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Old 09-21-2011, 11:01 AM   #1
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adding reflector to roof..

Has anybody actually added a reflector panel to the roof of their airstream. thinking about curved ribs (maybe a 1" squre tubing and white aluminum panels with a 3" gap at the center, top for venting and some sort of rolled edge on all the edges. Screwed down to the channel so that they could be removed if stuff lodges under them? Multiple panels small enough for 1 person to handle. maybe 4 or 6 panels on a 25 footer. Anybody done anything like this. Did it help- reduce the temperature inside in the sun? I have done some high temperature thermal measurments and shields like this are very useful. Do not know if they are effective at low temperatures though.

Or do I just paint the roof white? That is what got me to thinking about the reflector.
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Old 09-21-2011, 12:45 PM   #2
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Why not cover the roof with mirror tiles?
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Old 09-21-2011, 12:52 PM   #3
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Just Paint it White

I used a product called Snow Coat.
It will drop the inside temperature about 10 degrees F based on readings taken about 2 inches from the ceiling during a week of readings taken with the trailer parked in direct sun before and after snow coating.

The snow coat will provide about 1/8 inch build up, which also seals seams and rivet heads, and provides some protection from hail. Here's what it looked like on my 1976 inside Airstream Haven.
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Old 09-21-2011, 12:59 PM   #4
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Already White?

Bill,
Aren't '88s already white? Mine '99 is, but I don't know when they started doing that.
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Old 09-21-2011, 01:25 PM   #5
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In order for a reflector panel to work there needs to be an air gap between it and the roof. Polished aluminum has about 90% reflectance and so does white paint but the white paint will soon mildew and loose reflectivity. The coating may have some R-Factor but it is small. A tarp made of the bubble wrap radiation barrier would work well. You don't want it to touch the skin or you might get corrosion where water collects. There are very few things that have a better reflectance than polished aluminum. If someone wants to send me some samples of polished, mill finish with clear coat and white cool seal etc. I can measure the reflectance (alpha) of them. We have an instrument here at NASA called a spectroreflectometer what we use for measuring paints and coatings for space craft.

Perry
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Old 09-21-2011, 02:26 PM   #6
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My '67 Land Rover had the "Safari" roof. That was another piece of aluminum about an inch or two over the real roof. Air could pass between the two pieces. It worked pretty well. Except that because it was designed for hot weather it didn't come with a heater. That didn't work too well in New York.
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Old 09-21-2011, 02:42 PM   #7
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White has a Higher Reflectivity

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Originally Posted by perryg114 View Post
In order for a reflector panel to work there needs to be an air gap between it and the roof. Polished aluminum has about 90% reflectance and so does white paint but the white paint will soon mildew and loose reflectivity. The coating may have some R-Factor but it is small.
Perry
On the tests we did, white has a much higher reflectivity than polished aluminum.
In fact, to the reflectometer, polished aluminum looked more like black.
This was counter-intuitive but consistent with the drop in inside temperature after the Snow Coat application.
Snow Coat has almost no R-factor, but it does seal the seams and rivet heads.
Neither white paint nor Snow Coat show any signs of mildew here in NM. I didn't notice any mildew when we lived in NC either but I keep my Airstreams pretty clean.
An second skin painted white with an air gap would be superb but challenging to install and maintain.
I have no data to support that costly an alternative.
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Old 09-21-2011, 03:12 PM   #8
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Show me data for your stuff. Here is the reflectivity of aluminum.

Reflectivity - Wikipedia, the free encyclopedia

Here is some data on white paint.

http://www.aztechnology.com/PDFs/AZ-93.pdf

You can't have better than 100% reflectance and that is what you are telling me. Physics is physics.

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Old 09-21-2011, 03:30 PM   #9
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Quote:
Originally Posted by perryg114 View Post
Show me data for your stuff. Here is the reflectivity of aluminum.

