Help! My Windows Are Melting My Grass!

[Ben]

Actually they're not melting my grass, but I ran into this on reddit, and did not know it was a thing that energy efficient windows, when reflecting the sun, can melt artificial turf, ruin car paint, and melt vinyl siding. It was of interest to me because this time of year, when I take the dog out for pre-bedtime business, the setting sun is directly in line with my office window, with the back patio in between. When I walk through that line, heat hits my head and it feels like I'm an ant under a magnifying glass.

I did not know this was common and that there are films specifically designed to stop it, which I am now considering putting on that window.

https://windowfilmforturf.com/pages/what-causes-vinyl-siding-to-melt#:~:text=Melted%20Vinyl%20Siding,-Experts%20call%20the&text=Energy%20efficient%20windows%20can%20melt,here%20is%20much%20the%20same.

[WLJ]

Can't speak to the effectiveness of the flims but this may be of some related interest
From 2013

'Walkie-Talkie' skyscraper melts Jaguar car parts
https://www.bbc.com/news/uk-england-london-23930675

No more Walkie Scorchie! London skyscraper which melted cars by reflecting sunlight is fitted with shading
https://www.dailymail.co.uk/news/article-2786723/London-skyscraper-Walkie-Talkie-melted-cars-reflecting-sunlight-fitted-shading.html

[Brad Johnson]

Go with a Fresnel lens and make the most of it. Everyone needs their own personal Death Ray.

Brad

[HankB]

When an entire building is curved, it acts like a huge concave mirror and can indeed damage whatever lies near the focus.

Individual windows acting as individual mirrors? Well, sure, but a flat mirror won't give you any more light on a surface than the original source. Necessarily LESS, of course.

Color me skeptical when it comes to individual flat windows being sufficiently concave to act as solar concentrators. A quick search shows that vacuum insulated glass has pillars every couple of inches between the 2 layers of glass to support the glazing against atmospheric pressure, limiting the concavity.

[Ben]

When an entire building is curved, it acts like a huge concave mirror and can indeed damage whatever lies near the focus.

Individual windows acting as individual mirrors? Well, sure, but a flat mirror won't give you any more light on a surface than the original source. Necessarily LESS, of course.

Color me skeptical when it comes to individual flat windows being sufficiently concave to act as solar concentrators. A quick search shows that vacuum insulated glass has pillars every couple of inches between the 2 layers of glass to support the glazing against atmospheric pressure, limiting the concavity.

Sorry, personal experience says you're wrong. My window is flat, and it is HOT when I get between it and the sun when the angle is right. There are a ton of articles on this.

[HankB]

Sorry, personal experience says you're wrong. My window is flat, and it is HOT when I get between it and the sun when the angle is right. There are a ton of articles on this.

For a FLAT window -

If you get between your window and the sun, your window is shaded by your shadow, and won't reflect any direct sunlight.

If you position yourself so that you're in both direct sunlight and whatever fraction of sunlight the window reflects, you'll note a modest increase of insolation. (Note the spelling.)

If you position yourself so that you're shaded from direct sunlight but in whatever fraction of sunlight the window reflects, you'll note some insolation, but necessarily LESS than from direct sunlight.

If a LOT of windows are reflecting sunlight to the SAME area . . . that's a solar concentrator. Don't stand there.

[Kingcreek]

We went with window tint on big windows for our greatroom, west and south with sliding glass doors on the north.
Only problem we have is the damn birds. 3 different breeds have been beating the bell out of them and making a mess for about 8 weeks. We did it 3 or 4 years ago but this year has been terrible.

[Ben]

you'll note some insolation, but necessarily LESS than from direct sunlight.

Again, single flat double pane window and more - much more - heat than direct sunlight.

Understanding the Problem
The Science Behind the Reflection
Low-E windows, designed for energy efficiency, inadvertently create hot spots due to their reflective properties. These windows can act as magnifying glasses, focusing the sun's rays onto a small area, which can significantly increase the temperature of the surface it hits.

