“carport” over your roof saves AC costs?
I had to do some work on a house in Riverside CA last August when temps were running over 100*F. I noticed that the temp outside under a tree was way less than it was in the garage. It got me thinking.
Most of the heat input to a house comes from insolation, not from air temp differences, so that if you could shade the house your AC would not have to work so hard to keep it cool; like the difference between day and night AC loads. Air temp in a carport will be lower than in a closed garage because the slightest breeze carries off the heat absorbed and re-radiated by the roof.
In your house, insolation during the day pumps huge amounts of heat into the attic which your ceiling insulation has to try to keep out of living spaces. What if the house had a second roof with a 1′ air space to allow air to flow freely and take away the excess heat. Would there not be huge reductions in energy consumption by the AC system?
Replies
That close you would still get a lot radiated heat. Maybe 6" and large gap at the top to for hot air and a radiant barrier on the bottom of the over roof would made a big difference. But is is cost affective.
I suspect that putting a radiant barrier on the underside of the roof would be more cost effective.
But don't have any numbers to prove any of this.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
That was 1 foot, not 1 inch.
BruceT
I think theres something to your theory.
Carports are huge here in Austin. These folks have been battling the heat here for many years, so they must know something that I don't know....yet.
I have seen some retro mod homes with carport type shades built over them. I suspect they have the same idea although it might have been done for looks rather than function.
fka (formerly known as) blue
I am confused by much of what you say, but think we agree on your conclusions.
"Most of the heat input to a house comes from insolation, not from air temp differences,"
???????????????????
Do you mean isolation or insulation?
Either way, I don't get your drift there. Insulation does not put any heat into a house. The purpose of insulation is to keep heat energy wherever it is.
"In your house, insolation during the day pumps huge amounts of heat into the attic which your ceiling insulation has to try to keep out of living spaces"
Yes ceiling insulation is there to keep attic heat out of the living space.
"What if the house had a second roof with a 1' air space to allow air to flow freely and take away the excess heat. Would there not be huge reductions in energy consumption by the AC system?"
Now you are cooking!
This roof system is called a cold roof, used extensively in extremely cold climates to buffer the extremes of temperature, but ti also works in hot climates as the sort of buffer you envision.
It is the basis of vented attics and those are sometimes modified and effectiveness increased by stapling a radiant barrier to the bottoms of roof rafters.
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"Insolation (from INcoming SOLar radiATION) is a measure of solar energy received on a given surface area in a given time. It is commonly expressed in kilowatt-hours per square meter per day (kW•h/m²/day). The surface may be a planet or a terrestrial object inside the atmosphere, or any object exposed to solar rays including spacecraft. Some of the solar radiation will be absorbed, causing radiant heating of the object, and the remainder will be reflected. The proportion of radiation reflected or absorbed depends on the object's reflectivity or albedo. Sometimes, as in the text below, a long-term average intensity of incoming solar radiation will be given in units such as watts per square meter (W/m2 or Wm-2) and called insolation, with the duration (such as daily, annual, or historical) stated or only implied.".
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
OK, Now what he said makes sense.But why not speak english instead of using made up terms that are designed to confuse being so close to other common terms in the lexicon?Or at least use all caps to indicate it is a made up termlike this - IN-Sol-Ationnow THERE is a WORD!;)
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Insolation is a common term in environmental thermodynamics.
Thermoply dynamite? I never knew it existed!fka (formerly known as) blue
i was think'n (dangerous i know) but on my flat built up roofs... one is to coat them white and two i was think'n some type shade sail... made out of that screen like material that allows water to pass thru but still provides shade... maybe 3ft above the roof surface? never seen it done... but opinions? i can't see the cost be'n too extreem..
thanks
P
"but on my flat built up roofs... one is to coat them white and two i was think'n some type shade sail..."Might reduce drains/scuppers clogging from leaves too.
BruceT
pretty urban area... i'm about 40ft above the trees...
last summer i built a penthouse and deck on the roof in august... kept a lawn sprinkler run'n on the roof with me... had to be 140 on the roof... the deck is about 40" above the roof... huge difference under it vers on the roof...
i was think'n (yeah i know) some type cable system to hold them taunt... a series of maybe 15' x 15ft "shade screens" (150 x 75 main roof) (80x 80 lower roof...) it would have to make the roof live longer... and have to make the building cooler...
right now i'm play'n with paints... did a few test patches... best so far is white traffic marking paint... just need to come up with a deal on 60 gallon... list on the stuff i'm testing is $35gal...
if it sticks to asphalt and you can drive on it and it's in the same sun as my roof... i'm think'n it might just work...
p
I made some ordinary window boxes, covered with ordinary screen. It surprised me the difference ordinary screen material made on the 'gain' I felt from the windows. ANY type of shade is a help. One critical part of this solution is to have free air flow. Ever wonder why desert robes come in two colors: all black or all white? Two approaches to the same problem. The guys in black are 'carrying' the shade with them.
I've read alot of ideas & opinions on how to cool a roof
one that i researched pretty deep was shallow planting boxes placed all over the roof...... 4'x8' maybe 6" deep on 30" or so legs on the roof planted with a grass that could withstand low water levels... lots of ideas on why these made the building cooler... i think 90% of it was the shade they created on the roof surface...
p
"Do you mean isolation or insulation?"Insolation is the term for heating by solar energy from direct sunlight as opposed to heating from air temperature. That's why your attic or a parked car gets much hotter than the air outside; solar energy from direct sunlight gets pumped in way faster than heat can radiate back out.
BruceTOops, I didn't notice that Bill Hartman had already explained insolation - better than I could.
Edited 10/26/2007 10:04 pm by brucet9
The tree turns some of that radiant energy from the sun into latent energy - by the water that evaporates from the leaves. Also, the leaves tend to overlap quite a bit, so there are multiple levels of shade from one tree. Some of the radiant energy that makes it through one leaf is stopped by the next one down.
"The tree turns some of that radiant energy from the sun into latent energy - by the water that evaporates from the leaves."Some of that energy would also be converted during photosynthesis as well. I guess If you can't shade the house with trees, you could plant a lawn on the roof. Of course during drought periods, you might be fined for watering your roof. :)
BruceT
What your suggesting can be accomplished by adding a thermostatically controlled, powered ventilation system to your attic space. This works best if you have a fully vented soffit. The vent fan should be installed close the ridge in a central location. Here's one source. http://www.airvent.com/homeowner/products/powerVents-powerRoof.shtml
You can also add reflective material to the inside of your roof, though I'm not convinced that it has much effect. A better reflective material is white shingles.
I am thinking about doing something like this for my steel garage, but would like to take advantage of winter warming. I would like to stretch shade cloth rated at blocking 90% of sunlight across my roof (say 4" above it) in summer and remove it in the fall. Some sort of roller & track system might work.
Yeah, I've installed some of those powered vents as well as passive "whirly" vents. They help, but those attics still stayed much hotter than outdoor temp.
BruceT
Yes, you would. I have experienced this first-hand. I have suggest such here a few times.
That "roof over a roof" idea is used in the following pictures. The system works pretty well, especially if painted white.
http://www.land-roverco.com/images/cars/blue3/blue3_02.jpg
http://www.land-roverco.com/images/cars/ed_scripps/ed_scripps001.JPG
The LA Times Home section did a short piece a couple years back about a house in Palm Springs that did just what you are thinking. There was a normal looking ranch house, and overtop was a big metal roof canopy, similar to a pole barn. It was about twice the height of the house. It looked odd, but still OK.
The woman who owned it said that she really reduced her AC use since it was put in.
Rebuilding my home in Cypress, CA
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