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N.Y. Times article on dealing with heat wave

Alan B | Posted in Energy Efficiency and Durability on

I read this article

And while most of it is silliness it did get me wondering if white or reflective window coverings would make an appreciable difference. Assuming high SHGC windows of course (not sure if my windows are or not).

Your thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    Cellular window shades (which are mentioned in the article) definitely work. They keep out a good fraction of the solar heat gain in summer and help retain heat in winter. More information here: "Insulating Window Shades."

    If you don't care whether your windows remain transparent or translucent, almost any method of covering the glass on the outside will work, including cardboard.

  2. Alan B | | #2

    Interesting, thanks Martin

    You mention on the outside, on the inside will these shades or any others make any difference?

    Light in itself does have some heating value but i assume most of the heat gain is in UV/IR? Would this be accurate?

  3. GBA Editor
    Martin Holladay | | #3

    Cellular shades are installed on the interior side of the window glazing. And yes, they work.

  4. Expert Member
    Dana Dorsett | | #4

    Alan B: "Light in itself does have some heating value but i assume most of the heat gain is in UV/IR? Would this be accurate?"

    Not really accurate. It depends on the type of window & coatings, but visible light is always THE major player.

    About half of the light transmitted heat gain through CLEAR glass windows is in the visible spectrum, with the bulk of the rest coming from near-visible IR radiation, but still some from from near-visible UV.

    Fully 2/3 to 3/4 of the light transmitted heat gain through typical low-E windows is visible spectrum, the remainder primarily from near-IR.

    With heat-rejection low-E coatings or heat-rejection window films, near ALL the transmitted gain is visible spectrum.

  5. Alan B | | #5

    @Martin, do those shades give R4 because of the foil reflection or because of the hollow core. Which would be the bigger component?

    @Dana. If visible light is the major component could i reduce solar gain in summer appreciably by using standard white blinds? Will they appreciably help with the partial UV and partial IR?
    My windows are dual pane vinyl, i tried to find out from the company if they are low or high solar heat gain but they could not tell me, they said check the receipt which i don't have since they were installed before i moved here.
    I suspect they may be interior coated Low E since i am near Toronto Ontario but they probably don't block anything beyond regular glass from the outside. Not sure if i can determine any of this somehow.

    For whats its worth i did see this recently saying one type of UV is blocked the other not.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Do those shades give R-4 because of the foil reflection or because of the hollow core?"

    A. Because of both.

    Q. "Which would be the bigger component?"

    A. Roughly speaking -- an educated guess here -- you'll get about R-2 from the R-value of the cellular shades and about R-2 from the R-value of the reflective surface + the adjacent air space.

  7. Lynda Robertson | | #7

    Where I live, it won't go below 100* during the day this week, and the sun is brutal.

    I've been working with reflective products for many years, and I disagree with the general consensus on GBA that they are not worth the cost. They aren't worth it in cooler climes, but they are very efficient where it gets really hot.

    Too bad the manufacturers of these products shot themselves in the foot from day-one by telling lies about high r-value, when the truth about how they work would have served them better.

    However, be careful of these products in windows. Used inside, they can reflect so much heat back into the window that they can occasionally cause damage. Be sure you allow for a bit of air-flow around the edges.

  8. Expert Member
    Dana Dorsett | | #8

    Alan B: Yes, standard white blinds will block and partially reflect IR & UV, but absorbing some amount of spectrum (depending on the characteristics of the material.) Aluminum / aluminized mylar reflects a very broad spectrum, absorbing very little.

    Most low-E windows sold in Canada (or even the northern US) are not heat rejecting types of coatings, but it still lets in only about half the solar gain of a clear glass double pane.

    With a candle or small flashlight you can view the reflections at night at an angle to figure out which surface has the coating, which will be visibly different in color/intensity than the other three reflections.

  9. Alan B | | #9

    @Lynda I know what you mean about cause damage, i used a whole windshield foil faced sun reflector in the car and one day it melted one side of the cell phone mount on the windshield. The sun must have been at just the wrong angle...

    @Dana I have mostly white coated aluminum blinds, i am curious what percent of all heat (light, UV/IR) they reflect if there is any data available.

    I will test the flashlight reflection tonight and see what i get from both sides.

    @Martin I have maybe half a dozen other questions about insulation/house efficiency, should i post a separate question of each one or just make one long question with all of them?

