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Community and Q&A

Windows that reflect infrared only when it’s hot?

brooklynite | Posted in Green Building Techniques on

I’m considering buying a home that faces south. The second floor windows get flooded with light for most of the daylight hours. The windows are single-paned, and if this house becomes mine, replacing them will be at the top of my list.

I’ve read about low-e glazing and how it reflects outdoor infrared while keeping heat inside. But what about in the winter, when I want the sunlight coming in to warm the house? Some considerations:

– The sun’s increased summertime angle will not significantly lessen the exposure.
– The house is landmarked, and exterior window treatments are not permitted.
– NYC is noisy, so sound insulation is a factor as well.

I’d love a flexible window solution that allows me to take advantage of the sun when I want it, and banishes it when I don’t. Does such a thing exist? Or should I get over it and just be satisfied with windows that effectively keep the boiler-produced heat inside during the winter?

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  1. Tom May | | #1

    "I’d love a flexible window solution that allows me to take advantage of the sun when I want it, and banishes it when I don’t. Does such a thing exist? "
    Yes they go by names such as blinds, shades or curtains.....

    1. brooklynite | | #2

      Well, sure, but don't most of those absorb infrared (or let it pass right through)?

      1. Tom May | | #4

        White reflects....dark absorbs....

    2. Burninate | | #19

      Also overhangs.

      In winter, the sun will be low in the sky at noon. In summer, it will be overhead at noon. You can tweak the angles so you get significant direct sunlight in winter, but little direct sunlight in summer.

      A similar effect is possible with the configuration of east (morning sun) versus west (afternoon sun)-facing windows.

    3. User avatar
      Eric Whetzel | | #23

      We used Suntuitive glass on our west facing windows. The glass works as advertised, and we have nothing but good things to say about it.

      The exterior surface temperature of the glass determines when the glass tints up, so on a cold and cloudy day you maximize the daylighting potential of your windows since the glass doesn't tint up. On cold, sunny days it may or may not tint up, but if it does it won't completely block sunlight --- it's more like sunglasses for your house; it tints when you need it, but it stays clear when the sun isn't an issue.

      Our goal was to minimize window treatments as much as possible, and the Suntuitive glass in our west facing windows has allowed us to achieve this goal --- unobstructed views to our backyard helping us to maintain a connection to the outdoors even in the dead of winter or (more importantly) on even the sunniest and hottest summer afternoons.

      The product is also ideal since it's "dumb", in the sense that once in place you never have to do anything, so there's no input or maintenance from the homeowner required.

      You can contact Suntuitive to see if they can help in your situation:

      You can also check out our blog post about our windows and our experience with the Suntuitive glass here:

  2. Jamie B | | #3

    You're asking for the magic pill.

    Motorized shades on an annual sun schedule, I personally think is an important part of an energy plan.

    But the shit part about blinds from a design perspective is if you or your occupants are in the house most of the time, ie work from home etc, it really blocks out out the sun light that makes a house fun and open feeling. There are semi blocking shades but they're still not great.

    To be honest, I'm waiting for electronically shades glass to become more mainstream. I think that's our future of fenestration glazing, where we can easily set schedules and opacities, not rely on loud mechanical parts, get rid altogether of the Interior design of curtains and blinds and really customize a plan that suites privacy, feel of the space and solar gain/energy efficiency.

    1. brooklynite | | #5

      > You're asking for the magic pill.

      I had a feeling I was. Never hurts to be sure the pill doesn't exist before I make any big decisions. What's your opinion on the best south-facing windows to use in a climate that gets extreme temperatures at both ends? I have a fetish for triple-paned, but I lack the experience to know whether it's worth it or not (not just in terms of the energy bill savings, but in terms of quality of life as well).

  3. User avatar
    Peter Engle | | #6

    Everyone is missing the point of how low-e windows function. Low-e coatings are "selective" surfaces, as the properties change with the wavelength of the light. IR is long wavelength and it is reflected from either side. Sunlight is shorter wavelength and it passes thru. So, when it is hot outside, the glass reflects the heat back to the exterior. When it is hot (warm) inside, the glass reflects the heat back to the interior. Sunlight always passes through and warms the interior when it is absorbed by the interior surfaces. Those surfaces then warm up and re-emit the energy as heat, which is kept inside by the glass.

    1. Zephyr7 | | #7

      Close, but not entirely. Sunlight INCLUDES IR light. All of the Lo-E coatings act as wavelength selective filters, usually with a bandpass characteristic which means they allow only a relatively narrow range of wavelength through. It is the nature of these type of optical filters to reflect the wavelengths they don't pass through, so anything not passed through gets reflected back to the side from which it originated.

