Painted white roof drops house temps 8-10 degrees
 

I just saw this over on builditsolar. This guy got an amazing difference from painting his roof white. I'd never even be hot in my house if I did this!

Experiment With White Roof Cooling Effect

http://www.builditsolar.com/Projects...nt/WhiteR2.jpg

Interesting. I wonder how less visually intrusive alternatives compare. Say a radiant barrier with increased attic airflow.

Yeah, I'd imagine you could achieve the same thing with an attic fan or two (possibly solar powered).

According to the FSEC (Florida State Energy Center) a radiant barrier in the attic can reduce home cooling needs by +/- 9% FSEC-CR-1231-01. A reflective white roof can reduce cooling needs by 18-35% FSEC-CR-1220-00. Those improvements include results for several homes that already had powered attic ventilation.

Thanks to BuilditSolar.com Passive Cooling Techniques for both links.

I've been very pleased with our cool roof (not white, but a good Solar Reflectance Index). We're midway through our second summer with the cool roof and it has made a big difference in our comfort level. :thumbup: I'm still looking to improve our home's passive cooling, but the roof definitely helped during a recent heatwave. We're Having a Heat Wave, a Tropical Heat Wave | EcoNewMexico.com

To be fair we need to look at the average cooling energy reduction data points, not just peak.

Radiant barrier (where thermostat temp was left unchanged between seasons): 19.7%, 11.3%, 16%, 27.2%, 5.3%, 8.4%, 0%.

Reflective white roofing: 18.5%, 21.5%, 24%.

Those two studies can't be used to compare the validity of both methods. The study on radiant barriers was performed on actual homes with people living in it with various insulations levels (R-9 to R-30) and ducking systems. The roofing study used 6 identical test homes.

Admittedly, the two studies aren't perfectly comparable. But, I think one can see the difference in heat reduction trends between a solar reflective roof and a radiant barrier.

If the ultimate goal is to reduce solar heat gain in an attic I think that a solar reflective roof surface would likely perform better than a "standard" roof surface with an underlying radiant heat barrier. The logic being that it's better to reflect solar heat from outside the roof structure (reflective cool roof) rather than from the inside (radiant barrier). That said, I'm certainly open to data to the contrary.

Of course, if you have a new(ish) roof surface a radiant barrier would probably be more appealing. But, if you need a new roof then choosing one with a high Solar Reflectance Index shouldn't cost any extra.

Here is a link to a manufacturer's metal roofing color charts with SRI ratings - Metal Sales - Colors (I have no affiliation with the manufacturer). I think it's interesting to compare the range of roof colors to their SRI. The color charts show that a cool roof doesn't need to be white.

FWIW,
Tim

Since you have a newer metal roof, can you comment on the changes you have noticed in the summer?

Unfortunately, this is mostly anecdotal as I wasn't tracking temperatures inside and outside the house as I do now. So, take this for whatever it's worth. :o

In the summer of 2003 we had a heatwave with high temperatures in the mid to upper '90's (record high of 99°F). Daily Averages for Santa Fe, NM (87505) - weather.com When I got home from work the house would be quite hot and I would open windows and use fans to vent the hot air (no AC or swamp cooler). The house would slowly cool as the outside temperature dropped. Often, I couldn't fall asleep until after midnight and slept with either no covers or at most one sheet on the bed due to the heat.

Flash forward to the heatwave of 2010 Monthly Weather Forecast for Santa Fe, NM (87505) - weather.com. We have a new "galvalum" propanel "cool" roof, cleared eave vents, a full ridge vent, and R-50 of cellulose insulation in the attic. We still use fans to vent hot air from the house (still no AC or swamp cooler), but now I work from home and watch the temps more closely. We've had days that have been just as hot, but the house doesn't heat up as much and it cools down more quickly. I also haven't had any sweaty and sleepless nights from the heat.

According to our indoor/outdoor thermometer the max. outdoor temp. has been 102°F and max. indoor temp. has been 85°F. While 85°F is warm the house cools down pretty quickly with fans as sun sets and the outdoor temp drops.

While I wish I had better data I am much more comfortable and I don't like the heat any more than I ever have.

FWIW,
Tim

Regardless of which one is better and the accuracy and validity of the tests, each method should be taken into consideration. Thanks for sharing this Daox.

The effect is due to the albedo of the surface. An albedo number close to 1 excludes virtually 100% of the heat, and lower albedo numbers closer to 0 absorb virtually all the heat.

hi
 

Its not so hot in your house if you paint it white. I think it is really cool and very neatly to an eyes.

