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Old 06-12-2018, 05:42 PM   #1
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Question Dual Pane Windows - Blowout?

I have read a few threads where RV owners have complained when covering their dual pane windows with exterior sun shades (e.g. solar bubble film) they experienced the seals to fail or "blowout" from trapping the heat.

Fact?
Fiction?

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Old 06-12-2018, 06:53 PM   #2
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Good question, but I think the concern is placing something reflective on the inside of the window which reflects the heat back into the glass driving the temperatures even higher. I can see where it's possible the additional heat load could be harmful to the seal. Since a double paned window contains air between the panes and this air expands or contracts with temperature changes additional heat could increase internal pressure. This additional pressure, combined with heat stress on the sealer may shorten the service life of the sealer. I was told by a glass guy that tempered glass is good up to around 190 degrees. Could temps reach that in the desert sun with a reflective insulator on the inside? Dunno.

The concerns you read about, and the answers you'll get to your question, like so many others questions of this type, are based mostly on anecdotal evidence. I just looked up the specs on the sealer I used to reseal my windows. Its made by CR Laurence, and here's the temperature tolerance: "Service Temperature -40˚F (-40˚C) to 200˚F (93˚C)" That seems to be right in line with the temperature tolerance of the tempered glass itself.

I think in reality there's no real way to know if placing reflectix on the inside of a window causes failure because there are so many factors involved. I think the greatest cause of seal failure is mechanical flexing and vibration, as well as drain holes getting plugged in the windows frame leading to the sealer staying wet for prolonged periods of time, but hey, that's complete supposition on my part too. As for the glass itself failing? It's certainly is more possible to have the glass fail from the additional heat if the glass already contains a flaw such as an edge chip, additional internal strain, or is pinched tighter in the frame than normal, but there's no way of knowing after the fact what was the true cause of the glass failure. The heat could be the straw that broke the camel's glass.

What I have done to enhance the insulating properties of my windows is use multi layered thermal shade material hanging in the windows with a tension rod. The middle layer of this material is aluminized mylar sandwiched between two white cloth layers. They insulate without reflecting the light back and they act as blackout shades to keep the room dark - great for sleeping in on summer mornings. It's also more attractive from the outside. Yes, it's a little more bulky than the reflectix but it works very well so far.

Ok, I've rambled enough. LEt's see what others have to say about your question.
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Old 06-12-2018, 07:51 PM   #3
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Quote:
Originally Posted by astrnmrtom View Post
Good question, but I think the concern is placing something reflective on the inside of the window which reflects the heat back into the glass driving the temperatures even higher. I can see where it's possible the additional heat load could be harmful to the seal. Since a double paned window contains air between the panes and this air expands or contracts with temperature changes additional heat could increase internal pressure. This additional pressure, combined with heat stress on the sealer may shorten the service life of the sealer. I was told by a glass guy that tempered glass is good up to around 190 degrees. Could temps reach that in the desert sun with a reflective insulator on the inside? Dunno.

The concerns you read about, and the answers you'll get to your question, like so many others questions of this type, are based mostly on anecdotal evidence. I just looked up the specs on the sealer I used to reseal my windows. Its made by CR Laurence, and here's the temperature tolerance: "Service Temperature -40˚F (-40˚C) to 200˚F (93˚C)" That seems to be right in line with the temperature tolerance of the tempered glass itself.

I think in reality there's no real way to know if placing reflectix on the inside of a window causes failure because there are so many factors involved. I think the greatest cause of seal failure is mechanical flexing and vibration, as well as drain holes getting plugged in the windows frame leading to the sealer staying wet for prolonged periods of time, but hey, that's complete supposition on my part too. As for the glass itself failing? It's certainly is more possible to have the glass fail from the additional heat if the glass already contains a flaw such as an edge chip, additional internal strain, or is pinched tighter in the frame than normal, but there's no way of knowing after the fact what was the true cause of the glass failure. The heat could be the straw that broke the camel's glass.

What I have done to enhance the insulating properties of my windows is use multi layered thermal shade material hanging in the windows with a tension rod. The middle layer of this material is aluminized mylar sandwiched between two white cloth layers. They insulate without reflecting the light back and they act as blackout shades to keep the room dark - great for sleeping in on summer mornings. It's also more attractive from the outside. Yes, it's a little more bulky than the reflectix but it works very well so far.

Ok, I've rambled enough. LEt's see what others have to say about your question.
I'm mystified by that last comment. "insulate without reflecting..." You either reflect energy or you absorb it. No other possible choices. So either it is reflecting or absorbing. The later would defeat the purpose of the thing, so it apparently is reflecting. Note that is not the sun "light" that is the problem. It is the sun's infra-red radiation you want to get away from.
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Old 06-12-2018, 07:52 PM   #4
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In my opinion it's true that if the windows are covered on the outside with a dark sun blocking material in a hot sunny location could damage the window seals. This picture was taken by the previous owner showing how he covered his windows each winter in Florida. I think the dark material not only absorbed heat , but over- heated the window units they covered, blowing the seals. I have used the aluminum bubble sheets at times on the inside since owning the RV, but the aluminum doesn't seem to absorb any heat and so don't seem to heat the windows which also have moving air on the outside. I discarded all the dark brown covers.
The front slide windows and drivers windows on the other side of the RV where also covered the same way as the passenger window unit. Every window, both sides, under these dark brown covers, fogged from the seals going bad, the windows not covered and the windshield were OK. The driver's and passenger window units had 3 window in each, the 2 windows units on the front slide had 4 windows, so I had a total of 10 windows that needed seal repairs. I did buy the seals & caulking required and repaired them myself using these instructions :

Don't wait too long

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Old 06-12-2018, 09:42 PM   #5
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Having trouble following the logic of the previous post about covered windows causing fogging, especially the windshield. Windshields are not dual pane and do not have fogging issues.

