Solar roof coverings- passive heat?

[Mumbly-Peg]

Hey everyone- first post here. I love these forums, and they've been an excellent resource, but I had an idea for something and I can't seem to find any posts about it.

My wife and I are planning to full time in an airstream in a year or so, and part of this time will be spent in colder than ideal temperatures. I've been researching skirting ideas, and the EZ snap RV skirts appeal to me because they can be installed without drilling, and they come in black for added passive heat.

My question is, couldn't the same idea be applied to the roof? (Given consideration for the vents, etc?) I could cut a custom black vinyl fabric cover to snap in place to pretty much act as a counter to the white roof on an airstream.

Has anyone tried something like this?

Thanks!

 

[MrNoodly]

There's insulation (I assume) between the roof and ceiling that's supposed to keep heat out to some degree. So the question is whether a black roof cover would be able to transmit enough heat to overcome the R value of the insulation. Meanwhile, heat wants to rise, not fall into the living space. I think it's easier and would probably work metter to position the trailer so that as much sun as possible comes in the windows.

 

[Mumbly-Peg]

That's true, and those are good points. At the same time, new airstreams come with white roofs, and plenty of people paint the the older ones white, so you'd have to think the roof color makes a difference. It shouldn't just go one way, right? The windows would surely also make a difference during the day as well (and then hurt at night, ha.)

As someone who has owned a black car and a white car, both the same model, and in immediate succession, I can tell you color makes a huge difference. Granted, a car doesn't have much insulation.

 

[vtwinkicker]

That's true, and those are good points. At the same time, new airstreams come with white roofs, and plenty of people paint the the older ones white, so you'd have to think the roof color makes a difference. It shouldn't just go one way, right? The windows would surely also make a difference during the day as well (and then hurt at night, ha.)

As someone who has owned a black car and a white car, both the same model, and in immediate succession, I can tell you color makes a huge difference. Granted, a car doesn't have much insulation.

My thoughts on this......black absorbs heat and white reflects so it will probably have some positive effect. I doubt the insulation is so adequate that it will not allow some heat transfer. If anything a black roof might slow the loss of heat from the Airstream to some degree.

I don't remember the exact numbers but I had lab data on the temperature difference of cars of different colors. The difference from white to black was in the range of about 130* for white car and about 180* for a black car in the sun with colors in between the two. Pretty amazing.

 

[Mumbly-Peg]

Thanks for that, Greg. I totally believe those numbers. I love driving a white car for that reason, actually. I argue with my brother in law who prefers the "murdered out" look. (Black paint, dark tint, black interior, black rims)

So suppose the roof of the airstream could be 60 degrees instead of 40, the heat loss would have to be significantly reduced, even if it would only be effective for 3-4 hours of the warmest part of the day, I'd have to think the propane usage and comfort factor would be positively effected.

Unless someone else pipes up, I might be the guenea pig!

 

[TrainChaser]

I'm with vtwinkicker -- The black should absorb some heat if it's sunny -- it doesn't have to overcome the R value of the insulation or anything else. But it should be useful just to help reduce the absorption of cold from the outside metal, as well as slowing the loss of heat from the inside.  IMO.

 

[SternWake]

A black surface will absorb more heat in sunlight, and also radiate more heat at night due to higher emmissivity of dark colors.

For a black covering to effectively transmit more heat in wintertme to interior, it would have to be in direct contact with the roof, heating that, and then at night one would remove the black covering to minimize heat loss.

Lots of work, little result, perhaps not even measureable, depending on amount of existing insulation.

 

[Mumbly-Peg]

I don't see removing every night as a benefit. If it's a thick vinyl/canvas material, it should almost act as a thin, mostly ineffectual layer of insulation. I don't see how it could cause any heat loss when the sun is down.

 

[SternWake]

I don't see removing every night as a benefit. If it's a thick vinyl/canvas material, it should almost act as a thin, mostly ineffectual layer of insulation. I don't see how it could cause any heat loss when the sun is down.