When I was installing my own solar system last July I took a laser pointer thermometer gauge thingy up the ladder with me and started pointing it a various surfaces. I have a picture of the thermometer pointing at my medium grey flat concrete roof tile and the number 165 is reading on the gauge!! My feet weren't tingling because of "falling asleep",.....they we're burning!! I am so going up on the roof and painting a tile or two white and reporting back to you guys on the delta of the temperature sensor.

Blue Skies,

Jeff

You might get conductive heat from nearby tiles, that makes the white tiles warmer.

When I was in Bermuda, (in the Navy) we had hills on the base that were covered
in concrete and painted bright white. Looked like many of the roofs in Bermuda.
Those white roofs and the white mountains on the base were rain catchment systems.
(There are very few fresh water wells on the island).

In the typical home, there is a little manhole in the kitchen floor.
Lift it up and find the 'basement' is a giant water tank with catfish swimming around..

http://www.everettpotter.com/wp-cont...970c-800wi.jpg


Anyways, while I was there (4-15-63 to 4-15-65), it got pretty hot in the summer time.
But, inside of most homes wasn't uncomfortable, since the roof was bouncing
the sunlight away, and a concrete floor sitting on tons of cool water tends to keep it nice.. :)
I didn't know anyone that had AC in their home..

Sounds great but in mixed temperature climates it's probably not such a good idea. I can live with a hot house easier than a cold house. I can survive 90 degrees inside but 0 degrees would be difficult.

Energy solutions need geographic concern
 

To S-F,
So true. It's easy for us to forget that not every solution will work in EVERY place.

I like the idea of a large aquarium acting as a heat sink as my foundation. We could just sweep my daughters high-chair scraps right into a floor grate!

Imagine where our building codes would be if we didn't have several decades of relatively cheap fossil fuels. In the SW roofs might be mandated to be white, and every roof would have 500ft2 of south facing pitch w/o chimneys and soil pipes for solar retro-fit, maybe even conduit stubbed out.

Painting surfaces white in hot climates and dark colors in cold climates is ancient technology that was lost in this country. My only concern with painting my roof white is (current) cultural unacceptance. Who knows, maybe I'll be a trend-setter on the block!

I'll let you know when I paint the tiles the temp delta. I'm going to do more than a few adjacent tiles to thwart conductive transfer.

"cultural unacceptance".. That's kinda what I got from my wife before we had
the roofers come in and remove/replace the old shingles.

I wanted to paint the roof with some weather-proof stuff, with white roofing paint over it.
"Nope, that wouldn't look like all the other roofs on the street".. :(


But, I kinda get my way for a while, every winter.. :rolleyes:

http://i46.photobucket.com/albums/f1...0slot/roof.jpg

i have added radiant barriers to both my attics. omg. talk about work! it all sounds good until you're sliding through an attic with a flashlight in your hand, a mask on your face and a backpack full of stapling supplies and attempting to pull and kick an unruly roll of radiant barrier through while balancing on the 2x4's. (word to the wise--take a phone in case you need to ask for something.)

did it help? YES! absolutely had more impact on temperature than anything else short term.

we don't need need to turn on central air until the outside temperature is about 105--closer to 110 if we have done everything perfectly that day. without the radiant barrier i don't think that would be possible.

however, if painting the roof white would be as effective i would gladly do that instead and not really care about what it looks like.

I could be misunderstanding here, but I'm not sure conductive transfer applies in this situation. When I installed reflective I did it over 3 years without worrying about full coverage. Unlike insulation, reflective methods work independently adjacent areas.

Painting the roof would still be a good idea and would help. It will still get hot and the radiant barrier will still help once the roof has heated up.

I wonder how many BTUh are absorbed by dark colored roof shingles,
as compared to shingles that had a bright white coat of paint.?.

My guess is, the painted roof would not be re-radiating heat inside (under the roof)
at anywhere near the rate of the dark roof.

One web hit on the topic..
Energy Efficiency Delivered by “Cool Roof” Reflectivity - Cool Zone - Duro-Last Roofing, Inc.

Sources of roof paint
 

A few years ago I looked into roof paint that incorporates ceramic reflective beads which could be applied to asphalt shingles from a company called Hy-Tech Insulating Roof Paint. I never could find information about an actual case of such an application, or what it looked like. The paint is expensive and you would prefer to have an attractive result. Has anyone experience with thermal reflective paint? Has the discussion here so far only involved white latex paint?

