Roof paint for a car? (Repel heat / keep interior cooler)

[Jez77]

Originally Posted by oldtamiyaphile

That's because most of the heat comes in through the windows of a typical car.

I think in hot climates like OZ there's simply nothing can be done to keep the heat out. You may be able to slow the heat rise a little but after an hour there will be little difference. The sun simply has too much energy and short of making all the windows and panels 100mm thick your never going to keep it out.
Even the old school safari roof won't help a lot of the time as the sun is rarely directly overhead.

According to the article leaving windows down was the only effective method to keep heat down. It's the method I use even though it's a security and rain risk.

[freebeard]

Even the old school safari roof won't help a lot of the time as the sun is rarely directly overhead.

Heat re-radiates off the ground.

[Galane]

Spaz Stix Ultimate Mirror Chrome will make it shine like a mirror, if the surface it's sprayed on is mirror smooth. It shines on both sides so if sprayed on glass you get reflection both on the paint side and through the glass. It also works on plastics. It was developed to be sprayed on the inside of vacuum formed Lexan radio control car bodies. It will convert a CD jewel case lid into a styrene mirror.

For the brightest shine you'd want to apply this Spaz Stix paint over a bright white, high gloss paint.

Rust-Oleum makes a similar paint but it only reflects through what it's applied to. The paint side remains dull. It doesn't work on styrene because the carrier solvent used attacks that plastic. They also make a gold version. Not as shiny but it's the only paint I've found that shows a smooth metal finish through a transparent substrate. All other "gold" paints I've tried show a speckly 'metallic' finish even when on the paint side they appear to have a fairly uniform and shiny metal look.

[mad_b]

Originally Posted by Daox

There are other options for paint out there. There is stuff that has additives to reflect more heat. I doubt its worth the extra cash looking into it for an automotive application. However, it is interesting stuff.

A quick google search found this company:

https://www.ultimatecoatings.net/

Looking at the photo, where the thermometer reads IR radiation to determine heat, I became a bit confused whether: if the temp readout is higher would not meat that it is reflecting more heat than the one that shows less IR temperature...

[aardvarcus]

There are two large factors that will influence the external temperature of the vehicle for a given surface coating, reflectivity and emissivity. In a nutshell, reflectivity is how much of the energy striking the surface is reflected back versus how much is absorbed. Emissivity has to do with the amount of energy that is re-radiated back off of the surface as it heats up. (Yes I understand this is a very simplified explanation and that is is actually more complicated.)

Even for darker paint colors, the overall reflectivity may be different due to reflectivity coefficients in non-visible spectrums. (E.g. cool roof shingles that aren't white.) The other complicating factor is that even if a material has high reflectivity if it has low emissivity it may reach a higher temperature because it can not shed heat as effectively. That is why some specialty coatings or polished metals are actually worse than a typical painted surface.

The other factor to consider is how much the temperature of the outside surface of a car impacts the temperature of the cabin. This is where insulation and/or ventilation comes into play.

This hits home with me, as my new (1999) 4Runner is black, which means I am at a reflectivity handicap. Don't really plan on painting this ones roof white. I have some peel and stick foam, which I plan to apply to the interior of the roof surface to slow the thermal transfer of heat into the cabin. I also plan to put thinsulate in between the headliner and the roof for the same purpose.

Being able to park the vehicle inside or under cover makes a huge difference obviously. A solar powered fan would be ideal for ventilation, but cracking the windows (assuming low chance of rain) can also be effective. Simple things like opening all the doors for a minute or two prior to getting in the vehicle can also make a huge difference in comfort and how quickly the AC can catch up.

[ToddT58]

I'm currently running an experiment with three identical 40' shipping containers sitting next to each other. I painted one with "mobile home" white coating. Stuff from Home Depot, total material cost about $100. The second unit I painted with the fancy NASA-spin off "ceramic" white paint. Material cost on this was around $800! The third unit was left uncoated as a control.

The white roofs were 36 degrees F cooler than the uncoated unit. This was measured on the exterior with a laser thermometer. The interior temperatures were 16 degrees cooler for the "mobile home" white and 18 degrees cooler for the NASA-spin off white.

Important to note: I did not coat the exterior walls. I think that would make a big difference... similar to the heat gain through the windows in a car. That said, back in the '70s I put chrome film on the windows. Much cooler. I later went to a very dark window tint and it seemed cooler.

By the way, I live in Louisiana and have been driving white trucks for 20 years.

[Shaneajanderson]

Originally Posted by ToddT58

I'm currently running an experiment with three identical 40' shipping containers sitting next to each other. I painted one with "mobile home" white coating. Stuff from Home Depot, total material cost about $100. The second unit I painted with the fancy NASA-spin off "ceramic" white paint. Material cost on this was around $800! The third unit was left uncoated as a control.

The white roofs were 36 degrees F cooler than the uncoated unit. This was measured on the exterior with a laser thermometer. The interior temperatures were 16 degrees cooler for the "mobile home" white and 18 degrees cooler for the NASA-spin off white.

Important to note: I did not coat the exterior walls. I think that would make a big difference... similar to the heat gain through the windows in a car. That said, back in the '70s I put chrome film on the windows. Much cooler. I later went to a very dark window tint and it seemed cooler.

By the way, I live in Louisiana and have been driving white trucks for 20 years.

So was the chrome on the windows better, or the dark black tint?

Are you going to try painting the sides too to see what difference it makes?

[ToddT58]

I remember the black window tint (window van) being cooler. It was much darker inside the van. However, I added spray foam insulation to the rest of the interior so that kills any apples-to-apples comparison.

As an aside, I also remember buying a solar powered fan that fit in the side window of a vehicle. The fan pulled out a little of the vehicle's hot air but the greenhouse effect of the car made hot air faster than the wimpy fan could suck it out!

[California98Civic]

FWIW, this was already discussed once on EM:
https://ecomodder.com/forum/showthre...tml#post308497

[strix]

I can help you here.

I did paint the roof of my own car white.
As its an old car, i used white metal paint with anti rust properties too - usually ppl paint fences with that paint.

Before that car roof was british racing green. Every time i got in the car i feelt like i was in sauna. The heat accumulated by the roof severely radiatet inwards. It was like being 5 cm away from an electric 5kw heater on full blast. Not even an extra layer of heat reflecting insullation tucked between metal and rooflining helped. It was quire severe situation during summers.

Now this problem is solved. No more furnace effect from roof when entering the car. I cannot comment on specific numbers, or the total drop in temperature of the cabin as ive not measured it before and after.

What i can tell you exactly - before painting, roof lining was uncomfortably too warm to hold your hand on. Now its barely warm.

White color works miracles. My next car will be white due to AC economy and comfort considerations.