The Impact on Different Surfaces
Siding: Various types of siding, particularly vinyl, are susceptible to warping and melting when exposed to high temperatures.
Artificial Turf: Synthetic materials in artificial turf can melt or deform, altering the appearance and functionality of the landscaping.
Other Surfaces: Concentrated reflections can also affect automobiles, patio furniture, and other outdoor items.

Reports show the coating used on the windows to reflect the sun, combined with a concave effect in some windows, magnifies and intensifies the beam up to 200 degrees Fahrenheit. The problem is worse in late fall, winter and early spring due to the low angle of the sun.

Problems with low-e windows have been reported across the country. The beams have melted vinyl siding, plastic and paneling on cars and garbage cans. Reflections from the windows burned people at a pool at the Vdara hotel, a high-rise in Las Vegas. The windows on a London skyscraper were blamed for melting part of a Jaguar XJ. The Consumer Product Safety Commission said beams from sunroom roofs have started fires on cedar shingles in at least four homes.

[HankB]

Again, single flat double pane window and more - much more - heat than direct sunlight.

A simple flat reflector won't focus light.

Reflections from the windows burned people at a pool at the Vdara hotel, a high-rise in Las Vegas.

The wall of windows at the Vdara hotel is not flat. Overlapping reflections from MANY windows concentrate sunlight.

The windows on a London skyscraper were blamed for melting part of a Jaguar XJ.

The wall of windows at the London skyscraper is not flat. Overlapping reflections from MANY windows concentrate sunlight.

[Ben]

It's single Low-E windows melting siding and starting fires. I already told you I personally experienced the heat effect and that there are literally dozens of articles on it you can find with a simple google search.

https://apnews.com/article/business-fires-8a4779a964566e84ac73d97aa32b517b
https://abc7ny.com/7-on-your-side-consumer-low-e-windows-emission/2264513/
https://www.fireengineering.com/fire-prevention-protection/the-dangers-of-sunflight-reflected-off-energy-efficient-windows/

[Ben]

Just now took the thermo out to get window readings. One showed 264F, and once I got it lined up (difficult without going blind) I hit 381F. This was at 845PM.

[K Frame]

"A simple flat reflector won't focus light."

Generally I'd agree with that, but we're talking about low-E windows, and they are not simple flat reflectors. There's a lot more going on in a low-E window than just a couple of panes of glass.

The new windows that I have have a reflective barrier in them that's designed to reflect higher angle sunlight (summer sun) to keep it from entering the home, but will allow lower angle sunlight (winter sun) to pass through into the home as a means of passive solar heating.

I've not read the articles that Ben has linked yet, but I suspect that this may have something to do with these reports. These coatings may be acting partially as concentrators when they reflect the high-angle sun. 

[Ben]

I've not read the articles that Ben has linked yet, but I suspect that this may have something to do with these reports. These coatings may be acting partially as concentrators when they reflect the high-angle sun.

That's basically what they're saying, and that also they may be creating a slight internal concavity, or a concavity effect. I can't visually see such in my window, at least not enough to create a multiplier effect from the physical surface. Both sides that I can hold a straightedge to are flat.

[K Frame]

Well, remember, a Fresnel lens can be flat as a pancake over its entire structure, it's the angles that are incorporated into that flat structure that create the lens effect. I suspect that is what is going on here. Yes, the membrane or coating or whatever it is inside the window is flat, but it's the how the structure is arranged.

[K Frame]

In your case, Ben, I think you may want to put up some roller blinds on that porch to block the summer sun from hitting that window.

[Ben]

In your case, Ben, I think you may want to put up some roller blinds on that porch to block the summer sun from hitting that window.

That would be preferable to me versus dealing with film that will have potential bubbling, etc. The problem is that the window is aligned directly with the prevailing wind whenever we get wind events. I would need to figure out someway to attach a blind that I could secure at the bottom as well, to keep it from blowing around. If not to the point of it just flying off (the wind will lift siding here), to the point that it doesn't whack against the house all night long.

[HankB]

Just now took the thermo out to get window readings. One showed 264F, and once I got it lined up (difficult without going blind) I hit 381F. This was at 845PM.