Jim in Smithfield, VA

if only the hoa would feel the same way :rolleyes:.

yea i thing my Nazi hoa will gas me for painting my roof white... maybe ill do it just to be a rebel, ive been contemplating it for about 2 years now lol.

well im preparing to go to my HOA and ask for premission to paint my roof white

So if heating load is greater than cooling load, it would be advantageous to have a dark roof that heats up in winter than a light roof that reflects heat in summer? What about greenhouse paint that is white but designed to last only through the warm part of the growing season and is mostly washed off by rain by the following winter when the sun is needed to heat the greenhouse?

It doesn't work that way because your roof needs to stay cold in the winter or you'll have ice dams. This is one reason why there are issues if attics aren't either ventilated or directly insulated. I've been in my attic when its 30 degrees outside with no clouds and the sun up for hours when it wasn't covered in snow, you still need a coat. Don't forget once the black roof is covered in white snow, it doesn't matter what color your roof is anymore and there is less sun in the winter too which is why it is a much bigger deal in the summer.

What if you have a cathedral ceiling with no attic?

"or directly insulated." The roof has insulation directly against it(between the roof decking and ceiling) which will be similar. In a situation where there is a large cavity such as a flat ceiling and a sloped roof, that extra space needs to be vented unless the roof itself has insulation directly against it. In a case where there is a flat ceiling and an open cavity with insulation against the roof(called a 'hot roof'), the entire cavity is considered conditioned space since normally you wouldn't put insulation against both the roof and between the ceiling and the living space, its one or the other, otherwise issues with moisture become a real problem unless that space is vented and if its vented you wasted some money on insulated both sections. ..unless the 'insulation' against the roof is a reflective heat barrier but I don't consider those insulation like some people say they are. I like to use insulation as a term used for a barrier to conductive heat transfer, hopefully in conjunction with a proper convective heat barrier as part of the system.

Yeah I agree that this should be only academic. If you have a vented roof I think there should be R-60 up there and then it doesn't matter what color your roof is.... unless you need to go up in the attic in the summer. In the summers here i New England where everyone has back shingles I am SUFFERING. Sure the home owners talk about how cool the house is after R-60 is in place but putting it there is miserable. Nothing like blowing cellulose in a cramped space in 130 degree heat. Even if you use good cellulose that isn't too dusty, like the Home Depot stuff, you still walk away completely covered is grey glue. When I put a metal roof on my house it's not going to be black! Also on the menu for this weekend is to install three lights in the attic with a switch So I can see what I'm doing when I air seal it to the hit in the summer. Beats the hell outta carrying a drop light around.

It's almost 8PM and my attic is still 91 deg F.
Musta been cooking up there this afternoon!

Several years ago, I was involved in a yearlong study of white roof coating. We used a product called Acrymax. We painted all the roofs in an entire city block in Philadelphia. Temperature sensors were installed in various places throughout the block; light poles, roofs, peoples houses, etc to generate data. The same was done in an adjacent city block, whose houses had traditional black tar roofs.

In general, it was found that the white roofs were 50-80 degrees cooler than typical asphalt roofs. The top floor of the homes about 5 to 15 degrees cooler (depending on insulation levels of the houses). This meant that a strategy of summer cooling whole house fans rather than A/C could be used. Average energy reductions were in excess of 30%.

Another interesting point I remember was the average street temp in the height of the summer was 5 degrees LESS that the adjacent (black roofed) street (urban heat island effect).

+++

A have just recoated my white roofs (over metal roof) with the initial coating lasting about 10 years. Recoating cost about 70 cents sq ft.

I was repainting my house bright white with sky blue trim (it was brown with a dark brown roof when I brought it) and thought why stop at the roof line, so I bought two 40lbs bags of hydrated lime (at $12 each, much cheaper than paint) and whitewashed my roof and it is now a bright white to match the walls. The advantage of whitewash is that it will gradually wear off, so by late winter the roof should be back to its original color and absorbing the winter's sun for warmth. It only took 2 hours to whitewash the roof with a paint roller, so the whitewashing process is quick and cheap enough to be an annual spring event. Also as it weathers, whitewash only releases calcium carbonate onto the surrounding landscape rather than whatever unknown chemicals are released from paint as it weathers. It should also help extend the roof life by greatly reducing the 24 hour temperature swing experienced by the shingles.

