Wrapping the roof in white vinyl for heat rejection

In Honolulu where sun angles are higher than in the rest of the U.S., solar heating through the black roof feels more problematic than through the windows other than the windshield, so window tinting seems less important unless one is tinting primarily for appearance. I use a reflective window shade on the windshield which works pretty well. But after parking in the sun, I can feel heat radiating down from the headliner making my head warm while my body is cooled by the A/C. It takes longer than my usual trip for the roof to cool, so it would be nice to prevent the black roof from absorbing heat.

I'm considering wrapping the roof in white vinyl to reflect most of the solar radiation. Unfortunately, with an Arravani Gray car, a white roof would make it a 3-tone car which isn't a great look. If I also wrapped the black hood and glass hatch with white vinyl, almost all of the glossy black would be wrapped, so the car would be mostly a 2-tone car. A secondary advantage might be to make the glass hatch less prone to breakage from minor scratches.

If you've wrapped your roof in a reflective color, has it helped keep your car cooler? What do you think about the appearance of a white roof? I should use an image editor to preview any color changes that I am considering.

For maximum efficiency, a reflective barrier needs an air gap below it, otherwise, conductive heat will win in the end.

I bought the (overpriced!, but on sale) i3 climate cover earlier this year and was surprised at how efficient it is at keeping the cabin cool. Not being intimately in contact and the reflective coating would probably make it more efficient (although less convenient!) than wrapping the roof.

On my car the roof isn't black in the same way as the bonnet (hood) but is bare 'black' Carbon Fibre - I guess they left it bare to save weight.

banner said:

On my car the roof isn't black in the same way as the bonnet (hood) but is bare 'black' Carbon Fibre - I guess they left it bare to save weight.

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That's true of our i3 as well, but its very dark color absorbs solar radiation quite well. I don't know why BMW didn't paint the roof like they did the hood (bonnet). The roof is CFRP while the hood is thermoplastic, but that wouldn't have prevented the roof from being painted. The roof is finished with a clear glossy coating of some sort unlike the exposed CFRP in the door jams. Maybe BMW decided that the roof looked "black enough" without paint.

I have my roof, spoiler, bonnet, wing mirrors, bottom of A pillars, and the black spot on the sill wrapped in corporate purple. I think it looks good.

Had to wrap all the bmwi blue bits too, as they clashed with the corporate colour. Didn't bother doing anything with the tailgate, as I didn't want to mess about with its lines.

I got it done when very new, so can't comment on whether it changed the inside temperature, but I imagine wrapping in white would be reasonably effective . White cars always feel cooler than black ones in the sun, although I haven't got hard data to prove that.

Instead of concentrating on the roof, you're going to be better off using some type of heat blocking film over the glass (windows & windshield) as cars are like ovens and the sun will heat up the car more through the windows than the roof.

Something like a 70% 3M Crystalline window film/tint (if you don't want any dark tint) all around will greatly reduce the heat inside the car.

SteveMondragon said:

Instead of concentrating on the roof, you're going to be better off using some type of heat blocking film over the glass (windows & windshield) as cars are like ovens and the sun will heat up the car more through the windows than the roof.

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My problem isn't the typical temperate zone hot interior temperature after parking in the sun but heat radiating from the headliner while driving after having parked in the high tropical sun that shines almost straight down on the roof. Except for the windshield, very little sunlight enters the interior through the side and hatch windows due to Honolulu's very high sun angle during the heat of the day. I use a reflective windshield sun shield which works well to reflect sunlight so that it can't heat interior surfaces.

So unlike in temperate regions, my problem would be dealt with better by reflecting as much solar radiation off the roof as possible rather than blocking the little solar radiation that enters the side and hatch windows.

Hello Art,

Did you end up wrapping the center roof panel of your i3? I am in a similar predicament. While tinting the windows with Ceramic tint helped, you can really feel the heat radiate from the roof - this might be because the interior roof feels closer to the top of my head compared to other cars (including the Gen 1 Insight)

EVBob said:

Did you end up wrapping the center roof panel of your i3? I am in a similar predicament. While tinting the windows with Ceramic tint helped, you can really feel the heat radiate from the roof - this might be because the interior roof feels closer to the top of my head compared to other cars (including the Gen 1 Insight)

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I haven't done so yet because we spend only ~6 winter months per year in Honolulu with our i3, park inside while at home, and don't drive much (i.e., heat radiation from the roof hasn't been a serious enough problem yet). Also, I'm not certain that I would like a white roof on an Arravani Gray i3 (i.e., 3-tone paint). I have considered also wrapping the frunk and glass tailgate in white which would cover most of the black parts thus reducing it to mostly 2-tone paint, but I'm concerned about the sun reflection off a white frunk while driving.