  10. Expert Member
    Dana Dorsett | | #10

    Alan- you would need to know the material of the dye to even take a WAG at how much of the IR & UV it was absorbing vs. reflecting. The aluminum blocks all of it, but whether the finish is absorbing or not depends on the finish. Titanium dioxide white reflects most of the solar spectrum, but there are many "white" paints in the visible spectrum that absorb some of the near IR & UV.

    Regarding which surface the low-E is on, for any given coating, when the low-E is on surface #2 the heat gain is lower than when it's on #3, but that's somewhat less important than the coating type. Windows targeted toward the Canadian market are more likely to have a pyrolytic coating on surface #3 than #2 to improve it's the net heat capture. characteristics. Windows with soft coat low-E heat rejecting coatings sold into the southern US market are more likely to have it on surface #2. But the manufacturers buck those trends all the time.

    Double low-E glass designed for cooler climates usually have a hard coat on both surfaces #2 and #4 (the surface in contact with the room air), and some of those can still have fairly high solar gain depending on what is used for surface #2. Some of Cardinal's double low-E glass has an SHGC greater than 0.5, barely lower than the 0.6-ish delivered by clear glass, despite having coatings on surfaces #2 & #4. That's twice the gain of single low-E heat rejection glass that would meet Energy Star criteria for the southern US.

  11. Alan B | | #11

    @Dana, that makes a lot of sense, they are cheap white Venetian blinds probably from walmart.

    I assume surface you mean inside or outside panes and their sides?
    Where is surface 1, inside pane inside facing or outside pane outside facing?

  12. Expert Member
    Dana Dorsett | | #12

    Surface #1 is in contact with the outdoor air. Surface #2 is the other side of that exact same sheet of glass in the assembly.

    With a single pane window surface #2 is in contact with the indoor air, but with a double pane assembly surface #4 is in contact with indoor air, and surfaces #2 & #3 are separated by the air or other gas used inside the assembly.

    With a triple pane surface #6 is in contact with the indoor air.

  13. Alan B | | #13

    Interesting, thanks.
    I have double pane vinyl windows which are maybe 10 years old, but i will check them tonight for coatings

  14. Alan B | | #14

    This is a photo from inside, when done from outside the middle and right reflections are reversed

  15. Expert Member
    Dana Dorsett | | #15

    It's hard to see what's going on in a picture when there are multiple LEDs. With a more compact/narrow light source viewing at an angle you'll see two pairs of images- the reflection surfaces #4 & #3 close to one another, separated at some distance from the reflections from surfaces #2 & #1.

    It may take a broader spectrum than a cheap LED flashlight to see it clearly.

    This picture was taken from indoors using a butane lighter as the light source, with the brightest being the nearest surface (#4) on the left:

    If you look carefully you'll see that the reflection from surface #3 has a greenish tint to it compared to the others, and the reflections surface #2 & #1 look very similar to one another in color and intensity. So this window's low-E coating is on surface #3.

    The change in color isn't always greenish as seen in the picture- it will vary depending on the exact coating type & thickness used, but it's usually pretty easy to tell which surface is the "strange" one.

  16. Alan B | | #16

    @Dana, it aint a cheap LED light ;)
    I tried a single LED flashlight and got nowhere so i tried a BBQ lighter and could not figure it out either. I also tried my only remaining incandescent light but it had a dead battery and is now recharging.

    Basically when tested from inside and outside the indoor pane reflects the same colour as the light and the outdoor pane has a purple tint. I can't seem to determine which side of the glass the tint is on, but i think it would make more sense to be on surface 1. That said i would be surprised if these are low SHGC windows, being in Canada i would expect Low E on the inside. Perhaps i should test both sides with an IR thermometer?

  17. Alan B | | #17

    This seems to be about the best photo i can get.
    Surface 1 and 2 appear purple, surface 3 appears green and surface 4 is white (same colour as the light).
    This is taken from inside, since its raining outside i can't repeat the exterior test till at least tomorrow night

    One thing i learned is low light level is needed, i was using the LED flashlight at too high a brightness and needed to get down to under 5 lumens.
    I was able to see the same green with a lighter instead of the BBQ lighter, but it did not photograph well.

  18. Expert Member
    Dana Dorsett | | #18

    It's definitely surface #3 has the low-E coating in your most recent picture.

  19. Alan B | | #19

    So am i correct in interpreting that this means it redirects IR/UV thats in the house back into the house but is not low SHGC?