Go argue with physics.

https://en.wikipedia.org/wiki/Emissivity

Whether it makes a measurable difference to your prospective interior's night time temperature, is of course arguable, until some actual data proves it one way or the other.

 

[highdesertranger]

x2 on what Stern is trying to tell you, he is right. another point if you lay a tarp flat on your roof, if there is any dirt on your vehicle or on the tarp and the wind kicks up it's going to be like a big sheet of sandpaper. highdesertranger

 

[Gary68]

not sure it would absorb heat but would be a windbreak,dead air space and protection from rain

 

[akrvbob]

When I live in Alaska, I was in a travel trailer for a year in an RV Park. It was regularly -30 so I bought a bunch of 4x8 pink styrofoam and cut it to fit all over the roof with a minimum of 4 inches and in some places 6 inches. I then covered the entire roof with a super heavy duty tarp and tied that off to the RV to keep it on in the wind and protect from the snow, rain and ice. It was still miserably cold but I like to think it kept it from being deadly cold in there!

 

[TrainChaser]

This guy must have seen Bob's van covered with insulation and said to himself, "Self, I can outdo HIM!"
http://g3.dcdn.lt/images/pix/file70063286_cud-eastbound-van-17.jpg

 

[Mumbly-Peg]

I dare not argue with physics, Stern. I think if we're taking radiative cooling of black surfaces into consideration, we also should consider convection. Having an extra layer of anything at night, black, white, blue, purple, pink, would add some isolation from convective cooling.

At any rate, I think the effects that a black roof would have at night are negligible compared to the difference it could make during the day, assuming I'm in a part of the country that gets a lot of sunlight in the winter.

The sandpaper effect might be a more real-world detriment though.

 

[DannyB1954]

lots of videos on Youtube about solar air heaters. You can attach them to windows, and just close them off at night. People put different thing in them, (metal screen, painted soda cans, painted dryer vent hose, etc). One video I seen the guy said he tried a number of things and they were all about the same. https://www.youtube.com/results?search_query=solar+air+heaters

 

[wayne49]

This guy must have seen Bob's van covered with insulation and said to himself, "Self, I can outdo HIM!"
http://g3.dcdn.lt/images/pix/file70063286_cud-eastbound-van-17.jpg

Funny thing. Last night I was thinking about that guy and wondering about what happens to the straw in the spring melt? How much of a mess it is, etc.

And how that can only be done where winter comes in and stays hard until Spring. The straw pile would have issues in areas that have snow and rain during the winter

My mind went to having a few inches of gravel and good drainage away from the structure and protecting the straw from direct precipitation.

Seemed like it could be more trouble than benefit.

 

[TrainChaser]

Wayne, yeah, I saw 'issues' with it, myself. I usually buy straw for my duck house, and put a fresh one out in early summer to use as a garden seat. By spring of the next year, it's a MESS!

 

[tripper]

I had an RV cover on my trailer for a year, it had zipper access holes so I could open the door when the cover was installed. I left the cover partially installed for a while (covering maybe 2/3 of it) and noticed it kept the trailer much warmer in the cold, it was like having a jacket on it. Having the catalytic heater on low kept the entire trailer toasty warm when the cover was on. Not really convenient for short stays, but if you were to boondock for a while in a cold spot it sure helps.

 

[TrainChaser]

Wayne, much of the U.S. only does one-step thinking, and they don't understand cause & effect. There ARE people who will sit on a branch and cut it with a saw between them and the tree trunk, just because it's looks like a simple way to do it.

 

[Vanada]

The only experience I can share, and this is kind of a no-brainer, is that with the solar panels on the roof up front and no insulation in I was getting condensation on the bare metal inside but only at the rear. Snow and frost could settle there and obviously made it a lot colder whereas beneath the panels there wasn't a lick of condensation (which was freezing obviously... Nightmare). If you could suspend something (like a tarp) over the roof enough to create an air gap it should do just enough to make a difference.