It was suggested that I wrap the roof with a clear infrared-blocking film that would not affect the roof's color, but I don't know if such a film exists. It would be like 3M’s Crystalline tint but designed for exterior application. I believe that exterior window films exist for installation on building windows, but these would be designed for perfectly flat windows, so I don't know how well they would work on an i3's slightly curved roof.

I plan to investigate further when we return to Honolulu in ~5 months after which we will be spending significantly more time each year in Honolulu.

alohart said:

...
It was suggested that I wrap the roof with a clear infrared-blocking film that would not affect the roof's color, but I don't know if such a film exists. It would be like 3M’s Crystalline tint but designed for exterior application. I believe that exterior window films exist for installation on building windows, but these would be designed for perfectly flat windows, so I don't know how well they would work on an i3's slightly curved roof.
...

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As far as 3M's product offerings go for exterior window film - this might work: http://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Sun-Control-Window-Film-Prestige-Exterior-Series-for-Residential?N=5002385+8709316+8710654+8710938+8711017+8721691+8730573+3292123810&rt=rud

Brochure: http://multimedia.3m.com/mws/media/1173227O/3m-sun-control-window-film-prestige-ext-family-card.pdf

Its not the Crystalline product, but from the brochure the clear film (PR40x) can reduce solar heat by 53% and is rated for exterior applications.

Reading the brochure again - it looks like you can double up on this product - as in tinting/filming both sides of a window - if using this in conjunction with crystalline tinting on the interior side, it looks like one could reduce solar heat by an additional 5-10%...

EVBob said:

As far as 3M's product offerings go for exterior window film - this might work: http://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Sun-Control-Window-Film-Prestige-Exterior-Series-for-Residential?N=5002385+8709316+8710654+8710938+8711017+8721691+8730573+3292123810&rt=rud

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That looks promising. I have sent a message to 3M describing what I want. I'll report what I learn.

Here is an alternative that has worked very well for me on my ice car (one car garage which the i3 gets):
http://www.capoptop.com

It uses the wipers, side mirrors, and an adjustable strap on the rear to secure the cover to the car, Takes only a few minutes to install and folds up into a very small package. Keeps the interior cool and in your case Art also keeps the top cool preventing the headliner from getting hot. Best part it does not alter the look of your i3.

Noticed you have your Insight for sale. Trust me, you'll miss it. After 12+ years sold mine and still miss it.

Archie

archieb said:

Here is an alternative that has worked very well for me on my ice car (one car garage which the i3 gets):
http://www.capoptop.com

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The problem I have with an exterior cover is that rain, usually brief, occurs very often in Honolulu. I prefer not to deal with folding and storing a wet cover while returning home and then opening it up to dry after reaching home (it would mildew here very quickly if stored damp). Parking in an apartment parking structure, I don't have a place to leave a wet cover while it dries.

archieb said:

Best part it does not alter the look of your i3.

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I don't think a white wrap would look good, either. I'm hoping to find a clear infrared-blocking film that wouldn't change the appearance of the roof.

archieb said:

Noticed you have your Insight for sale. Trust me, you'll miss it. After 12+ years sold mine and still miss it.

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I know that I will miss it, but we're selling our apartment in Sweden where our Insight has been since 2009 and we don't need it in Honolulu now that we have an i3, so I hope that I can find a good home for it in Europe.

As I see it, there are two ways to avoid heat buildup:
- reflect it away
- absorb it and block it.

The problem with the later is how do you then prevent the absorbed heat from getting through via conduction. A reflector won't be perfect, and for that type of device to function well, requires an air gap, otherwise, it will still transmit some of the heat, since it, too, will become heated. I have 3M Crystalline film on my i3, and it does a pretty good job of keeping the interior heat buildup, at least through the windows. But, it at least feels like the actual glass is now hotter on the outside surface than it was prior to adding the film. In effect, the glass gets heated by the incoming rays, and then again by that going out that is blocked by the film.

What may be a superior solution would be to ventilate the car, as the ambient air is likely cooler than the IR heated surfaces. There are a few solar cell powered ventilation fans around. If BMW were to add a solar cell to the vehicle, it would be a viable option to use it to power the ventilation system, and if that weren't required, to potentially offset any parasitic losses on the 12vdc system running the alarms, and computers in their sleep mode verses having to get power from the main, propulsion battery pack.