    If thats correct then white or reflective window coverings on the inside would be ineffective, because they can get in the window but can't be reflected out?

  20. GBA Editor
    Martin Holladay | | #20

    The issue is complicated. A window expert who posts information using the screen name "Oberon" has posted the following comment on an earlier Q&A thread:

    "A high solar heat gain (HSHG) LowE coating will allow direct solar energy to pass thru the coating and will prevent heat from passing back thru the coating to the outdoors when the sun isn’t shining. This coating offers the advantages of passive solar energy gain with the advantages of the heat-blocking ability of the LowE.  Placing this coating on the #3 surface of the IG (the outer side of the interior lite) affords a bit more solar gain advantage as well. But, placing the coating on that surface will make the coating less effective in the summer months in keeping outside heat outside, so there is a trade-off. LowE coatings are generally placed on surface #2 – interior of the exterior lite – except in extreme northern climates."

    Another window expert, Tex McLeod, posted the following comment:
    "In very general terms, you would look for surface 2 in an air conditioning climate and surface 3 in a heating climate. The U value [U-factor] of the window remains constant in both placements but the SHGC varies. According to lab work conducted by LOF, solar heat gain through Energy Advantage glass is reduced by about 10% by installing the sealed unit so that the coating is on surface 2 rather than surface 3.

    "In theory, you can advise your window manufacturer which way you want the sealed unit to be installed in the frame by virtue of whether you want the additional solar heat gain or not. In other words, what is more important to your client - passive solar heat or reduced air conditioning loads.

    "As mentioned earlier, it is not that easy because of the range of glass products (hard coat, soft coat, suspended films etc.), their properties and the way in which manufacturers use the products. For example, some manufacturers have a low E coating on both surface 2 and surface 3 to virtually eliminate solar heat gain. Most builders haven't done a very good job of managing solar heat gain and as a result, many manufacturers have way more problems with overheating than general comfort. Therefore the bulk of glass offerings are adjusted accordingly to design out solar gains, much like you would find in commercial installations."

  21. Alan B | | #21

    So my takeaway is that if i have a low E window solar gains get in but not out.

    "A high solar heat gain (HSHG) LowE coating will allow direct solar energy to pass thru the coating and will prevent heat from passing back thru the coating to the outdoors when the sun isn’t shining"

    This does say it prevents heat from getting out at night and i'm assuming that heat is UV/IR. But unless the coating is advanced enough (or self aware) to know the difference between day and night it should keep UV/IR in the house 24/7.

  22. GBA Editor
    Martin Holladay | | #22

    Q. "If I have a low E window solar gains get in but not out."

    A. Not quite true. Heat can flow in both directions through a window, whether it is low-e or not. Under certain conditions, you'll get solar heat gain (whether there is a low-e coating or not). Under other conditions, heat will flow from the interior to the exterior (whether there is a low-e coating or not).

  23. Alan B | | #23

    Are you speaking of conduction or radiation or both?

  24. Expert Member
    Peter Engle | | #24


    In all of these discussions, we're talking about radiation, and the wavelength (color) of the radiation is very important. When we talk about "heat flow," we are talking primarily about long-wave IR radiation. Not UV.

    Broad spectrum sunlight contains everything from long wave IR to very short wave UV. Most of the long wave IR and short wave UV are filtered by the glass and coatings, so the remainder that gets into the house is mostly visible light, with a bit of near visible IR and UV. That energy is considered the 'solar heat gain.' When that light is absorbed by furniture and other surfaces inside the house, they heat up, and warm surfaces emit heat as long-wave IR radiation.

    Low-e coatings reflect long-wave IR. So when the low-e coating is on surface #2, it doesn't do much to stop visible light from coming through the window, but it does reflect "heat" (long-wave IR) back into the house, keeping the house warmer 24/7.

    So it's more accurate to say that with a low-E window, visible light gets in, but heat doesn't get out.

    Going back to the OP, reflective materials inside the window can be very effective, because they reflect the light at the same wavelengths that they receive it. If the window is relatively clear (HSHG), the light passing through the window from outside to inside can be reflected right back out again.

    Going to the other extreme, black window shades won't work. They will still block the light, but they will absorb it, heat up, and re-emit the light as long-wave IR. That long-wave IR is reflected by the low-e coating right back at the shade and the net result is that all of the heat gain still enters the room because the shade is emitting heat from both sides and only the side facing the room actually ends up losing heat.