      In the case of a bright, sunny day, a big chunk (but not quite all, which is why aggressive lo-E coatings look tinted) of the visible spectrum is passed through to the indoors, but wavelengths outside of the pass range are reflected back out. Much of the "feels warm in the sun" energy is in the IR range, probably some UV, and that is what is reflected and the primary contributor to the "solar gain" SHGC (Solar Heat Gain Coefficient).

      It is true that visible light will still warm things up, but if you've blocked 30% of the incoming energy, you still have 30% less "heat" coming through regardless of which specific wavelength you block. The basic fact that all Lo-E coatings exploit is that the sun's energy is distributed over a pretty wide array of wavelengths, but we only see a narrow range of these wavelengths. By reflecting wavelengths we don't need for vision, it's possible to block energy without impacting the visible spectrum. The visual tint of the more aggressive Lo-E coatings is because the filters aren't entirely perfect.

      Regarding the original question about adaptive coatings, I've always liked venetian blinds for this. If you have white blinds, you can use the adjustable angle of the slats to adjust how the light comes through, essentially varying from direct, to somewhat diffused, to mostly blocked. The thinner blinds with aluminum slats work best for this, and you need the horizontal style for best effect.


      1. Tom May | | #8

        Right Bill, as well as having them overlap facing the inside or the outside, just like scales on a fish...

  4. User avatar
    Dana Dorsett | | #9

    Peter Engle has it right. Most of the energy in solar spectrum is in the visible light and NEAR infra-red. Heat radiating from the landscape or from the stuff in your house is in the DEEP infrared.

    Hard coat low-E coatings reflect primarily the deep infra red and almost nothing in the visible spectrum. Some of the softer low-E coatings are broader spectrum than that, and most low-E coatings that reject solar heat well also reduce the visible light transmittance.

    That said, ANY low-E coating will lower the direct solar gains a bit, while improving heat retention when the indoors is warmer than the outdoors. In NYC you definitely want to go with a higher gain/lower heat rejection hard coat, not a soft-coat low-E.

    Indium tin oxide hard coat is harder than the glass substrate, and can be used on single pane glass without much risk of visible scratching, with very minimum impact on visible light transmittance.

    If this is a historic district that won't allow exterior storms, re-glazing with hard coat low-E glass (with the low-E facing the indoors) will improve the overall window performance. Adding INTERIOR storms can bring the overall performance up to nearly the current IRC code minimum, as long as either the interior storms or the or the original sashes have a hard coat low-E coating. To minimize wintertime condensation issues between the panes it's better to put the low-E glass on the original sash, not the interior storm.

    For a lot more money it's possible to re-work the sashes to accommodate thin double-pane glass.
    This company in CT reglazes antique sashes with thin double pane glass:

    They rout out the interior side of the sash & muntin bars to allow for the thicker insulated glass units, leaving the exterior looking pretty much the same:

    1. User avatar
      Dana Dorsett | | #10

      BTW: Heat rejecting low-E coatings don't reflect the infra-red- they work by not emitting infra-red. The "E" in "low-E" is for "emittance". If it worked primarily by reflecting heat it would probably have been a "high-R" (for "reflectance") coating.

      While emittance and reflectivity are closely related, it's the difference in spectrum between what's being radiated in from outdoors, and what's being radiated out from the indoors that gives it the heat retention effect. All low-E coatings will behave as heat retainers when the indoor temp is warmer than the outdoor temps. But hard coat low-E coatings will allow more energy in, since it doesn't reflect as much of the solar spectrum (including visible light) as the better heat rejecting coatings.

      1. User avatar
        Peter Engle | | #13

        I'd quibble with that one just a bit.

        Any light striking an object is either reflected, absorbed, or transmitted. For any given material, the sum of Reflectivity (R), Absorbtivity (A) and Transmissivity (T) is one. Or,


        For a body in equilibrium with its environment, absorbtivity = emissivity. So, if a low-e coating has a very low Emissivity, it also has a low Absorbtivity. This means that it must be high in either reflectivity or transmissivity (or both). We don't want a coating that is high in transmissivity in IR wavelengths, because we're trying to block the heat. So this means that low-E coatings must also be high-R coatings. They do, in fact, reflect light in long IR wavelengths and they could just as accurately have been called "high-R" coatings.