The solar shades we had custom made all the way around for our coach do a fantastic job of keeping the sunlight away from the windows thereby keeping the heat from getting inside the coach. They attach to the outside, are nice and tight, offer privacy without blocking views and look great. All you have to do is feel the windows from the inside with and without the shades to know they are doing their job. Couldn't imagine not having solar shades on this rig now, especially on that front windshield.

We had foggy windows when we bought the coach and the previous owners had never covered their windows with anything, inside or out. It's just the nature of dual pane windows and has nothing to do with heat or anything else. The seals fail, the desiccant wears out and moisture sets in. Repair them and move on to the next maintenance item.
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Old 06-12-2018, 11:19 PM   #6
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Bob said, Quote: "I'm mystified by that last comment. "insulate without reflecting..." You either reflect energy or you absorb it. No other possible choices. So either it is reflecting or absorbing. The later would defeat the purpose of the thing, so it apparently is reflecting. Note that is not the sun "light" that is the problem. It is the sun's infra-red radiation you want to get away from."

I knew that particular comment would be questioned by someone but decided to leave it as written as is because I'd rambled on long enough. I tried to think of a better way to put it but just decided to leave it be. A better choice of words would have been "insulating while reflecting less energy back at the glass."

To explain what I meant - Yes, the internal reflective film still reflects some infrared right back through the fabric back at the window because that's what it's designed to do, and the fabric is probably pretty transparent at those longer wavelengths. Being covered by the fabric, the film it isn't reflecting other wavelengths as much as a bare reflective film such as "Reflectix" does. The fabric probably is slightly opaque in the near infrared and absorbs some both incoming and returning from the film. Without the fabric covering, other wavelengths add to the internal heating between the panes in a mini greenhouse effect. The double pane RV windows I've seen have a darker tinted outer pane and a clear inner one. The darker outer pane is dark because it absorbs light and this light is energy, whatever light that passes through the glass, when reflected back is subject to more absorption, some of this absorbed energy becomes heat, and some of that heat gets trapped between the panes, so it is about the sunlight too. The same happens to a lesser extent with two clear panes. That's why I used the general description of "insulating without reflecting", in retrospect a poor choice of words. Phew!

On a different note, and circling back to the OP question, I see a reply that confirms the covering on the outside of a window causing seal failure, and this is the first I've read of this happening. All other threads I've read for years, speaks of placing reflective material on the inside of the window and the problems it may cause with overheating the glass. I thought maybe the OP had heard it wrong or had misinterpreted the rumor but I see I was wrong in my assumption - my mistake, now corrected.

I wonder if there are other examples of this happening? I can't imagine how a shade that physically blocks 30% or more of the light can cause damage to the window seals. These types of shades are used for reducing heat load through windows in greenhouses and buildings, and are even sold to reduce the heat load on RV walls. One possibility I can think of off the top of my head and it's a reach at that - and I'm thinking out loud here - is the dark shade plastic itself absorbing heat, getting hot, and transferring this heat to the glass by contact? I it absorbing the sunlight and re-radiating in the infrared which the darker outer pane of glass absorbs? Even here there would still be a net loss in energy because the conversion/transfer isn't 100%. Could it be interfering with the glass' ability to shed heat by radiating back out into open air or by interfering with convective cooling because all the little holes become a stagnant boundary layer? Huummmm. . . . this is a head scratcher. Could using something as a spacer to create a small air gap between the shade and the glass be a simple solution???? Ok, enough thinking out loud. . . .

Since this is new to me I'm certainly curious about this claim and I'll be checking back here to see if others have had a similar experience. I'm fascinated to learn more about this and may do some "googling" too. My wife and I have discussed purchasing shade cloth shades of this type and depending on where this discussion leads, I may chose lighter colored shades and use a spacer for an air gap as one way to play it safe. I doubt a spacer would be needed on the windshield since this is laminated glass and not double pane. Interesting stuff for sure.
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Old 06-13-2018, 09:11 AM   #7
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As I mentioned, the windshield did not have a problem from being covered. The previous owner made the window covers himself and they where a solid material, you could not see through them, no light passed through them, maybe even no air movement under them. I never used the covers after buying the RV
after seeing what they appeared to have done to the windows. Only the Windows they had the covers over, fogged. I could be wrong, but my conclusion is these covers contributed to these window seals leaking. If you can see through your covers, they may be a much better material for this application.
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Old 06-13-2018, 09:15 AM   #8
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Actually TomM, the reply by Gregory doesn't confirm anything. It's his opinion only. Only certified independent lab testing would confirm that placing solar shades on the outside of a dual pane RV window causes seal failure. I put solar shades all the way around on our coach 9 years ago and I've never had a seal failure since. Had 3 windows done by Dave Root when we first bought the coach that had creepy seals/foggy windows.

My windshield in direct sunlight is 50 degrees cooler with shades than without. This argument that shades cause seal failure is preposterous.
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Old 06-15-2018, 08:11 PM   #9
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Originally Posted by astrnmrtom View Post
...Ok, I've rambled enough. Let's see what others have to say about your question.
Hey, ramble on, dude.
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