  25. GBA Editor
    Martin Holladay | | #25

    You wrote, "So it's more accurate to say that with a low-E window, visible light gets in, but heat doesn't get out."

    Sorry -- that doesn't pass my accuracy test. The rate of outward heat flow through a low-e window is definitely measurable. Clearly, on a cold winter night, or even a cool spring night, "heat gets out" a low-e window. In fact, the low-e windows on a code-minimum house will be leaking heat at a much higher rate than the solid walls and ceiling.

  26. Jon R | | #26

    With Peter's opening qualifier "In all of these discussions, we're talking about radiation...", it's reasonably accurate.

    On the other hand, conduction is important - include it and air tight R2 black shades will work to some extent.

  27. Expert Member
    Peter Engle | | #27

    OK,OK, if you want to be picky.

    Martin: You are right. Heat does get out because the coatings aren't perfect, and because of conduction (see below). But the point of the low-E coating is that it reflects heat (radiant long-wave IR) back towards the house, so that less heat gets out than with an uncoated window.

    FWIW, this is also why you feel more comfortable standing in front of a low-E window when it's cold outside than an uncoated window. The low-E coating reflects your body heat back at you, so you don't "feel" the cold outside. I think there are some articles around here that discuss mean radiant temperature...

    Jon: You are also right, in that I completely ignored conduction, because this thread has been primarily about coatings and radiant heat transfer and I was answering Alan's question on line 23.

    The OP was about cellular shades, and they work on both conduction and radiation. R2 black shades will conduct (or block) heat at the same rate as R2 reflective shades. But, the outer surface will get hotter because it absorbs all of the visible light being transmitted through the window. Eventually, all of that absorbed heat will be transmitted through to the interior via conduction (and some convection around and behind the shade). With reflective shades, the visible light (or most of it since we're being picky) is reflected back to the exterior, and it leaves the house entirely. The net effect is that more heat will penetrate an R2 black shade than an R2 reflective shade. So I think that makes me mostly right ;-)

  28. Jon R | | #28

    > Eventually, all of that absorbed heat will be transmitted through to the interior

    Probably not - if the space between shade and window is 120F and it's 90F outside, some solar heat gain will move back to the exterior via conduction.

    But we agree, reflective shades work much better.

  29. Alan B | | #29

    Sorry it took me a while to get back to this thread.

    I am ignoring conductive heat movement because i'm not replacing the windows. If i were then everything would be on the table.

    I am assuming the conductive heat gain is from the temperature difference from outside to inside on a hot summer day since hot chases cold and the cellular shades adding R2 is inconsequential because the heat is still inside the building, though it may concentrate and push some outward but its not very tight so i'm rating them as a wash for conductive insulation.

    As for radiative @Peter makes a very interesting argument.

    First are we sure any glass window filters long wave IR and short wave UV?

    The video i posted above, what kind of UV is it?

    Are we sure that all that gets in assuming a high SHGC window is visible light, with a bit of near visible IR and long wave UV?.

    Do we have confirmation that heated up objects emit only long wave IR?

    What percent efficiency is a low E coating?

    Does a low E coating on surface 3 coating block anything from getting in from outside?

    @ Peter I find your post very interesting because it suggests we can block most of a low E window's (non conductive) solar gain with white titanium dioxide shades or blinds.

    Finally any idea how strong what gets in the window is, can it be measured without expensive equipment, how much does it vary from winter to summer?

    My apologies for asking so many questions but this is a very interesting topic to me and may make a big difference in window heat gain and whats posted here suggests most of the radiative gain can be eliminated even without low SHGC windows!

  30. Expert Member
    Peter Engle | | #30


    The "efficiency" of reflectivity, emissivity, and/or absorptivity of any surface or coating is a function of the basic material properties of the surface or coating, and also its surface characteristics. These values are often expressed in curves, because they are also related to the wavelength of light. Low-E coatings are selected because they are nearly transparent to visible light, but reflective in long wave IR. When we talk about "reflecting heat," we are really talking about reflecting a high percentage of long-wave IR. It is always a percentage. The answers for most of your questions above exist in the technical literature, but you would have to read quite a few articles to find them. The answers are also not yes/no, but "it depends on wavelength and other things."

    Here is a link to an article by Alpen that shows the light transmission of some different types of window glass:

    You can see that clear glass is a pretty good UV filter, but not very good at filtering near-visible IR. Low-E glass is better at filtering IR, because it reflects that portion of IR back at its source. "spectrally selective' glass is better in both UV and IR, but uncommon in windows because it is expensive.