        FWIW, this is why windows with interior-side low-e coatings "feel" warmer in cold weather even when the glass temperature is low. They are reflecting your own body heat back at you and you perceive that as warmth. Or, you could say that they don't transmit or absorb your body heat and therefore, they don't steal the heat through the glass.

        Going total geek here, this is also why you can "see" your reflected heat signature in an IR camera when try to look through low-e glass. The coating is reflecting the IR that you are emitting.

        Like I said, just a quibble. The point is that low-e windows let most of the visible light in, and they keep most of the heat out from whichever side is warmer. They do also stop the part of sunlight that is in the long-range IR (heat) spectrum, but the remainder is available to warm the inside of the house.

        1. Tyler Keniston | | #14

          Thanks for that Peter. I've always been confused when in the building world folks say that technically the heat isn't reflected
          (such as in this Alex Wilson article:

          I've approached it from the same %R+%A+%T= 100% (btw you wrote E when I think you meant R)

          If it didn't reflect (or transmit) the energy it doesn't emit, the low-e coating would heat up, and up, and up...
          unless of course we are allowing for greater transmittance, as you state, which we don't really want.

          So maybe there is a technical reason that low emissivity shouldn't be equated with high reflectivity, but I can't see why, unless its because transmissivity is allowed to be variable.

          This issue is also interesting when viewed in the context of radiative night-sky cooling, since the emissivity of the objects and of the air is relevant to whether or not something will cool beyond 'ambient.'

          ^ a short and concise explanation on the emissivity-reflectivity-transmissivity relationship. Ivity ivity ivity.

        2. Tom May | | #15

          Yup, I'll say thank you too Peter for clearing up emissivity, perhaps next you can explain how a black body works.

          1. Tyler Keniston | | #16

            The basic version:
            A blackbody is a theoretical object that absorbs 100% of all electromagnetic radiation (entire spectrum), and therefore also emits 100% (in thermal equilibrium). It also emits it isotropically, meaning evenly diffuse.

            In the RAT equation it's basically A=100%, R +T = 0

            White body would be the theoretical opposite. Note that the color white isn't an indicator of something being a white body, for that's only relevant to the visible spectrum. Which is why white shades may still absorb and emit significant IR, though they will reflect the visible spectrum better than the color black.

            Lots more that could be said, but that's the gist, and about as far as I dare to speak on.

        3. Tyler Keniston | | #17

          I was thinking more about this... and I realized I was viewing this situation in terms of radiation only. How conductive (and to some degree convective) transfers play into the equation suddenly had me reeling.

          Is specifying that it's truly low-e as opposed to high reflectivity relevant when we consider the properties of the coating and that heat conducted to the coating, while it was never reflected, will still be emitted at a slower rate according to the emissivity values?
          But in that situation, it will still emit that energy as it seeks equilibirum, so no energy is ultimately rejected back inside, but it will emit more slowly...?

          So in the case of a double pane window with low-e on surface 3, the heat that is conducted to the coating via the interior pane will ultimately be emitted by that coating, more slowly than if it wasn't low-e, but ultimately still emitted (i suppose in both directions). However the working principle that makes it effective for our purposes would be that radiative energy emitted from objects in the room NOT TOUCHING the coating (I.e. not the glass) will be reflected and therefore not emitted. Does this sound right?

          1. User avatar
            Peter Engle | | #21

            "So in the case of a double pane window with low-e on surface 3, the heat that is conducted to the coating via the interior pane will ultimately be emitted by that coating, more slowly than if it wasn't low-e, but ultimately still emitted (i suppose in both directions). However the working principle that makes it effective for our purposes would be that radiative energy emitted from objects in the room NOT TOUCHING the coating (I.e. not the glass) will be reflected and therefore not emitted. Does this sound right?"

            That's about right. The glass will be physically warmer than glass without the low-e coating, because the glass is warmed by convection of indoor air, but the coating does not allow it to emit that warmth to the outdoors. At the same time, the coating reflects the IR from interior objects back at them. The first part reduces energy loss through convection and re-emission. The second reduces energy loss through IR transmission, and makes us feel warmer standing in front of the window.

  5. brooklynite | | #11

    First of all, this forum contains an astounding amount of information and knowledge. Thank you to everyone.


    > In NYC you definitely want to go with a higher gain/lower heat rejection hard coat, not a soft-coat low-E.

    I assume you're saying this because NYC is considered a colder climate. However, the past few winters have been light, while summers have been dreadful. But a hard coat is probably better than what I have on my apartment right now. Thanks for this advice.

