating? Anyone know if the having the Low E coating on both windows would add to the performance vs. just one low e coating?"

    A. Every additional low-e coating lowers the U-factor (raises the R-value) of the glazing. A triple-glazed window with two low-e coatings has a lower U-factor than a triple-glazed window with one low-e coating.

    Q. "There was a case being made about not using Low E at all in heating climates because the net effect of Low might be to block more solar gain in the winter than it saves in heat loss."

    A. Your analysis is headed in the right direction, but is a little misguided. There are two kinds of low-e coatings: those that result in low-solar-gain glass, and those that result in high-solar-gain glass. Not all low-e coatings are low-solar-gain. On the south side of a house in a cold climate, you want a low-e coating with a high SHGC. More information here: All About Glazing Options.

  19. Alex Wilson | | #19

    Two low-e coatings
    David and Martin,
    The other potential concern with low-e on both inner and outer windows is heat build-up in the inter-glazing space. My colleague Peter Yost has measured temperatures as high as 160°F when he combined an exterior low-e storm window (single-glazed) with an interior low-e (double-glazed) prime window. At that temperature, damage to some insulating glass unit (IGU) seals and other materials can occur. If we go this route, we will try to do some data collection to better understand this issue. Such a practice may argue for silicone IGU seals.

  20. David Jones | | #20

    Low E and Heat Build up
    High solar heat gain Low E seems to be (very) difficult to get in the real world. I had read the information you show the link for, but perhaps i came to the wrong conclusion.

    In our primarily heating climate in Connecticut I made the following assumptions.
    If low solar heat gain -Low E block more solar energy in the winter than it saves in the summer then high solar heat gain is better for our climate. Since high solar heat gain also blocks some energy in both directions and there would still be a net energy penalty. Therefore I concluded using clear glass (no low E film) would be even better for our climate. I wasn't comfortable with the conclusion, but I based it on the assumption that high solar gain Low E reduced the windows ability to block energy in both directions equally. I am not ready to advocate that my clients forgo Low E all together, but article did leave me wondering if no low E was actually the best option.

    I too have some concern about temperature build up with two windows, but I suspect it would not turn out to be significant in the real world.
    1. Door manufactures are concerned about the color of an exterior door (especially when it is behind a storm door). I am not aware of any door manufacturer being concerned about the their seals on doors with Low E glazing. The storm door over a primary door with a low E window is very similar to the double window assembly we are talking about.
    2. Unlike with the storm door - inner door assembly, the double window assembly can leave the inner windows open (or remove them) when the heating season ends. This requires some thought on the part of the homeowner, but its easy to do and there is a wide range of time in the swing season to do this before the temperatures get extreme.
    3. The solar /insect screen would significantly reduce the chance of overheating as well. (Manufacturer claims they block 80% or more of solar heat gain). Again installing and removing the screens requires the owner to be involved. I grew up an antique house with restored windows. We installed and removed wood windows every year. By comparison changing screens is easy.
    4. When Peter Yost measured the 160 degree temperature, it doesn't sound like he had seal failure. Those temperatures may exceed the limit set by the manufacturer but still not cause actual failure. I am not advocating that we intentionally exceed manufacturer recommendations, I am only pointing out that it seems unlikely that occasional high temperatures are going to cause actual failure.

    I really think this double window arrangement warrants some testing beyond the sort of arm chair engineering we are able to do here.

    We have all come to consider double window assemblies because the cost of triple pane windows is so high. I can't really see why making the triple pane window is such a significantly cost increase. (Yes they are thicker and heavier, but that doesn't seem like it should change the retail cost that significantly). I suspect it has less to do with the cost of production and more to do with limited production/demand/ and the ability of the manufacturer to add a "performance premium" to the price right now. In the long term if demand / production of triple pane windows goes up, the cost differential may go down and we can forget about the double window idea.

    If cost wasn't the driver the double window arrangement would probably not be worth doing, but I do suspect that the double air sealing, added sound reduction, redundant protection against impact do still have some value. The added hassle of opening two windows to let air in the room would probably outweigh those factors however.

  21. David Jones | | #21

    Combined U-Factor
    Can anyone take a stab at estimating the U-factor for the combination of the two window? Even if we disregard any benefit from the large air space between the two sashes, I can't determine how to "add" the benefits together to get a ball park figure for the total performance.