    And yes, you can avoid much of your solar heat gain by installing a reflective material on your window shades. But this also makes your windows opaque. Most people install windows so that they can see out of them and also for daylighting. If you install opaque shades, you might as well just have a wall which is less expensive. Of course, you then have to turn on electric lights, which generate heat....

    The reason high-efficiency windows are so complicated and expensive is that the window manufacturers are working very hard to find durable materials that pass as much visible light as possible (so that we can see out and get free daylighting) while filtering out UV and IR (reducing fading and overheating), and that also reflect long-wave IR (heat) back the way it came in. That's a tall order.

    The point of the NYT article was that reflective cellular shades can help cut heat gain in very hot weather, and that they can make a significant difference in cooling costs and/or room comfort. This is all true, but you sacrifice daylighting and view. If you just need to use them on occasion, or in rooms that are seldom occupied (so that they do not require light and views), these shades can make sense.

  31. Alan B | | #31

    Thanks for that link, that is very interesting.

    I agree that if your going to reflect things out with shades your going to lose the primary function of the window but you can cheat a bit, when your at work you can reflect it out without much consequence and you can use blinds that are partially open at other times knowing your at least reducing the gain.

    Not perfect solutions like low SHGC coatings but unless one wants to replace Low E windows one can work with them and at least get some heat load reductions.

    And its interesting to note that blackout curtains will make the problem worse by converting all wavelengths to long wave IR which will get trapped inside hence its white and preferably the proper white blinds/curtains you want.

  32. Alan B | | #32

    I would guess that the Low E coating reflects long wave IR in both directions?

  33. Expert Member
    Peter Engle | | #33


    Not necessarily in both directions. The material properties of emissivity, reflectivity and absorptivity are all surface properties. The values we find listed for glass products describe how different wavelengths of radiation (light) act at the interface between the surface of the material in question and a gas (or vacuum). They change when the surface is adjacent to another solid or liquid surface. The best example of this is a one-way mirror. It reflects visible light from one side but transmits it from the other. Low-e coatings can work the same way, just in IR rather than visible light.

    This all gets complicated, and that's why the advanced glass manufacturers have so many options to select from. They have optimized the coating type and location for different climate zones, shading, heating and cooling requirements.

  34. Expert Member
    Peter Engle | | #34

    One more thing about about blinds.

    The thread got me interested, so i went out and did some reading. As it turns out, low-e coated glazings can be more prone to thermal breakage than clear glass, because the low-e coatings also cause more of the incident light to be absorbed in the glass than with clear glass. This results in more heating of the glass, and temperature differences between the center of the glass and the edge spacers cause stress that can result in breakage.

    This gets worse when shades or blinds are used, because they reflect the incident light and heat back at the window, as we've discussed above. This causes additional heat stress on the glazing. Cardinal recommends against doing everything we've been talking about: Shades should be dark, not light. They should be spaced a couple of inches from the window, and the space between the window and shade should be vented to the interior to prevent heat buildup between the shade and the glass.

    The takeway is this: If you've got high-quality low-e windows properly selected for your climate, you probably don't need blinds to further reduce heat gain, and they might damage your very expensive windows. if you've got cheap clear glass windows, go ahead and use light-colored blinds when it's super hot outside - they will help a little bit.

  35. Alan B | | #35

    @Peter That is interesting about how it may work one way only but it all depends on which formulation they actually used. I have wondered if it would be worth someone developing an LED light source at various wavelengths to test windows with in the field and a detector to see which wavelengths are getting through a window. It doesn't have to cost much as LED chips at many different UV/visible/IR wavelengths are already available (though at every wavelength needed?) but not sure what the detector would be, IR temperature readers are cheap but their wavelengths detected is an interesting question

    That is interesting about window coverings. I am not interested in the cellular shades in the NYT article but i was thinking of white aluminum Venetian blinds coated with white titanium dioxide paint, assuming such blinds are available and not too expensive. These would not be at all airtight and can be adjusted to let some light in when needed (say before and after getting home from work). But they would reflect many wavelengths back at the window but i would guess (with no data, just gut feeling) that this would not break window panes since they are not at all airtight and aluminum is the opposite of a thermal insulator.

    Just for the record i am near Toronto Canada, so we often get 30ºC+ (95+F) summers and -25ºC (-13F) winters, but most windows here seem to be Low E but not low SHGC.

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