Effect: it hasn't been hot enough for A/C use since I whitewashed the roof, but on the previous day the attic vent fan (set to turn on at 120F) kicked on at 11am and shut off at 4pm, while on the two days following the whitewashing the fan never turned on. All 3 days were sunny with a high of 79F. On the original dark brown roof, I could feel the heat radiating (almost too hot to touch) off the shingles at noon, whereas the white roof just gets slightly warm to the touch.

I would do some research to check out what the potential interaction might be between whitewash and the material your roof is composed of. I know that a problem has recently been observed around the use of copper flashing & wood shake roofs; because of the wood preservative being used leeches out & reacts with the copper.

You might also consider if it matters that the whitewash will change the Ph of the ground surrounding your house, potentially having an effect on whatever is growing there.

Whitewash is just calcium carbonate (limestone) and has no detrimental effects on a composition shingle roof with no flashing. I live in an area with acidic soils and have to add lime to the garden soil from time to time to raise the pH. Any calcium carbonate leached from the whitewash on the roof would have the same effect as liming the soil.

According to this https://docs.google.com/viewer?a=v&q...tptvt4fFhUmBXA

an article from the 4th Int Symposium on Roof Technology; Re use of fly ash in shingle manufacture:

"A common specification among shingle manufacturers is a limit to the free lime content in filler. Roofing industry experience has shown that an excess amount of lime can react with certain asphalt constituents to greatly accelerate granule loss from the shingle, resulting in premature failure of the roof."

I'm using metal roofing supported by metal beams. I have vents in the ceiling located outside the house.

So I'm thinking that if I repaint the roof in white. it'll make a big difference in heat reduction is that right? or am I better off putting a layer of that foil wrapped foam sheets inside the ceiling (which may entail taking off the ceiling to put it in..fyi I have a low ceiling)

http://i43.photobucket.com/albums/e3...a/IMG_1834.jpg

I would white coat the roof with Acrymax Acrymax Technologies, Inc. - Flexible Solutiong in High-Perfomance Coating Systems! AND insulate the the space under the roof. Remember, heat travels by radiation, conduction & convection; so the best game plan addresses all three. WRC only really addresses radiant heat gain.

Perhaps a access hatch could be made in a closet ceiling to gain access.

I have metal low slope roofs on my 1880's Second Empire style house & it has made a bid difference.

I power washed the roof; put down the base coat (with roller). You then roll out a layer of fiberglass mesh which becomes embedded in the wet undercoat. Then you roll on the final reflective coat. I first did white roof coating in about 1984 (when I learned about it from the National Park Service), and did a top layer recoating last year.

I also tried EP Henry's & Ames Labs similar product, which are less expensive. That is because they have more liquid and less pigment. The roof I did with EP Henry lasted only about 4 years, Ames about 7. I have gone back to the Acrymax.

To maximize its efficiency, you have to keep it clean, so consider giving it a wash before hot weather.

PS: I have NO affiliation with any material supplier or manufacturer whatsoever. I am only interested in results.

The paragraph above refers to excess free lime in the finely pulverized mineral stabilizer filler used in the interior of the shingle during the manufacturing process. Filler is used inside the shingle to keep the asphalt from flowing when it gets hot on the roof. That article proposed using fly ash as replacement for the pulverized limestone traditionally used as filler. It isn't referring to the much larger sized grit imbedded on the surface of the shingle.

What I am doing involves just the upper exterior surface of the shingle. The granules on composition roofing are usually made of limestone (calcium carbonate), which is what hydrated lime turns into as it cures. I'm just adding a thin layer of unpigmented (white) limestone on top of the pigmented limestone grit that is already there. Calcium hydroxide has long been used as an additive to the asphalt used for paving roads to make pavement last longer.

Fly ash is a mix of silicon dioxide, aluminum oxide, and iron oxide, which chemically is very different from the calcium hydroxide in hydrated lime.

Thank you very much Sir! Will be looking for that. I just hope it'll handle up to extreme tropical weather. 7 months sun (100 degrees Fahrenheit at peak sun) and 5 months rain. I have several months to source it out since rainy season has begun already.

oh and I like that closet hatch idea, been contemplating If i had to make a hole in the bathroom or in the hallway..now that's definitely better and wont make the house look messy

When blowing insulation thru a ceiling hatch, I start at the farthest point I can get the hose and work back to the hatch. If you can't reach the entire crawlspace, consider cutting a 6" circular hole in the roof, blow in insulation then install a mushroom or turks head vent in the hole.