    I anticipate the combined performance would rival some high end triple pane window ratings. If it didn't then it may make sense to skip the complications of two windows in one rough opening and use the triple pane windows or drop back to a double glazed window with a single glazed low E triple track storm.

  22. GBA Editor
    Martin Holladay | | #22

    Response to David Jones
    You can't add U-factors, but you can add R-values.

    If a double-glazed window has a U-factor of 0.30, that means that it has an R-value of R-3.3.

    Two such windows have an R-value of 3.3 + 3.3 = 6.6

    A 1-inch vertical air space has an R-value of about R-1. So the above example yields a total window assembly R-value of about R-7.6.

    R-7.6 = U-0.13

  23. Sue Erlikh | | #23

    Storm window pictured?
    To Alex Wilson
    Alex, in the above article of july 26 2012, there is a picture of a storm window with the following caption:

    *** A low-e storm window at the Brattleboro home of my colleague Peter Yost. ***

    I like the slim lines of tis window, who is the manufacturer? My email is [email protected].

  24. Sue Erlikh | | #24

    Low E or Not
    David, in your post of 4/2/13 you raise the question of low E glass not being beneficial in the overall energy savings because it reduces the desirable winter solar heat gain as compared to standard glass, even if the low E coating is of high solar gain type. I am having similar doubts. In my Long Island home, the winter solar gain, occurring through my single pane DH windows plus old aluminum storms, is so significant that the heating does not go on during a sunny day even with the outside temps close to zero F. The undesirable solar gain in summer months is easily controlled with internal reflective shades fully lowered. Have you come to a final conclusion on this subject? I plan to install new storms soon and this info would be helpful.
    Anyone else is also welcome to chime in. My email is [email protected]

  25. GBA Editor
    Martin Holladay | | #25

    Response to Sue Erlikh
    When choosing glazing, there are usually trade-offs between SHGC and U-factor. We usually want a high SHGC during the winter, but it would be nice to have a low SHGC during the summer. To figure out the results of different glazing choices, you have to use an energy modeling program.

    For more information on these issues, see All About Glazing Options.

  26. Alex Wilson | | #26

    Low-e storm windows
    The low-e storm windows on Peter Yost's house are made by Harvey Industries: Apparently, they used to only sell to builder accounts; Peter isn't sure if that's still the case:

  27. Matt Walker | | #27

    Double glazed argon/low-e plus internal storm
    I too question the investment in triple glazed windows. I will soon be building a highly insulated house with foot-thick walls. I am considering using good double glazed windows (argon/low-e) mounted on the outside of the wall and supplementing that with a custom built single pane storm window fitted from the inside. I'd trim out the deep openings with flat trim with a built in lip of about an inch. This lip would provide a surface to the mount the internal storms to and would look good when the storms are not installed. I was thinking of about a 4 - 6 inch gap between the exterior window and the internal storm.

    Any thoughts or concerns about this idea would be most appreciated.

  28. GBA Editor
    Martin Holladay | | #28

    Response to Matt Walker
    The biggest problem with interior storms is condensation on the primary window, leading to water staining on the window stool, and in some cases to black mold.

    Here's the mechanism: the interior storm cools the primary window below the dew point of the air. Small cracks allow humid indoor air to get by the interior storm. When this humid indoor air contacts the cold primary window, you get condensation.

    The best way to address this problem is to make your primary window a little leaky -- and that can be hard -- and to make your interior storm window installation as airtight as possible.

    Exterior storms work better, because it's easier to make your primary windows airtight, and your exterior storm windows a little leaky, than vice-versa.

  29. David Jones | | #29

    Double primary windows also
    Double primary windows also means redundancy in the air sealing. One of the windows is likely to seal better than the other.

    The space between the windows may absorb some of energy of wind gusts, by pressurizing the space between windows. When the wind gust diminishes, the pressure in the space would leak back to the outdoors as well as the indoors. (potentially reducing the air leakage from a wind gust by 50%)

  30. David Jones | | #30

    Double window option worth more consideration
    With the price premium for triple glazed windows coming down, the cost benefit for a double layer of windows in a single opening is reduced somewhat. That said I do think this option is still worth more evaluation.

    i did try to interest one of the Building America research teams in studying this concept. There was little interest however as they felt condensation would be to much of a problem. They did not however cite trapped heat as a concern.

    RE: Condensation

    All windows can form condensation under the right circumstances.

    Storm windows produce a lot of condensation because the storm glazing is very cold. Also storm windows are generally installed over very leaky interior windows which means more air carrying more moisture into the space between primary window and storm window than would be the case with a double layer of modern windows.