OF COURSE you're going to get high readings using an IR thermometer that way - you're basically pointing it at the reflected image of the sun! You'd get even higher readings with a common mirror, but as it is you're getting a little visible light reflection (probably around 4% per surface) but with higher IR reflection from the low E coating.  Point it at the sun itself and you'll get even higher readings - if you don't damage the thermometer.

Try putting some heat absorber - say, a black cast iron fry pan - in the light reflected from the window. Measure the temp of the fry pan after a couple of minutes. Then try putting the same absorber in direct sunlight and repeat the measurement.

[230RN]

Thank you, Hank B! 

I have a bamboo drop-down curtain outside my west kitchen window.  It does get blown around  a bit, but since there are spaces between the bamboo strips, it is'nt as good a "sail" as a solid sheet of material.  Yet it block just enough to keep the kitchen cooler.  It would also limit outside reflections, although I don't have that concentration problem anyway.

I recall an S-F story where the fans of a soccer team were angry at bad calls from a particular referee.  Next game, all their game programs had mirrors on them.

Next time that ref made a bad call, they all shined their mirrored programs on him and he got incinerated.

I figure, say, 5000 fans with 10cm x 12cm mirrors shining the sun on him, gets him blasted with about 45 kiloWatts...more or less, kinda sorta, roughly, if my arithmetic is correct and assumptions anywhere near reality.  So call it the power of 450 100 Watt light bulbs.

Terry, 230RN

[Ben]

OF COURSE you're going to get high readings using an IR thermometer that way - you're basically pointing it at the reflected image of the sun! You'd get even higher readings with a common mirror, but as it is you're getting a little visible light reflection (probably around 4% per surface) but with higher IR reflection from the low E coating.  Point it at the sun itself and you'll get even higher readings - if you don't damage the thermometer.

Try putting some heat absorber - say, a black cast iron fry pan - in the light reflected from the window. Measure the temp of the fry pan after a couple of minutes. Then try putting the same absorber in direct sunlight and repeat the measurement.

So you still haven't read any of the articles.

[HankB]

. . . I recall an S-F story where the fans of a soccer team were angry at bad calls from a particular referee.  Next game, all their game programs had mirrosr on them.

Next time that ref made a bad call, they all shined their mirrored programs on him and he got incinerated.

It was a long time ago, but I remember reading that story myself as a kid . . . and wondering at the time how tens of thousands of fans would each be able to aim their individual reflections accurately.

[HankB]

So you still haven't read any of the articles.

Read them. The absence of scientific analysis - diagrams, images, measurements, etc. - makes them about as meaningful as a CNN story about polar bears drowning due to global warming.

[Ben]

Read them. The absence of scientific analysis - diagrams, images, measurements, etc. - makes them about as meaningful as a CNN story about polar bears drowning due to global warming.

There are images and videos documenting the phenomenon, as well as the link from a firefighting engineering site, and I gave you my own evidence to include actual felt heat on my body. So I'm done arguing with you. Have a nice day.

[K Frame]

Well, Ben, since it absolutely CAN'T be reflections from the windows because, despite numerous reported instances, windows are flat planes and can't reflect and concentrate sunlight, it sounds like you've got an invisible aliens using laser weapons problem.

Or Steve is *expletive deleted*ing with you.

[230RN]

It was a long time ago, but I remember reading that story myself as a kid . . . and wondering at the time how tens of thousands of fans would each be able to aim their individual reflections accurately.

That's why I picked "say, 5000" out of the assumed larger crowd. Other of my assumptions could be challenged, e.g., that sunshine amounted to about a horsepower a square yard.  Precise values depend on weather, latitude, time of year, etc.

I was thanking you for the explanation of the high readings from the IR thermometer.

[MechAg94]

The only thing that comes to my mind about this the internal reflective properties of the glass is supposed to scatter the reflection randomly, but sometimes the "scatter" lines up in one direction due to some issue in the manufacturing process.  .. Sort of like the Fresnel lens. 

Reminds me that I bought a couple fresnel lens to play with.  I need to test them out.