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Old 07-26-10, 02:47 PM   #1
Daox
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Default 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

Quote:
House temperatures with regular roof and AC OFF......... 87-92F.
WHITE ROOF (and ac OFF)........ 79-82F.

ATTIC temperatures with regular roof.................... 120 - 140F
WHITE roof ................... 90 - 97F .


Attic temperatures were reduced by about 30 to 40 degrees.... and house temperatures were reduced about 8 or 10 degrees.

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Old 07-26-10, 04:05 PM   #2
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Interesting. I wonder how less visually intrusive alternatives compare. Say a radiant barrier with increased attic airflow.
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Old 07-26-10, 04:55 PM   #3
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Yeah, I'd imagine you could achieve the same thing with an attic fan or two (possibly solar powered).
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Old 07-26-10, 05:20 PM   #4
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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. 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
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Last edited by TimJFowler; 07-26-10 at 05:29 PM.. Reason: added note about attic ventilation
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Old 07-27-10, 07:00 AM   #5
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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.
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Old 07-27-10, 01:10 PM   #6
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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
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Old 07-27-10, 02:22 PM   #7
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Since you have a newer metal roof, can you comment on the changes you have noticed in the summer?
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Old 07-28-10, 11:58 AM   #8
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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.

In the summer of 2003 we had a heatwave with high temperatures in the mid to upper '90's (record high of 99F). 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 102F and max. indoor temp. has been 85F. While 85F 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
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Old 11-04-10, 04:09 AM   #9
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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.
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Old 11-08-10, 10:25 AM   #10
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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.

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