    > If this is a historic district that won't allow exterior storms

    My mistake. Awnings are prohibited, but exterior storms are allowed.

    One additional note that probably strengthens your argument for a hard coat: the windows in question are oriel.

  6. Roger Berry | | #12


    Try reading this:

    Maybe Mr. Wetzel will weigh in on actual performance over time.

    1. User avatar
      Eric Whetzel | | #24

      Thanks for the link Roger!

      We're still very happy with the Suntuitive glass on the west side of our house.

      I should've noted in my comments above that you lose most of your potential solar heat gain with the Suntuitive. We have a significant amount of glass on our south side to maximize solar heat gain without overheating the home, so this wasn't an issue for us. More than made up for on summer afternoons when curtains or blinds are made unnecessary by the Suntuitive glass.

      There's more detailed info available directly on our blog at:

      We would definitely recommend the Suntuitive glass to family or friends, and we would definitely use it again. Makes window treatments totally unnecessary, apart from privacy issues.

  7. Keith Gustafson | | #18

    As a non tech side note, my dual hard coat windows facing southeast still make you feel warm when you stand in front of them. Not as warm as some of the yet to be replaced non loE on the same side, but enough to make you want to stand there on a sunny winter day.
    Point being LoE glass lowers solar gain, but does not eliminate it.

  8. Burninate | | #20

    Windows make for terrible walls. A modern take on PGH / net zero homes, with homeowner demand to be warmed and cooled and humidity-controlled, will likely reject the technique of passive solar gain entirely. It's hard to control, expensive, and creates temperature differentials between rooms.

  9. Greg Smith | | #22

    Great discussion!

    Martin wrote the following some years back, and it's still very much worth reading:

    Interesting quote from Martin's article:

    Lstiburek says, “Don’t do it”

    In a 2014 article titled “Zeroing In,” Lstiburek addressed passive solar design principles with his usual bluntness.

    “Don’t bother with the passive solar,” Lstiburek wrote. “Your house will overheat in the winter. Yes, you heard that right. Even in Chicago. … You should go with very, very low SHGCs, around 0.2, in your glazing. If this sounds familiar to those of you who are as old as me, it should. We were here in the late 1970s when ‘mass and glass’ took on ‘superinsulated.’ Superinsulated won. And superinsulated won with lousy windows compared to what we have today. What are you folks thinking? Today’s ‘ultra-efficient’ crushes the old ‘superinsulated,’ and you want to collect solar energy? Leave that to the PV.”


    1. User avatar
      Eric Whetzel | | #25

      I'm reluctant to disagree with Joe Lstiburek, but I can only relate our direct experience.

      The windows on our south side have .5 SHGC. It's a Passive House, so low air leakage with a Mitsubishi head in the main area on the south side (kitchen and family room) of the home. Behind the kitchen is our utility room. While it's true the temperatures do go up some, even on the coldest winter days as long as the sun is out, it's never felt like overheating.

      We keep our temperature set at 69F, so on the sunniest winter days temperatures in the kitchen and family room may rise to 73F, but this makes the space feel cozy, as comforting as any traditional fireplace. While I can imagine some may not like the sun hitting the quartz on our island in the kitchen, this too is quite welcome when trying to get through cold, often gray, Chicago winters.

      It's also true that we didn't go too crazy on the percentage of glazing on the south facade, with only about 15% of the south side covered in glass. This includes a 3'x5' window in our utility room, a 5'x4' window over the kitchen sink, a door with a significant amount of privacy glass, and a 9'x4' window in the family room.

      In fact, the biggest unanticipated surprise is how well all the south facing windows perform in terms of daylighting on even the grayest winter day --- including today, when there's complete cloud cover, it's snowing, and it's still possible to easily read a book while at the kitchen island or on the couch without any thought to turning on lights.

      In effect, I think it's possible to balance high solar heat gain in the winter while avoiding overheating, as long as the total amount of glass and its placement is carefully considered.

      One of our favorite outcomes regarding the building of our home is this passive solar effect in winter, along with little sun getting in during the summer. It does improve your mood, or your quality of life if you like.

      1. Tom May | | #26

        Totally agree with your Eric. As a bit of a summertime sun worshiper, I enjoy sitting in front of a nice sunny window in the winter, my plants and small indoor garden seem to enjoy it too. Not to mention the free heat and lighting thing. Just wish we could put more time and energy into having them stop spraying our skies and blocking our sun.

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