    A low E glass "storm" window would be less prone to condensation because the inside of the outer window would somewhat warmer than a single glazed storm. (Custom double pane storms are apparently being used successfully)

    A double window arrangement would still benefit from a drain. (A small hole covered by a moving flap similar to a triple track storm window).

    Since the double window approach would produce less condensation than than a traditional storm window, it seems likely that this issue can be managed successfully.

    RE: Heat trapped between two windows causing seal failure

    If trapped heat is a concern, the simplest solution is to open the inner window during the warmer months. This does require homeowner involvement, but is not a time sensitive task since it can be done anytime during the shoulder months.

    A low e full glass storm door is frequently installed over a 1/2 glass insulated door. This arrangement is very similar to the double window arrangement being discussed.

    The low E storm door would emit more of the trapped heat back to the outdoors because it does not have the double pane / argon gas of a typical primary window. However the insulated interior door would probably emit less of the trapped heat to the interior of the building because the solid section of the door has a higher R-value than a window.

    The double window arrangement would emit less of the trapped heat to the exterior and more into the building because the inner window has a lower R-value than the solid area of the door.

    These two factors may combine to make the trapped heat in the spaces (storm door/primary door as compared to double windows) roughly equal. Since doors don't seem to have seal failure on the glazed portion when there is a storm door installed, it seems reasonable to conclude that a double window would perform at least as well.

  31. David Jones | | #31

    Came across related
    Came across related information below. While Lawrence Berkeley National Laboratory results show a potential concern of temps reaching 185 degrees, this does not necessarily translate into seal failure. (My understanding is that LBNL tested temperatures and not for seal failure.) These temperature extremes must exist in a storm door / insulated door arrangement too.

    If high temps are the concern (vs. condensation) then the easy answer is to leave inner windows open during cooling season. This would only need to be done with windows exposed to direct sunlight. Windows protected by porches, overhangs, or on north walls would not be subject to overheating.

    Am anxious for more data /study on this concept since it has the potential to offer extremely high performance at a more reasonable cost.

    Modeling performed for LBNL by sustainability consultant Thomas Culp, Ph.D. has uncovered the potential for serious overheating problems when low-e storms are added to low-e windows: in hot weather, in direct sunlight, temperatures up to 185° F (85° C) may be reached. That kind of heat can cause premature aging or failure of the insulated glazing unit’s seals. Further testing will yield a better understanding of the exact conditions under which this can occur and possible solutions, but it would be wise to avoid using low-e storms in combination with low-e double-pane windows until more is known.

  32. Edward Cambridge | | #32

    Glazing Units for Interior Storms
    This informative series has me wondering. Did any of the previous contributors (1) Produce or source a double glazed interior storm, oR (2) Where can you buy glazing units? I am looking to provide interior storms for some large double glazed double hung windows (currently R3)


  33. GBA Editor
    Martin Holladay | | #33

    Response to Edward Cambridge
    I don't know of any manufacturers who make or sell double-glazed storm windows.

  34. Charlie Sullivan | | #34

    Double glazed interior storm for Edward Cambridge
    This company makes double glazed interior storms with two layers of heavy plastic film.

    There's a dealer/installer in my area (central VT/NH), and I've seen some installed. They look very good--you can't tell it's plastic and can barely see it at all. The plastic film is actually transparent to infrared, so they won't do as much to stop radiation as glass does, and are even further behind low-e coated glass, but the double layer does well to inhibit convection and the installation stops drafts through leaky windows very nicely.

  35. Sacie Lambertson | | #35

    interior vs exterior storms and the material used
    We are have our very old double hung windows rebuilt by a window preservationist. I am now tackling the storm window issue. Seems to me any interior storms obviously do not protect the exterior primary window so why other than the ease of installation why would one want these? Two I am most interested in Peter Yost's experience with aluminum storms. I understand vinyl is preferable in terms of heat transfer questions. Would appreciate a discussion about this.

  36. GBA Editor
    Martin Holladay | | #36

    Response to Sacie Lambertson
    Over the last 30 years, aluminum new-construction windows have been in steady decline, while vinyl windows have taken an increasing market share.

    The same process appears to be happening in the storm window market. It's true that vinyl frames are less conductive than aluminum frames -- but when it comes to storm windows, the energy penalty attributable to the difference in the frame conduction characteristics is probably pretty small.

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