Reflectivity - Wikipedia, the free encyclopedia

Here is some data on white paint.

http://www.aztechnology.com/PDFs/AZ-93.pdf

You can't have better than 100% reflectance and that is what you are telling me. Physics is physics.

Perry
The reflectivity data you're linking to is for an aluminum MIRROR. Even the most lovingly polished panel on an Airstream that's been exposed to the elements for years before it was polished isn't likely to match the reflectivity of something you'd consider a mirror from an optical perspective, and its reflectivity will begin to degrade immediately as soon as it's exposed to the elements. If you clear-coat it you're also likely to reduce its reflectivity significantly.

Physics is physics, but theoretical perfection is rarely matched in real-world materials.
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Old 09-21-2011, 03:49 PM   #10
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White Light Perhaps

Perhaps you are considering the reflectivity of the visible spectrum.
What about UV?
We took our measurements beneath the painted aluminum plates and measured what was not reflected.


Theory is theory. Direct experience is direct experience.
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Old 09-21-2011, 04:08 PM   #11
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no, the 88 have grey aluminum for the roof. maybe now that the shuttle is grounded I can get some to there ceramic tile and cement and paint it white? any body want a million dollar airstream?

since I posted I have been thinking about it. have decided a few degrees temperature is not worth the prospect of making the roof leak. might go the white paint route.

Ken, are you happy with the Snow coat in general. Would it be good on a trailer that is parked outside in a humid climate with a lot of pine needles and leaves falling on it? Thinking about mildew and staining.

Thanks for any information
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Old 09-21-2011, 04:56 PM   #12
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Informative Thread

In response to Perry's request for data, I did find this thread:
http://www.airforums.com/forums/f4/i...s-13953-3.html

Reponse #35 was what prompted me to do my own experiments with different colored, aluminum panels.

This is a great thread, because it shows how counter-intuitive, IR relectivity can be. (Yes, I know I said UV in my earlier post, but dammit Perry I'm not a scientist, I'm a Marine.)

There is another thread that deals with painted panels. Give me time to find it.

And Bill, I cannot vouch for Snow Coat in your hot, humid climate. I live in NM as does the gent to whom I sold that Airstream.
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Old 09-21-2011, 04:59 PM   #13
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Most of the sun's energy is in the visible. UV is only a very small portion of the total energy. The graphs I posted go from the UV to the far infrared. Show us some data. White paint is going to degrade over time just like aluminum. There are two kinds of reflectance. One is specular like a mirror and the other is diffuse. It does not matter which direction the light is reflected as long as it is away from the surface. There is also the issue of how much energy is emitted in the infrared. Your paint probably has better emmisivity than aluminum so it looses heat quicker.

Perry

Quote:
Originally Posted by Alumaholic View Post
Perhaps you are considering the reflectivity of the visible spectrum.
What about UV?
We took our measurements beneath the painted aluminum plates and measured what was not reflected.


Theory is theory. Direct experience is direct experience.
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Old 09-21-2011, 05:09 PM   #14
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Another Fascinating Thread on Topic

http://www.airforums.com/forums/f474...d-13363-7.html

As summed up by this quote from Zepelinium

"The other thing I am missing is any discussion of the effects of emissivity versus reflectivity. This applies in two areas. First, in measuring the temperatures with non-contact thermometers, they see total IR, which would be a combination of both the emissions from the surface and the energy from reflection, so it seems to me that the temperatures of the panels should be measured by shielding them temporarily from direct sun radiation and then quickly taking a measurement, in order to get true surface temperature.

Second, the reflectivity of the white elastomeric coating is the primary thermal feature when it's on the outside (top), but its emissivity is the primary feature when it's on the bottom (inside). Oh, plus conductivity, which applies in both cases. Seems to me you'd want high reflectivity if it was outside and low emissivity if it was inside, so maybe white is really optimal for both surfaces (actually, gold is optimal for IR reflectivity, but I think it has to actually be gold, not gold color--ouch--and I assume white is low emissivity because black is generally high emissivity, but performance in the visual spectrum is no predictor of IR performance)."
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